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[begin page]
Chapter
8
The Origin of Species
What Steps Lead to the Evolution of New
Species?
KEY CONCEPTS
After completing this chapter you will be
able to
∙ [t/k in order used in chapter]
∙
Darwin’s theory of evolution by natural selection and the evidence that
supports it have convinced biologists that Earth’s biodiversity—the millions of
species alive today, as well as the countless millions that have lived in the past
and are now extinct— are all products of evolutionary change.
Given our understanding of DNA, inheritance, and mutation, it is not
difficult to accept that the genetic makeup of a species might change over time.
And, given the human experience in plant and animal domestication, it is also
quite obvious that selection can lead to significant changes in species over
time. But how can selection and mutation result in major evolutionary changes
including the evolution of millions of different species? And why, if the theory
evolution is true, are there still so many “primitive species”? Why have all
species not evolved into advanced organisms like us? As you will learn, the
answer hinges on the word “different.”
Each of the millions of different species that are living and have lived on
Earth is unique. Each species has specialized features that make it well suited
to a particular ecological niche in a particular place and time. The mudskippers
in the photograph on the right are a wonderful living example of a specialized
species. Mudskippers are amphibious fish, spending part of their time
underwater and part on land. When tides are low, mudskippers move around
on the exposed beach, capturing small prey and defending territories. During
high tide or when they are threatened, they hide in burrows that they dig in
the soft substrate. Mudskippers have many adaptations that support this
lifestyle. Like many frogs, they can breathe through their damp skin and the
roof of their mouth. They also trap large bubbles of air inside their gill pouches
when on land to provide more oxygen to the gills than would be available in a
small pocket of water. It would be a mistake to think that these fish are “on
their way” to becoming fully terrestrial vertebrates. Instead, we should
recognize that they are filling an ecological niche that favours their success.
In this chapter, you will learn how different factors and circumstances
have led to both the evolution of species, including our own, and to mass
extinction events. You will also learn how understanding the evolutionary
history of a species can be of benefit today.
STARTING POINTS
Answer the following questions using your current knowledge. You will have an
opportunity to revisit these questions later, applying concepts and skills from the
chapter.
1. What human characteristics or adaptations do you think are most
responsible for our success as a species?
2. What human features do you consider least advantageous? In what ways
are some human features disadvantageous under certain conditions?
3. Brainstorm possible situations that you think might cause one species to
evolve into two different species.
4. Species have always gone extinct—it is a natural process. Given that this
is true, do you think there is any reason to be worried about species going
extinct today? Explain your reasoning.
[end page]
Investigation X.X.X 1
[begin page]
[Formatter: this is the chapter opener photo, and should fill the page up to the
Mini Investigation (below)]
[CATCH C08-P042-OB11USB; Size CO; Research. Photo of a mudskipper partly out
of water.]
Mini Investigation
Simplicity, Complexity, and Diversity
Skills: t/k
[catch Skills Handbook icon: t/k]
The word “primitive” is often used incorrectly from a biological perspective. You
might hear someone describe a snail as being “more primitive” than a mammal,
or single-celled organisms being more primitive than multicellular organisms. In
biology, however, the word “primitive” means ancient. Primitive species are
those that lived millions of years ago. In contrast, all organisms that are alive
today—from bacteria to tulips, and snails to whales—are all “modern” species.
The biological distinction is that some lineages have remained simple, while
others have evolved to be highly complex. You might wonder why some
species have remained very simple while others have become very complex.
You might also wonder why species are so different. If natural selection favours
“beneficial traits.” why do all species not have the same traits? In this activity,
you will have an opportunity to ponder answers to these questions and gain
insights into the factors that are responsible for the formation of new species.
Equipment and Materials: Selection Scenario Cards
1. Obtain a Selection Scenario Card (Table 1). Each card describes a particular
situation or scenario and one characteristic that would likely be strongly
influenced by natural selection in that situation. The card also lists two species.
2. In discussion with your partner or group, brainstorm the natural selection
pressures that might influence the chosen characteristic in this particular
scenario. Create a bulleted list of your thoughts.
3. Now carefully consider the particular characteristics of the two species.
Decide if you think one species would be significantly more successful in this
particular scenario. Record your decision and your reasoning.
4. Exchange Selection Scenario Cards with another student group and repeat
the exercise.
A. Were complex species always best suited to take advantage of a particular
set of environmental conditions? Use examples to support your answer. [T/I]
B. Compare your results with others’ results. Did all student groups reach the
same conclusions? [T/I]
C. What did your conclusions suggest about the relationship between different
species and their success under different environmental conditions? [T/I] [A]
Table 1 Samples of Scenarios
Example scenarios
Trait of interest
Species
animal-pollinated
ability to feed on nectar butterflies,
Investigation X.X.X 2
flowering plants
growing high in the
mountains where the
winds are strong and
temperature remains
cool all summer long.
an area of open
savannah where there
are many large
carnivores with a highly
developed sense of
smell and keen
eyesight
a large area of land and
lakes in Northern
Ontario during the
month of January
a hot desert with very
low productivity where
rains occur less than
once a year
a large animal dies and
sinks to the bottom of a
deep lake
[end page]
and pollinate plants
hummingbirds
the ability of young
individuals to avoid
predation
chimpanzees, antelope
ability to survive by
staying warm and
obtaining food
chickadee, loon
ability to endure long
periods without a large
food supply
snake, rodent
ability of take full
advantage of this
sudden food supply
bacteria, trout
Investigation X.X.X 3
8.1
catch C08-F01b-OB11USB;
Size D; New. Map of African
showing areas with the sicklecell allele]
[catch C08-F01a-OB11USB;
Size D; New. Map of Africa
showing areas with malaria]
Figure 1 (a) The sickle-cell
anemia allele is more prevalent
in (b) populations where there is
a high level of malaria.
Natural Selection <Catch: 8 pages>
Evolution occurs when natural selection acts on the genetic variability within
populations. Genetic variation arises by chance through genetic mutations and
recombination. The process of natural selection, however, does not occur by
chance, because the environment favours certain individuals or others. Just as
human breeders have selected individuals in artificial selection of
domesticated plants and animals, the environment selects individuals that are
better suited to their environment .
Sickle-cell anemia is a useful example of how mutation, genetic variation,
and natural selection can lead to a change in a population. In humans, the
sickle-cell allele results from a single base mutation in the DNA coding for
hemoglobin. Individuals who are heterozygous for the allele are resistant to
malaria and thus have a better chance of surviving than those who lack the
allele. Figure 1(a) shows the distribution of the dreaded disease malaria in
Africa. People living in this region who were born with the sickle-cell mutation
are more likely to survive than those living elsewhere. With survival comes
reproduction and the passing on of the sickle-cell allele to the next generation.
Over time, the result has been an increase in the frequency of the allele within
those populations (Figure 1(b)). The sickle-cell mutation may never have
occurred in the populations living in the malarial regions of north Africa and
Madagascar island.
Types of Selection
Selective pressures may result from any number of abiotic or biotic factors:
diseases, climatic conditions, food availability, or predators—or even your
mate! These selective pressures can result in different patterns of natural
selection.
directional selection selection
that favours an increase or
decrease in the value of a trait
from the current population
average
[CATCH C08-P001-OB11USB;
Size D; Research. Photo of
hummingbirds. Ideally the photo
should show both the long and
short-billed species. Preferably
around flowers (i.e. not at an
artifical feed as in example).]
Figure 2 There are more than
300 species of hummingbirds.
Their bill lengths can vary
dramatically from species to
species.
Directional Selection
Directional selection occurs when selection favours individuals with a more
extreme variation of a trait. The result is a shift away from the average
condition. Directional selection is very common in artificial breeding, where
individuals with an enhanced trait are often selected. Strawberries have been
selected for larger and sweeter fruits, chili peppers for hotter flavour, and
thoroughbred horses for their running speed.
Consider the following example of directional selection in nature.
Hummingbirds use their bills to feed on nectar (Figure 2). Suppose a
population of hummingbirds enters into a new habitat with plants that have
longer flowers. The hummingbird population includes individuals with a
variety of bill lengths, though most have a bill best suited to medium-length
flowers (Figure 3(a), before selection, next page). In the new habitat,
individuals with slightly longer bills are favoured by the environment and will
be more successful than those with medium-length and shorter bills. Longerbilled birds will obtain more food and contribute more offspring to later
generations (Figure 3(a), after selection, next page). Eventually the bill length
of the population will increase.
[END PAGE]
[new page]
Investigation X.X.X 4
Stabilizing Selection
stabilizing selection selection
against individuals exhibiting
variation in a trait that deviates
from the current population
average
Stabilizing selection occurs when the most common phenotype within a
population is favoured by the environment. For example, imagine an initial
population of hummingbirds that lives in an unchanging environment with
medium-sized flowers (Figure 3(b), before selection). The most common
medium-billed hummingbirds will be favoured. A longer bill requires more
nutrients and energy to grow and carry around, while a shorter bill may
reduce a bird’s ability to reach food within the flowers. Selective pressures will
reduce the reproductive success of individuals that exhibit extremely long or
short bills (Figure 3(b), after selection).
[catch C08-F02-OB11USB; Size B; New. Diagram illustrating directional
selection.]
[catch C08-F03-OB11USB; Size
D; New. Graph showing two
different sets of data for Birth
mass (kilograms). ]
Figure 4 Human babies with
average birth weights have a
higher rate of survival.
disruptive selection selection
that favours two or more
variations or forms of a trait that
differs from the current
population average
Figure 3 Examples of selection in a population of hummingbirds. (a) In a new
environment with longer flowers, directional selection will favour individuals with
longer bills. (b) In stabilizing selection, individuals with an average bill length are
favoured. (c) In disruptive selection, the environment has long and short
flowers, which favour individuals with long and short bills over individuals with
average bill lengths.
Human birth weights are also subject to stabilizing selection. Birth
weights are variable, and part of this variability is heritable. According to the
theory of evolution by natural selection, babies born at weights offering the
best chance of surviving birth should be more numerous. More human babies
are born weighing just over 3 kg than any other weight. Babies with
significantly lower weights are often developmentally premature and less
likely to survive, while heavier babies often experience birth-related
complications that threaten the life of both baby and mother (Figure 4).
Disruptive Selection
Disruptive selection favours individuals with variations at opposite extremes
of a trait over individuals with intermediate variations. Sometimes,
environmental conditions favour more than one phenotype. For example, two
species of plants with different-sized flowers may be available as a food source
for the hummingbird population (Figure 3(c), before selection). Each species is
a good source of nectar, but neither is well suited to a hummingbird with a
medium-length bill. Birds with longer and shorter bills will be more successful
and will contribute more offspring to later generations (Figure 3(c), after
selection).
[END PAGE]
[new page]
Investigation X.X.X 5
Sexual Selection
sexual selection differential
reproductive success that
results from variation in the
ability to obtain mates; results in
sexual dimorphism and mating
and courtship behaviours
INVESTIGATION 8.1.1
Bird Monogamy and Sexual
Dimorphism
[TK]
[CATCH C08-P003-OB11USB;
Size D; Research. Photo of a
bat eating a tungara frog.]
Figure 6
[CATCH C08-P004-OB11USB;
Size D; Research. Photo of a
flower being pollinated by a
butterfly or other unusual insect.
Try to find an image of a
monarch butterfly pollinating
milkweed. Do not show a bee.]
Natural selection favours the reproductive success of individuals with certain
traits over others. While reproductive success is enhanced by being healthy, an
even more essential requirement is finding a mate. Sexual selection is the
favouring of any trait that specifically enhances the mating success of an
individual. Sexual selection often leads to the males and females of a species
looking different and behaving quite differently.
The most common forms of sexual selection are female mate choice and
male-versus-male competition. In many species, females choose mates based
on physical traits, such as bright coloration or behaviours (Figure 5(a)). In
other species, males have evolved larger body size and other physical
attributes such as antlers that are often used in direct competition (Figure
5(b)). The males often fight each other to establish control over a territory that
is home to females with which they can mate. The difference between success
and failure can be dramatic. For example, a very successful male elephant seal
may mate with dozens of females each year and hundreds of females in his
lifetime, while a weak male may live a long life without ever reproducing. In
this case the genes of the shorter-lived but dominant male are destined to
become more common in succeeding generations.
[CATCH C08-P002a-OB11USB; Size B1; Research. Photo showing an Ontario
bird with bright male and duller female mate (e.g. a goldfinch). Bird should be
brightly coloured. Photo must be of a pair of birds; second example for
reference only.]
[CATCH C08-P002b-OB11USB; Size B1; Research. Photo of a male animal
with antlers or large horns. Please use either a male elk or a big-horned sheep.
DO not use a moose. Preferred photo of big-horned sheep about to crash into
each other]
Figure 5 (a) Male goldfinches use brightly coloured plumage and song to attract
females. (b) Male bighorn sheep compete head to head, using their horns for
head-on clashes. Female bighorn sheep have much smaller horns.
While traits such as bright coloration and large antlers can be favoured by
sexual selection, they are often a disadvantage when it comes to longevity.
Avoiding predators is not made easier by brilliant plumage or a distinctive
song. Fringe-lipped bats, for example, locate male tungara frogs by listening for
their mating calls. Male frogs that call frequently are more likely to be eaten.
Male frogs that never call remain safe but are unable to attract a mate.
Sexual selection is not limited to animal populations. Colourful flowers
and scents are the most obvious sexual features of plants (Figure 6). Rather
than attracting mates, these features attract pollinators. By maximizing their
chances of being pollinated, plants have a greater likelihood of contributing
more alleles to the next generation’s gene pool.
Natural Selection in Action
Natural selection results in evolutionary changes within populations. Examples
of such changes can be observed in nature and demonstrated under controlled
experimental conditions.
Geneticists have recently revealed an example of directional selection in a
human population. Tibetan people have inhabited the Himalayan mountains
for thousands of years (Figure 7). At this elevation, the oxygen level is only
40 % of that at sea level. When people from lowlands move to this elevation,
their bodies exhibit a physiological response. Over a period of days and weeks,
Investigation X.X.X 6
Figure 6 Attracting more
pollinators may ensure greater
seed production.
[CATCH C08-P005-OB11USB;
Size D; Research. Photo of a
Tibetan mountain scene. There
should be Tibetans in the photo
as well.]
Figure 7 Tibetans living at high
elevation have a high oxygencarrying capacity in their blood.
their red blood cell count increases, helping them obtain adequate oxygen. This
survival response, however, is not ideal, because the increased red blood cell
count makes blood more viscous. This places stress on the heart and results in
reduced fertility and increased child mortality. Tibetans do not exhibit
elevated red blood cell counts yet have no difficulty coping with the low
oxygen levels. Instead, directional selection has favoured a number of genetic
mutations that increase the oxygen-carrying capacity of their blood while
maintaining normal red blood cell counts. Geneticists have documented more
than 30 genes that have been selected within the Tibetan population. One
allele was almost 10 times more common in Tibetans in the study group than
among people of lowland descent.
[END PAGE]
[new page]
Under controlled experimental conditions, researchers at the University
of Wisconsin tested the hypothesis that certain behaviours might have an
inherited component and could be influenced by natural selection. The
researchers modelled directional selection in populations of mice by choosing
individuals for breeding that ran the longest distances and had the highest
speed on exercise wheels. After only 10 generations, the population descended
from the chosen mice exhibited much higher running distances and average
speeds when compared to a control population (Figure 8).
[catch C08-F04a-OB11USB; Size B1; MPU. Graph showing the distance run in
revolutions/minute against Generation with two lines being measured.]
[catch C08-F04b-OB11USB; Size B1; New. Graph showing the average speed
in revolutions/minute against Generation with two lines being measured.]
Table 1 Possible Selection
Pressures that Resulted in
Specific Animal Traits
Animal
trait
hawk:
acute
vision
polar bear:
white fur
elephant:
long trunk
lobster:
large
claws
wolf: keen
sense of
smell
human:
large brain
Selective pressure
• ability to spot prey
over long distances
• ability to sneak up on
seals on snow-covered
ice
• ability to reach for
food and water while
minimizing the
movement of its
massive body
• ability to crush large
shells and other prey
items
• ability to locate and
track the movements
of prey
• ability to reason and
communicate
• ability to construction
and manipulate tools
Figure 8 A controlled experiment in mice suggests that some behaviours have
a genetic component and can be influenced by directional selection.
This heritable change in mice behaviour is an example of rapid evolution.
It happened quickly—in a matter of 10 generations. While there are many
other examples of rapid and observable evolution, most major evolutionary
changes are slow, occurring over hundreds of generations and thousands of
years. In such cases, we can observe the product of the lengthy and ongoing
process of natural selection. It is often easy to speculate about the selective
pressures that have been at work. Table 1 provides some examples of wellknown animal traits and a selective pressure that has contributed to their
evolution.
What is less obvious is how natural selection produces complex
structures. Imagining the various stages and selective pressures on species
over millions of years is not easy, and unless there is fossil evidence, it may be
impossible to know how a particular trait evolved. Nonetheless, it is possible
and useful to hypothesize about scenarios of how complex features could have
evolved. The following tutorial presents one such case and challenges you to
generate a working hypothesis.
Investigation X.X.X 7
[FORMAT FULL PAGE WIDTH] Tutorial 1: Cumulative Selection
Evolutionary biologists have the unique and extraordinary challenge of not only
studying the characteristics of living things but of attempting to unravel how
living things came into existence—how they evolved. As part of this process,
evolutionary biologists often hypothesize a possible scenario that might have
led to the evolution of a particular trait and then look for evidence to support or
refute their hypothesis. In this tutorial we will outline a hypothesis for the
evolution of insect pollination in plants. This represents one of the most
significant adaptations in the history of life on Earth. Insect-pollinated plants are
among the most diverse and successful of all living things.
To begin we must base our hypothesis on a set of assumptions, and we
must choose an appropriate starting point. We not attempting to outline the
entire evolutionary history of plants themselves—only a particular trait—in this
case insect pollination.
Our starting assumptions are:
1. Insect-pollinated plants evolved from simple flowering plants that were wind
pollinated. Wind-pollinated plants produce drab greenish flowers and relatively
large quantities of pollen that is not sticky. This is a reasonable assumption
given that the simpler non-flowering seed plants—the gymnosperms—are wind
pollinated, and modern flowering plants that are wind pollinated have simple
flowers and produce large quantities of non-sticky pollen. They are typically
small and green in colour.
[END PAGE]
[new page]
2. Insects that fed on plants were very abundant during this evolutionary
process. Some of these insects fed on flower pollen. This is a reasonable
assumption because we know that many different insects visit various flowers
and cones as a source of food.
3. All new genetic variations must arise from mutation events.
4. It will take a number of different mutations to evolve such a complex
adaptation.
5. Each and every mutation must be beneficial if it is to be favoured by natural
selection.
6. The process may take millions of years.
Our evolutionary scenario describes a series of plausible beneficial
mutations and the advantage they would have offered the evolving plant. Our
goal is to present a scenario that meets all of our assumptions.
Evolutionary Scenario:
Original plant: wind-pollinated flowering plant producing millions of pollen
grains; flowers drab and odourless
Original insect: seek out flowers and feed on the pollen
Mutation 1: Sticky Pollen
Natural Selection
• A slight modification of
• Insects that visit this plant feed on its pollen
the outer surface of the
and get a small amount of pollen stuck to their
pollen grains makes the
body.
pollen slightly sticky.
• Transported pollen is more likely to reach
another flower.
• Flowers with sticky pollen have more pollen
transferred to other flowers and therefore
produce more offspring.
Additional mutations that enhance the sticky quality of the pollen will be
favoured for the same reason and over time will accumulate, resulting in sticky
pollen.
Mutation 2: Less
Natural Selection
Chlorophyll in Flowers
• Flower tissues produce
• Insects are more likely to locate these
slightly less chlorophyll.
flowers.
Investigation X.X.X 8
genetic drift changes to allele
frequency as a result of chance;
such changes are much more
pronounced in small populations
• Plant does not rely on
• Flowers are more likely to have their sticky
flowers for photosynthesis,
pollen carried from one flower to another.
so this change does not
• Flowers become more visible to insects and
affect food production.
are more likely to receive pollen.
• Plant cells contain many
pigments; with less
chlorophyll the other
pigments are slightly more
visible.
Additional mutations that enhance the colour of flower parts will make them
more visible to insects and therefore more likely to be pollinated.
Mutation 3: Hairy Insects
Natural Selection
• An insect has slightly
• Insects are favoured because they are more
longer bristles on its body. efficient at pollinating the flowers.
• Longer bristles are more
• The trait does not benefit the insect directly
likely to get covered in
but increases the success of the flowers they
sticky pollen.
feed on, resulting in a greater food supply.
• Hairy insects are more likely to get covered in
sticky pollen.
Additional mutations that enhance the pollen-transferring ability of the insect will
favour both the plant and insect.
Additional Mutations
Natural Selection
• Flower size, colour, or
• The flower is more likely to attract insects
fragrance is enhanced.
and be pollinated.
• The insect is better able to • Insects are more likely to find food.
transfer pollen or find the
flowers.
• The plant has the ability to • The plant is more likely to attract more
release small amounts of
insects and therefore increase pollination.
sap from its flowers.
• The earliest rudimentary
nectar is produced.
Scenario Challenge:
With a partner, brainstorm a possible set of simple mutations and natural
selection processes that might have led to the evolution of one or more of the
following features:
(a) Binocular vision in primates. Most mammals have eyes set out to the sides
and have very poor depth perception but better peripheral vision. Consider what
you know about the habitat and behaviour of most primates.
(b) Poison arrow frogs. These colourful frogs are highly toxic and easy to spot.
Note that both the males and females are coloured, so this is not an example of
sexual selection. Consider which feature might have evolved first—coloration or
toxicity.
[END tutorial]
Evolutionary Change without Selection
Not all evolutionary changes are the result of natural selection. They are
changes in the genetic makeup of the population that are not influenced by the
traits of individuals. As you will see, each of these changes tends to reduce
genetic diversity within a population.
[END PAGE]
[new page]
Genetic Drift
The genetic makeup of a population can change simply by chance. When
individuals produce offspring, the chances of passing on any particular allele to
Investigation X.X.X 9
their offspring is subject to random chance. The smaller the number of
individuals in a population, the greater the influence of genetic drift—the
random shifting of the genetic makeup of the next generation. In small
populations, genetic drift can result in a particular allele becoming either very
common or disappearing entirely over a number of generations (Figure 9).
Any lost alleles result in a net reduction in the genetic diversity of the
population.
genetic bottleneck a dramatic,
often temporary, reduction in
population size, usually resulting
in significant genetic drift
[catch C08-F05a-OB11USB; Size B1; New. Graph showing the frequency of
Allele A over generations.]
[catch C08-F05b-OB11USB; Size B1; New. Graph showing the frequency of
allele A while Allele A is neither lost nor fixed.]
[CATCH C08-P006-OB11USB;
Size D; Research. Photo of
cheetahs. Try to find image of a
mother with cub(s)]
Figure 9 (a) In small populations, genetic drift can result in dramatic changes
in allele frequency. (b) In larger populations, genetic drift is not usually
significant.
Bottlenecks and the Founder Effect
Figure 10 Cheetahs have very
little genetic variation because
their population was subject to a
genetic bottleneck.
founder effect genetic drift that
results when a small number of
individuals separate from their
original population and establish
a new population
Genetic bottlenecks result in a loss in the genetic diversity following an
extreme reduction in the size of a population. For example, if an initial
population of 10 000 individuals is reduced to only 50 individuals, they are
unlikely to contain all of the alleles found in the larger population. Many
alleles, and in particular rarer alleles, are likely to be eliminated in this
bottleneck event. If the population is allowed to recover, the genetic makeup of
future generations will be limited to the alleles carried by those 50 surviving
individuals and any new mutations. Bottlenecks can have adverse
consequences for populations. Cheetahs, for example, have very little genetic
variability. As a result they are vulnerable to diseases, have low reproductive
success, and have high juvenile mortality rates. All cheetahs are thought to be
descendents of a population that experienced a severe bottleneck event—
perhaps only seven individuals—about 10 000 years ago (Figure 10).
The founder effect occurs when a small number of individuals establish a
new population. For example, a small number of finches from the coast of
South America established a founding population on the Galapagos Islands.
The initial population would—by chance alone—have a different mix of alleles
than the overall population. By chance, an allele that was common in the large
population might be uncommon in the founding population, or a rare allele
might be much more common in the new population. For example, suppose an
allele is found in only 1 in 1000 (0.1 %) finches in the mainland population.
Now suppose that by chance, 1 of only 20 finches that reach the Galapagos
Islands carries the same allele. This represents 5 % of the founding
population—an increase of 50. While such a change does not increase the
diversity of the population, it does mean that the new population will begin
with a different gene pool than the original population’s gene pool.
Small populations that result from a bottleneck or founder effect are also
subject to the effects of genetic drift. This will further increase the chances that
their gene pool will differ from that of the original population.
Although genetic drift and bottlenecks can be important in some cases,
natural selection is usually the major driver behind changes that result in the
Investigation X.X.X 10
evolution of a significant adaptation to the environment. Natural selection is
the only mechanism known that is able to shape a species to its environment.
[END PAGE]
[new page]
The Hardy–Weinberg Principle
Hardy–Weinberg principle in
large populations in which only
random chance is at work, allele
frequencies are expected to
remain constant from generation
to generation
To modern biologists, evolution is the change in the genetic makeup (or gene
pool) of a population over time. Mathematically, a gene pool can be described
by the frequency of each of the alleles within the population. Two
mathematicians, Hardy and Weinberg, used mathematical reasoning to explain
the relationships between allele frequencies within a population and the
chances of those frequencies remaining constant. This relationship, often
represented by a mathematical equation, is referred to as the Hardy–
Weinberg principle.
Any factor that causes allele frequencies to change leads to evolutionary
change. Based on the Hardy–Weinberg principle, biologists recognize that the
following conditions result in evolution:
• natural selection: favours the passing on of some alleles over others
• small population size: increases the likelihood of genetic drift
• mutation: introduces new alleles to a population
• immigration or emigration: introduces or removes alleles in a population
• horizontal gene transfer: the gaining of new alleles from a different species
These five conditions are known to occur in many populations and
inevitably result in evolutionary changes over time. Knowing the influence that
each of these factors can have on a population allows biologists to predict
which populations are likely to exhibit the most evolutionary change. Biologist
must also take into account the particular biology of each species. For example,
a species that has high genetic diversity and reproduces very quickly will
respond to natural selection more rapidly than a species with little genetic
diversity and that reproduces very slowly. Such factors account for how insects
and bacteria have rapidly evolved resistance to pesticides and antibiotics.
[FORMAT IN 2 COLUMNS; FULL PAGE] Mini Investigation: Modelling
Genetic Drift
Skills: Predicting, Performing, Observing, Analyzing
Genetic drift is the random change in the gene frequencies of populations. Such
random changes are most pronounced and more likely to occur in small
populations. In nature, this happens when there is a genetic bottleneck and a
population is reduced to a small number of individuals or when a small
population is separated from a larger population and goes on to found a new
population. In this investigation you will model how such events can influence
the allele frequencies of a population.
Equipment and Materials: Initial containers of “Population A” and “Population
B” (pop-it bead organisms); calculator
1. Remove all the individuals from the Population A container. Each individual is
represented by two pop-it beads joined together. Each pop-it bead represents a
single allele; different colours represent different alleles. Together the two pop-it
beads represent the genotype of the individual.
2. Without separating the beads, count the total number of each type of allele
(bead colour). Record this number as a percentage of the total number of
alleles in the population gene pool. These values represent the initial allele
frequencies. For example, if there are 50 individuals in the container and 10 are
red and 40 are blue, then the gene pool contains 100 alleles and the allele
frequencies are 20 % red and 80 % blue.
3. Return all the individuals to the container and mix them thoroughly.
Investigation X.X.X 11
4. Without looking, you are going to randomly remove 10 individuals from the
container. Before doing so, make a prediction about the allele frequencies of a
these ten individuals. These ten individuals represent the survivors of a
bottleneck event (or a small founder population).
5. Count and record the allele frequencies of this population, and then return
the 10 individuals to the container.
6. Repeat Steps 3 through 5 twice.
7. Repeat Steps 1 through 6, beginning with Population B.
A. Was there evidence of genetic drift? Did the allele frequencies change when
the populations were reduced from 50 to 10 individuals? Were your predictions
accurate? [K/U]
B. Were any changes in allele frequencies consistent? For example, did the
allele frequencies of the most common allele always decrease? [K/U]
C. Were any alleles ever lost entirely in a new population? [K/U]
D. According to the Hardy–Weinberg Principle, there are five conditions that
can result in changes in allele frequencies. Which of these conditions were
modelled in this investigation? Which were not? [K/U] [T/I]
E. Based on your results, do you think genetic drift is likely to cause an increase
or a decrease in the genetic diversity of a population? [T/I] [A]
[END Mini Investigation]
Consequences of Human Influence
Humans interact with all other species, either directly or indirectly. We
commercially harvest many species from the wild; we alter habitats by
clearing land for agriculture, urban expansion, and mining operations; and we
pollute the air, soil, and water. Our production of greenhouse gases is changing
Earth’s climate and the chemistry of the oceans. We also set aside large areas
as parks and intervene to protect endangered species. All of these interactions
act as agents of natural selection and have the potential to influence the
evolution of species. Table 2 describes some consequences of human activities
on the evolution of species.
[END PAGE]
[new page]
<Formatter set Table 2 as full page width>
Table 2 Consequences of Human Activities on Evolution
Human selective Pressure
Evolutionary change and
consequences
- Commercial fishing
- The average adult size of many
targets large fish and often valuable commercial fish species,
allows smaller fish to
including cod, has declined
escape. Some fishing
dramatically.
regulations even require
-The alleles that code for large
the release of small fish.
adult size are being lost from the
- Fish that reach maturity
gene pool of the population.
at a smaller size are more
likely to escape and
reproduce than individuals
that reach sexual maturity
at a larger size.
Exampl
e
[CATCH
C08P043OB11U
SB; Size
E;
Researc
h. Photo
of a cod
fish]
Investigation X.X.X 12
- Habitat loss, the
introduction of invasive
species, and
overharvesting have
reduced the population
sizes of many species to
extremely low levels.
- This has created genetic
bottlenecks, reducing the
genetic diversity of the
species
- Populations with little genetic
variability are less able to survive
environmental changes and
diseases.
- Even if their size recovers,
populations will not recover their
genetic diversity and will remain
vulnerable to diseases and other
threats.
- The population of northern
elephant seals was reduced by
hunting from over 100 000 to just
24 individuals by the 1890s. The
population in now over 30 000,
but the seals have very little
genetic diversity.
- Many migratory bird species are
expected to begin migrating
shorter distances and eventually
stop migrating entirely.
- Species living in alpine and
arctic environments may not be
able to adapt quickly enough to
survive.
[CATCH
C08P044OB11U
SB; Size
E; Photo
of
Norther
n
elephant
seal]
- Selective hunting of prize
animals favours individuals
with less desirable traits.
For example, elephants
that grow smaller tusks are
less likely to be shot for
their ivory. Bighorn sheep
that grow smaller horns are
less likely to be shot as
trophy animals.
Individuals that exhibit prized
traits become less common in the
population.
- The average tusk size of mature
African elephants is decreasing.
- Close to 50% of all male Asian
elephants are tuskless. This may
be have resulted from selective
hunting practices in the past.
[CATCH
C08P046OB11U
SB; Size
E; Photo
of
tuskless
male
Asian
elephant
s]
- Use of insecticides and
herbicides is widespread.
- Resistant insects and
weed plants are more likely
to survive and reproduce.
- Many insects and plants, such
as bedbugs and pigweed, are
becoming resistant to pesticides.
- The cost of controlling these
pests and the economic losses
they cause are increasing.
[CATCH
C08P047OB11U
SB; Size
E; Photo
of
bedbugs
]
- Climate change is altering
selection pressures on
species in many ways.
- In some situations
changes may happen too
rapidly for species to
adapt.
[CATCH
C08P045OB11U
SB; Size
E; Photo
of a
caribou]
Investigation X.X.X 13
- Use of antibiotics and
antimicrobial cleaners is
widespread.
- Many infectious bacteria, such
as methicillin-resistant
Staphylococcus aurea (MRSA),
are becoming resistant to multiple
varieties of antibiotics, making it
more difficult and expensive to
treat patients.
- In the home, antimicrobial soaps
and cleaners rapidly kill off weak
bacteria that may be replaced by
more resistant forms.
[CATCH
C08P048OB11U
SB; Size
E; Photo
(microgr
aph) of
methicilli
nresistant
Staphyl
ococcus
aurea
(MRSA)]
The Driving Force of Evolution
A variable and changing environment, mutations, and natural selection all play
important roles in evolution. Whenever a mutation occurs that is beneficial to
a population, it produces a trait that will be favoured by natural selection. This
favouring of a particular trait can result in the trait, and the associated allele,
becoming widespread in the population. Beneficial mutations acted upon by
natural selection is the key driving force of evolution whereby populations
become better adapted to survive and reproduce in their habitat.
8.1 Summary
• Directional and disruptive selection produce evolutionary changes by
favouring individuals that are differ from the population norm.
• Stabilizing selection acts to limit evolutionary change by favouring the
current population norms.
• Sexual selection is a form of natural selection in which traits are favoured
that specifically enhance mating success.
• Evolutionary changes produced by natural selection can accumulate over
time and result in major adaptations and the formation of new descendant
species.
• Genetic drift produces evolutionary changes independently of natural
selection.
• Bottlenecks and the founder effect enhance the influence of genetic drift.
• The Hardy–Weinberg principle can be used to identify those factors that will
result in evolution change.
• Human activities have a very strong selective influence on many species and
therefore influence their evolution.
8.1 Questions
1. Biologists often describe evolution as a change in the frequency of alleles
in a population. How does this definition relate to the process of natural
selection? [K/U]
2. Describe the way in which natural selection has influenced the genetic
makeup of Tibetan human populations. [K/U]
3. Use the example of the sickle-cell allele to illustrate how natural selection
can cause a mutation to be beneficial in one environment and harmful in
another. [K/U] [A]
4. How is it possible for evolution to favour behavioural traits in males of
Investigation X.X.X 14
some species that cause them to risk their life in fights with other males over
mates? [K/U] [A]
5. Suggest which type of selection led to the following characteristics: [K/U]
T/I]
(a) hollow and very lightweight bones in birds
(b) hundreds of different but genetically very similar species of fruit flies
living in the Hawaiian Islands
(c) the fact that some turtles species have changed little over millions of
years
(d) the fact that males of many frog species “sing” every spring, while
females are silent
6. Account for the fact that in stable environments species often show little or
no evolutionary change. [K/U] [T/I]
7. If species are not changing, is it true to say that natural selection is not
happening? Explain your reasoning. [T/I]
8. Both male and female blue jays are quite brightly and similarly coloured. Is
this an example of sexual selection? Explain. [T/I]
9. Genetic drift leads to evolutionary change in the absence of natural
selection. Explain how this is possible. Provide an example to support your
answer. [K/U] [T/I]
10. The human population of Iceland was founded by a relatively small initial
population more than 1000 years ago. Would you expect the genetic diversity of
Icelanders to be more or less than the genetic diversity of Canadians? Explain
your reasoning. [T/I] [A]
11. Antibiotic-resistant bacteria may need to expend extra energy and
resources to produce special compounds and carry extra genetic material in
order to protect themselves against antibiotics. Predict what might happen to
these bacteria if they are not exposed to any antibiotics for many generations.
[T/I] [A]
12. Do online research and complete the following: [catch web link icon][T/I] [A]
(a) Describe the key steps that are thought to have occurred in the
evolution of eyes. Include labelled diagrams to illustrate the steps.
(b) Have eyes evolved once or more than once in the evolutionary history
of life on Earth?
(c) Explain how having simple or even poor eyesight might be
advantageous for a species compared to having no eyesight at all.
13. In each of the following situations, based on the Hardy–Weinberg principle,
determine whether or not evolution would be expected to take place. Explain
your choice. [K/U] [A]
(a) A very large population of mosquitoes lives in a stable environment.
(b) A small population of lizards inhabits a remote island.
(c) Climate change is influencing flowering time of a species of wildflower.
14. Provide three examples of how human activity is directly influencing the
evolution of wild (non-domesticated) species. [T/I] [A]
[catch web banner]
Investigation X.X.X 15
8.2
Speciation <Catch 5 pages>
microevolution changes in
gene (allele) frequencies and
phenotypic traits within a
population and species;
microevolution can result in the
formation of new species
Natural selection is continuously at work shaping the evolution of species.
Scientists know that species are able to evolve based on a wide range of
evidence, including the direct observations of evolution in nature and the
dramatic examples of change in domesticated species of plants and animals.
Changes that occur within species, sometimes referred to as microevolution,
are often relatively easy to understand. Natural selection favours fasterrunning cheetahs, colourful displays of male songbirds, or the larger brains in
humans. But what factors lead to speciation—how does an entirely new
species evolve? How did cheetahs, giraffes, and humans evolve in the first
place?
speciation the formation of new
species
What Is a Species?
[CATCH C08-P007-OB11USB;
Size D; Research. Photo of orca
whales swimming. Should not
be visibly in a tank or other
confined area.]
Figure 1 Recent genetic
evidence suggests there are at
least three different species of
orcas.
A biological species, according to one definition, includes all the members of a
population that interbreed or have the ability to interbreed with each other
under natural conditions. Individuals of different species are incapable of
interbreeding under natural conditions and are described as being
reproductively isolated from one another. On a genetic level, populations of
different species will not exchange genetic information—they will have
different gene pools. Defining a species as all members of a population that
share a common gene pool has advantages. Evidence of interbreeding between
individuals and groups can be difficult or impossible to examine and cannot be
applied to species that rarely or never reproduce sexually. Some species are
quite distinct from all others and can be readily identified based on their
morphology or physical appearance, while two other species might be
outwardly indistinguishable from each other. As a result, biologists must
employ a variety of methods to help distinguish species.
Today biologists are able to sample and compare the genetic makeup of
different populations to determine whether or not they represent different
species. This has led to some surprising results. Until very recently, orcas were
thought to be a single species, but genetic comparisons suggest they represent
at least three different species (Figure 1).
Modes of Speciation
reproductive isolating
mechanism any behavioural,
structural, or biochemical trait
that prevents individuals of
different species from
reproducing successfully
together
prezygotic mechanism a
reproductive isolating
mechanism that prevents
interspecies mating and
fertilization (e.g., ecological
isolation, temporal isolation, and
behavioural isolation
postzygotic mechanism a
reproductive isolating
New species can evolve under a variety of circumstances. One factor that is
common, however, is the eventual evolution of distinct features within a
population that separate it from all other species. These features result in the
new species becoming reproductively, and therefore genetically, isolated from
other species.
Mechanisms of Reproductive Isolation
For a new species to form, individuals from the original species must evolve to
become reproductively isolated from the remainder of the population and they
must establish a new interbreeding population. A reproductive isolating
mechanism is any biological factor that prevents the two populations from
interbreeding when living in the same region. Factors include differences in
breeding season, physical or behavioural traits, habitat preferences, and the
incompatibility of gametes. These are all prezygotic mechanisms—they
prevent fertilization and zygote formation. Still others, called postzygotic
mechanisms, can prevent a fertilized egg from growing into a viable and
reproducing adult. Table 1 (next page) lists and provides examples of each of
Investigation X.X.X 16
mechanism that prevents
maturation and reproduction in
offspring from interspecies
reproduction
these mechanisms. As you will learn, any mechanism that prevents two
populations from interbreeding can give rise to new species, but how do such
reproductive isolating mechanisms evolve?
[END page]
[Start page]
Table 1 Reproductive Isolating Mechanisms [set table as full page width]
Mechanism
Prezygotic
behavioural
isolation
Description
Example
Different species use
different courtship and
other mating clues to
find and attract a mate.
temporal
isolation
Different species
breed at different times
of the year.
ecological
isolation
Even very similar
species may prefer
occupying different
habitats within a
region.
mechanical
isolation
Differences in
morphological features
may make two species
incompatible.
gametic
isolation
Male gametes may not
be able to recognize
and fertilized an egg of
a different species.
[CATCH C08-P049-OB11USB;
Size E; Photo of Canadian species
of tree frog – a spring peeper or
wood frog would be good]
Male frogs of different species have
unique calls that attract only
females of their own species.
[CATCH C08-P050-OB11USB;
Size E; Photo of pussy willows.]
Pussy willows produce flowers in
the early spring. They are
reproductively isolated from plant
species that produces flowers at a
different time of year.
[CATCH C08-P051-OB11USB;
Size E; Photo of either the
Mountain bluebird (Sialia
currucoides) or the Eastern bluebird
(Sialia sialis)]
The Mountain bluebird (Sialia
currucoides) lives at high
elevations, while the Eastern
bluebird (Sialia sialis) prefers lower
elevations and does not encounter
the mountain species.
[CATCH C08-P052-OB11USB;
Size E; Photo of dragon flies or
damsel flies in “mating pair”)]
Male dragonflies transfer sperm
during an unusual mating flight. The
male and female genitalia of each
species are uniquely shaped and
are physically incompatible with
other species.
[CATCH C08-P053-OB11USB;
Size E; Photo of corals (or other
marine organisms – could be
sponges or sea cucumbers) with
sperm and or eggs being released
into the water)]
Many marine animals including
corals and clams release their
sperm and eggs into open water.
The sperm find eggs of their
species by recognizing chemical
markers on their surface.
Postzygotic
Investigation X.X.X 17
zygotic
mortality
Mating and fertilization
are possible, but
genetic differences
result in a zygote that
is unable to develop
properly.
hybrid
inviability
A hybrid individual
develops but either
dies before birth or, if
born alive, is unable to
survive to maturity.
hybrid
infertility
Hybrid offspring
remain healthy and
viable but are sterile.
Mules are the sterile
hybrid offspring of a
horse–donkey cross.
[CATCH C08-P054-OB11USB;
Size E; Photo of sheep and goat
together, or if not possible, one of
them)]
Some species of sheep and goat
are able to mate, and but the
zygote is not viable.
[CATCH C08-P055-OB11USB;
Size E; Photo of a leopard]
When tigers and leopards are
crossed, the zygote begins to
develop but the pregnancy ends in
a miscarriage or stillborn offspring.
[CATCH C08-P056-OB11USB;
Size E; Photo of a mule.]
Mules are the sterile hybrid
offspring of a horse–donkey cross.
Allopatric Speciation
allopatric speciation the formation of a
new species as a result of evolutionary
changes following a period of
geographic isolation
Most new species form after an original species becomes separated into two
geographically isolated populations. This is called allopatric speciation. Once
populations are physically separated, they can no longer exchange genetic
information. Over generations the populations will gradually become less and
less alike. Any beneficial mutation that arises in one population is not shared
with the other population. Any differences in the environments of the two
populations will also lead to different forms of natural selection. Changes that
result from genetic drift may also cause the populations to become
increasingly different. Once enough time has passed, there will be a good
chance that individuals from the two populations, even if they did meet, will
have evolved some sort of reproductive isolating mechanism. Perhaps their
courtship rituals will have changed, the time of year they breed or produce
pollen will have shifted, or they may no longer be physically compatible
(Figure 2, next page).
[END page]
[Start page]
[catch C08-F06a-OB11USB; Size C3; New. Diagram of the first stage of
allopatric speciation.]
[catch C08-F06b-OB11USB; Size C3; New. Diagram of the second stage of
allopatric speciation.]
[catch C08-F06c-OB11USB; Size C3; New. Diagram of the third stage of
allopatric speciation.]
[catch C08-F06d-OB11USB; Size C3; New. Diagram of the fourth stage of
allopatric speciation.]
Figure 2 caption t/k
[CATCH C08-P008-OB11USB;
Size D; Research. Photo of a
flightless cormorant from the
Galapagos showing small,
The geographic splitting of a species into two populations can occur in a
number of ways. As you have learned, populations may become isolated on
remote islands where they are far from the original population. New selective
pressures can then cause them to evolve dramatically new features like that of
Investigation X.X.X 18
outstretched wings]
Figure 3 The population of
cormorants that established on
the Galapagos Islands lost their
ability to fly.
[catch C08-F07-OB11USB; Size
D. New. Art of the Isthmus of
Panama with wrass species
shown along the sides. ]
Figure 4 Genetic testing
indicates that the Pacific and
Caribbean wrasse species
shown here evolved from single
species that became separated
into two populations by the
isthmus.
the flightless cormorant of the Galapagos Islands (Figure 3). Mountain ranges
may form, separating populations of species whose members do not travel
over mountains. Powerful rivers may erode deep canyons that might not
separate bird species but might easily separate species of small animals such
as snakes, mice, or land snails. Continental drift has split apart entire
continents and separated countless species into separate populations.
A clear example of allopatric speciation events followed the formation of
the Isthmus of Panama—a thin strip of land that now separates the Caribbean
Sea from the Pacific Ocean. Prior to its formation, the Caribbean Sea was
connected by a wide channel to the Pacific ocean. Many species of marine
organism inhabited this region. Two million years ago, the isthmus formed,
permanently dividing species into separate Pacific and Caribbean populations.
Now, the species on both sides are distinct and cannot successfully interbreed,
even when placed together (Figure 4). Perhaps the best examples are the
seven different species of snapper shrimp on each side of the isthmus—each
having its closest relative on the other side. Since being separated, seven
original species have evolved into 14 different species.
Geologic changes can directly influence the natural selection pressures on
species. Consider the effect that the formation of the Rocky Mountains had on
the evolution of species. As this mountain range formed, it divided many
widely distributed species into separate western and eastern populations and
also produced profound changes in their environment. West of the Rocky
Mountains, the climate is moderate with cool summers and warm winters, and
there is heavy precipitation. East of the mountains, there are hot summers,
cold winters, and little precipitation. As a result, the western and eastern
populations of the original species experienced different environmental
conditions and different selective pressures. Plants, for example, would be
selected for their ability to thrive in wet conditions on the coast and arid
conditions in the prairies (Figure 5). Over millions of years these populations
evolved into different species.
[CATCH C08-P009a-OB11USB; Size B1; Research. Photo of a temperate
rainforest on Canada’s west coast]
[C08-P009b-OB11USB; Size B1; Research. Photo of the dry foot hill ecosystem
on the eastern side of the rockies.]
Figure 5 The mountain ranges in Western Canada create two strikingly
different environments. (a) On the west coast, natural selection favours species
that thrive in a wet, mild climate. (b) On the eastern side of the mountains,
natural selection favours species able to survive a dry climate.
A USEFUL ANALOGY
It is often useful, in trying to understand a difficult concept, to consider a
similar or analogous process that is more familiar. Understanding speciation is
difficult in part because of the long times involved and the formation of
isolating mechanisms with which we are unfamiliar. However, consider how
the process of speciation is analogous to the formation of a new language. In a
non-biological but analogous way, we as individuals inherit our language from
our parents and pass it on to our children. When one original human
population becomes geographically separated into two groups over long
periods of time, their spoken languages start to diverge. Regional accents and
slang develop and spellings change, as do common phrases. New words are
Investigation X.X.X 19
also introduced in each population. You have no doubt noticed these
differences yourself, for example, in the English spoken by people from
Australia, England, the United States, and Canada. Given enough time and
continued isolation, languages become distinct and individuals from the
different regions are no longer able to understand each other. The earliest
Germanic language, for example, spoken in northern Europe during the first
millennium BCE, has since given rise to more than 50 distinct languages.
[END page]
[Start page]
sympatric speciation the
evolution of populations within
the same geographic area into
separate species
[CATCH C08-P010-OB11USB;
Size D; PU. PU Fig 18.22 from
Biology: Exploring the Diversity
of Life, p. 412 (0176440941),
photo of hawthorn flies on
hawthorn tree]
Figure 6 Disruptive selection is
resulting in the sympatric
evolution of a new species of fly.
The original species now
consists of two distinct
populations. The original form,
seen here, mates and lays eggs
on native hawthorn fruit. The
recently evolved form lays eggs
on the fruit of introduced apple
trees.
[CATCH C08-P011-OB11USB;
Size D; Research. Photo of a
gray tree frog]
Figure 7 Ontario’s eastern gray
treefrog is a tetraploid species
that is almost indistinguishable
from the diploid Cope’s gray
treefrog.
Sympatric Speciation
A new species can also evolve from within a large population. This process,
called sympatric speciation, occurs when individuals within a population
become genetically isolated from the larger population. Such isolation may
occur gradually or suddenly.
One example of gradual sympatric speciation appears to be under way as
a direct result of human action. The hawthorn fly is native to North America.
The original population of hawthorn flies laid their eggs in the small fruits of
hawthorn trees. Between 1800 and 1850, after the introduction of apple trees
from Europe to North America, some of these flies began laying their eggs on
apples (Figure 6). Today, the species consists of two very distinct populations.
One population, now called apple maggot flies, feeds almost exclusively on
apples, while the other feeds almost exclusively on hawthorns. It is likely that
disruptive selection favoured mutations that enhanced either feeding
behaviour. Genetic testing suggests there is still a small amount of
interbreeding between the populations but that they are on their way to
becoming reproductively isolated and separate species.
Sudden sympatric speciation is also possible. Even a single mutation can
render an individual unable to reproduce with other members of the
population. If two such individuals share the same mutation or if a single
individual is able to reproduce asexually, a reproductively isolated population,
a new species, may result. This is thought to have occurred many times in the
evolution of plants. Many plants are able to reproduce both sexually and
asexually. If an individual plant has a mutation that prevents successful sexual
reproduction, it might still be able to produce large numbers of offspring
asexually. These offspring would be a sexually compatible population.
POLYPLOIDY AND HYBRID SPECIES
One way a mutation can produce a new species is when it results in polyploidy.
Mutations causing polyploidy result in a doubling of chromosome number
within an individual. Polyploid individuals are able to produce fertile offspring
when mated with each other but produce only sterile offspring when mated
with the original species. The evolution of about 30 % to 70 % of all flowering
plant species have involved polyploidy. Ontario’s eastern gray treefrog (Hyla
versicolor) is a polyploidy species (Figure 7). It was once thought to be the
same species as the Cope’s gray treefrog (Hyla chrysoscelis), which is virtually
identical in appearance. Genetic tests reveal that H. versicolor is a tetraploid
species having 4 sets of chromosomes (4n), while H. chrysoscelis is diploid
(2n). The chromosomes of both species are near perfect matches to each other,
suggesting that the eastern treefrog formed as a result of a polyploidy
mutation of the Cope’s species.
Human Influence on Speciation
Investigation X.X.X 20
[catch C08-P012a-OB11USB;
Size D; Research. Photo of a
highway corridor separating
forested area]
Figure 8 The construction of
wide roadways may divide and
isolate populations of species
that are not very mobile. This
can result in reductions in
genetic diversity.
[CATCH C08-P012b-OB11USB;
Size D; Research. Photo of a
wildlife corridor]
Figure 9 Wildlife corridors
prevent isolation, increasing
gene flow and maintaining
genetic diversity.
[CATCH C08-P013-OB11USB;
Size D; Research. Image of
tetraploid daylilies]
Figure 10 Many varieties of
polyploid daylilies have been
created by using colchicine to
prevent chromosomes from
separating.
The separation of a single population into two isolated populations can lead to
speciation, but it can also threaten the survival of species. Many human
activities are causing once large habitats to be fragmented into smaller areas.
Forested regions and the populations they contain may become smaller and be
separated by agricultural and urban expansion and the construction of roads
(Figure 8). For some species these actions are enough to effectively isolate
populations. A recent study of timber rattlesnake populations in the state of
New York revealed that roadways were a significant barrier to gene flow and
that genetic diversity within each isolated population was low. Populations
with low genetic diversity are at greater risk when threatened by disease or
changing environmental conditions such as climate change. Similar research in
China has shown that the survival of giant pandas, a critically endangered
species, is being threatened by their separation into small isolated populations
in patches of bamboo forest.
[END page]
[Start page]
These concerns can be addressed by conservation practices. Gene flow
can be maintained and enhanced if the connectivity between habitats is
ensured. Fragmented habitats can be joined by wildlife corridors such as
highway underpasses with strips of connecting forest that are left standing
(Figure 9).
Humans can also influence the formation of new species. New polyploid
plant species have been created intentionally and as a consequence of human
actions. To produce a tetraploid plant, biologists apply the chemical colchicine
to a growing bud of a diploid plant. Colchicine prevents the chromosomes from
separating during mitosis. When the cells divide, one daughter cell lacks
chromosomes and dies, while the other is tetraploid. These cells are allowed to
continue to grow and reproduce with no further addition of colchicine. All leaf,
stem, and flower cells on this growing branch will be tetraploid, and therefore
the gametes produced by flowers on this branch will be diploid instead of
haploid. Many varieties of daylilies and orchids have been produced in this
way (Figure 10). New polyploidy species are also known to have formed in the
wild when a native polyploidy species has hybridized with an introduced
polyploidy species.
Any human actions that result in the formation of a new species are of
concern. It is impossible to predict the possible success of a new species,
should it become established in the wild. There is a risk that it will outcompete native species and become invasive, disrupt food webs, or be harmful
in other ways.
8.2 Summary
• New species form as a result of the evolution of a reproductive isolating
mechanism that prevents members of two populations from interbreeding.
• Populations of the same species evolve independently when separated by a
geographic barrier.
• Differences in selective pressures and genetic drift can lead to the evolution of
reproductive isolating mechanisms and the formation of new species.
• New species can evolve within a population when mutations result in
immediate reproductive isolation or when disruptive selective pressures cause
one species to gradually separate into distinct and reproductively isolated
populations.
• Human activities can influence both the evolution of new species and the
survival of current species.
8.2 Questions
Investigation X.X.X 21
1. It is not possible to know if two similar fossils represent individuals that
were reproductively isolated from each other. Suggest other methods or
characteristics that paleontologists might use to determine if two specimens
represent different species. [T/I]
2. Explain which type of reproductive isolating mechanism is at work in each
of the following situations: [K/U]
(a)
Zebroids, the hybrid offspring of matings between horses and
zebras, are sterile.
(b)
Asian lions were once common and lived in open grasslands,
while Asian tiger species preferred forested habitats.
(c)
Female fireflies of different species identify males by the
pattern of light flashes they produce.
(d)
Male geese have a penis, while male herons do not. Male
herons are unable to fertilize female geese.
(e)
Two species of fruit flies are known to live in the same region.
One mates in the morning, while the other mates in the late afternoon.
(f) Domestic goats and sheep can mate and fertilize one another’s eggs,
but their embryos do not develop and die before birth.
(g)
When pollen grains from white pine trees land on the female
cones of red pine trees, fertilization does not occur.
3. Plant and animal breeders make a concerted effort to keep their breeding
lines “pure” to prevent unwanted crosses with their breeding stock. How is this
human activity analogous to the conditions that cause allopatric speciation?
[K/U] [T/I]
4. Would you expect to find a more unique species on a large remote island
like Iceland, or on a smaller island that is close to a large continent, such as
Prince Edward Island? Explain your reasoning. [T/I]
5. Most speciation events are thought to occur over many thousands of
years. Describe two examples of speciation events that have occurred or are
under way much more rapidly. [K/U]
6. Parasitic wasps that feed on the hawthorn fly now appear to be evolving
into two separate species. One lays eggs on maggots in hawthorn fruit, and the
other lays eggs on maggots in apples. Do you find this surprising? Why or why
not? [C] [A]
7. How can the separation of two populations lead to the following: [T/I] [A]
(a)
the formation of a new species
(b)
a reduction in genetic diversity of the populations and a
possible threat to their survival
8. A very unusual and endangered Tasmanian plant, Lomatia tasmanica, is a
triploid plant (3n = 33 chromosomes) and is completely sterile. It can reproduce
only asexually, and all known individuals are genetically identical clones of each
other. Fossil evidence suggests this clone has existed for more than 43 000
years. [K/U] [T/I] [A]
(a)
How many chromosomes do you think would be found in the
plant species from which it evolved?
(b)
Do you think this species arose by allopatric or sympatric
speciation? Explain.
(c)
In what way does this plant not fit a typical definition of a
species?
Investigation X.X.X 22
8.3
Patterns of Evolution <Catch: 5
pages>
Natural selection leads to predictable outcomes:
• Closely related species share many homologous structures, even though they
no longer serve the same function.
• Species have vestigial structures and pseudogenes that once served a useful
purpose in their ancestors.
• Remote islands are inhabited by unique species that are descended from a
few individuals of species able to reach them across wide expanses of ocean.
When considered on a grander scale, these and other predictable outcomes
lead to discernable patterns.
adaptive radiation the
relatively rapid evolution of a
single species into many new
species, filling a variety of
formerly empty ecological
niches
[CATCH C08-P014a-OB11USB;
Size D; Research. Photo of a
moist, rich forest the Galapagos
Islands]
[CATCH C08-P014b-OB11USB;
Size D; Research. Photo of an
arid desert area of the
Galapagos Islands]
Adaptive Radiation
Adaptive radiation occurs when a single species evolves into a number of
distinct but closely related species. Each new species fills a different ecological
niche. This process usually occurs when a variety of new resources become
available—resources that are not being utilized by other species.
Consider the example of Darwin’s finches (Figure 1). Here, a group of 13
species that live in the Galapagos Islands evolved from a single species. We can
assume that the original species of finch (Tiaris sp.), living on the mainland of
South America, had a medium-sized bill ideally suited to feed on certain
medium-sized seeds. Individuals born with slightly smaller bills might have
been better at eating smaller seeds, but they might have faced stiff competition
from other bird species that were already specialized in feeding on the smaller
seeds. Finches eating larger seeds would also face similar competition. The
result was stabilizing selection on the mainland finches to stay in their
specialized ecological niche. An entirely different fate awaited individuals of
this finch species once they reached the Galapagos Islands (Figure 2). Instead
of hundreds of other species of land bird, there were few or none. Their only
competition was with each other—individuals of the same species—for
medium-sized seeds.
[catch C08-F08-OB11USB; Size B; New. Tree diagram showing the fourteen
species of Darwin’s finches]
Figure 2 The Galapagos
Islands are home to a rich
diversity of habitats, from (a)
moist forests to (b) dry deserts.
Figure 1 Thirteen species of Darwin’s finches have undergone recent adaptive
radiation to fill many different ecological niches. Genetic evidence shows they
all evolved from a single common ancestor species.
[End page]
[Start page]
The islands might already have been teaming with populations of many
Investigation X.X.X 23
[CATCH C08-P015-OB11USB;
Size D; Research. Photo of
Lake Malawi cichlids. Photo
must show a great deal of
diversity as well as many
species]
Figure 3 pending image
selection
plant and insect species that could have arrived long before. Different islands
and different habitats would have harboured a diverse array of food resources,
such as various sized seeds and an abundance of different insects. With no
other insect-eating birds on the islands, the finches had an opportunity to
exploit a new food with no competition. In such a setting, any finches born
with a different bill sizes or feeding behaviour would have been rewarded with
a rich supply of food and little or no competition from other birds. The result of
adaptive radiation was seven different seed-eating species (granivores), one of
which feeds primarily on other plant parts, and six-insect eating species
(insectivores).
The most spectacular case of adaptive radiation is witnessed in the cichlid
fishes of lakes Victoria, Malawi, and Tanganyika in Africa. Each lake is quite
isolated from other bodies of water, making it very difficult for new species to
arrive. Each lake, however, is home to hundreds of unique species, all
descended from one or a few initial species. Lake Malawi alone has nearly
1000 species of cichlid. All but two of these species are found nowhere else on
Earth (Figure 3).
In each case of adaptive radiation, an initial species evolves into a variety
of new species that differ to varying degrees from the original species. In this
way, adaptive radiation contributes to biodiversity. A similar pattern can be
seen on a much larger scale when we consider entire groups of organisms and
very large ecosystems.
Divergent Evolution
divergent evolution the largescale evolution of a group into
many different forms
In any ecosystem, there are a number of major ecological roles. All natural
ecosystems, for example, have producers, consumers, decomposers, and
scavengers. These major roles are never filled by a single species. Consider the
ecological role of herbivores. Not surprisingly, natural selection has favoured
the evolution of a wide variety of herbivores. For example, herbivorous
mammals come in a variety of shapes, sizes, and specialties. Natural selection
has favoured their divergent evolution into a great variety of species.
Northern Ontario forests are home to many rodents, the largest taxon of
herbivorous mammals (Figure 4). These rodents provide an excellent example
of divergent evolution. All of these species have evolved from a single common
ancestor millions of years ago.
[CATCH C08-P016a-OB11USB; Size D; Research. Image of a deer mouse
and/or a redbacked vole]
[CATCH C08-P016b-OB11USB; Size D; Research. Image of a flying squirrel in
flight]
[CATCH C08-P016c-OB11USB; Size D; Research. Image of a Porcupine – in
tree, if possible]
[CATCH C08-P016d-OB11USB; Size D; Research. Photo of a beaver]
Figure 4 Ontario has over 1000 species of closely related rodents, a group of
mammals that has undergone significant divergent evolution. Species include
the (a) deer mouse, (b) flying squirrel, (c) porcupine, and (d) beaver.
Red squirrels have evolved as tree-climbing seed specialists that are
active during the day, while northern flying squirrels fill a similar ecological
niche but are active only at night. Chipmunks are considered ground squirrels
and spend much of their time foraging for seeds at ground level. The smallest
forest rodents include deer mice and red-backed voles. Deer mice prefer small
seeds and insects and usually nest in trees, while voles nest on the ground and
prefer a diet of roots and buds. Porcupines are the largest tree-climbing
rodents and feed on twigs and the thin bark of conifers. Beavers, the largest of
Investigation X.X.X 24
all Canadian rodents, prefer the twigs and bark of angiosperm species and cut
them down and drag them into the water before feeding on them. The unique
characteristics of each of these species has been proven successful and been
selected for by the environment.
[End page]
[Start page]
Divergent evolution leads to two predictable outcomes:
• Competition between species is minimized as new species diverge to fill
specialized ecological niches.
• Given enough time, new species continue to evolve until most available
resources are utilized.
The result is an overall increase in biodiversity as a single species or group
evolves to fill many available ecological niches.
Convergent Evolution
convergent evolution the
evolution of similar traits in
distantly related species.
[C08-P017a-OB11USB; Size D;
Research. Photo of a cacti in
the desert. Photo should look
similar to photo of a euphorbia
below.]
[CATCH C08-P017b-OB11USB;
Size D; Research. Photo of a
euphorbia in the desert. Photo
should look similar to photo of a
cacti above]
Figure 5 (a) Cacti and (b)
euphorbia have evolved similar
features in response to their hot
dry environments.
Evolutionary biology predicts that when a single species is placed under two
different sets of selective pressures, it is likely to undergo divergent evolution.
What if the situation were reversed? What if two species were placed under
similar selective pressure?
Convergent evolution occurs when two different species, or taxa, evolve
to occupy similar ecological niches. Patterns of convergent evolution are often
most obvious when you compare different geographic regions.
One of the clearest examples is observed in two groups of plants (Figure
5). Cacti evolved in the deserts of South America and are native only to the
Americas. Euphorbia look similar to the cacti, but first evolved in the deserts of
South Africa and now occur in Africa, Eurasia, and Australia. Both groups have
species that display a set of features that have evolved in response to
extremely dry conditions. Many cacti and euphorbia species have sharp spines
and thick green stems that perform photosynthesis and store water. During
dry conditions, some euphorbia have no leaves but, unlike cacti, are able to
grow green leaves when ample water is available. Similarly, although both
plant groups have evolved spines that serve the same protective function, the
spines of cacti evolved from leaves, while those of euphorbes evolved from the
outward growth of stem tissues.
Sharks and dolphins are another example of convergent evolution. Both
have evolved very similar streamlined bodies well suited for their high-speed
carnivorous behaviour. Natural selection favoured the same body shapes in
two very distantly related ancestor species (Figure 6). Sharks evolved from a
primitive fish with a cartilaginous skeleton and a side-to-side body motion that
powers a vertical tail. Dolphins are recently evolved, warm-blooded marine
mammals with a bony skeleton. They power their horizontal tail flukes with an
up-and-down motion inherited from their land-living ancestors.
[CATCH C08-P018a-OB11USB; Size B1; PU. Photo of a shark. Fig. 1a from
Nelson Biology 12, p. 602 (0176259872)]
[CATCH C08-P018b-OB11USB; Size B1; PU. Photo of a dolphin Fig. 1b from
Nelson Biology 12, p. 602 (0176259872)]
Figure 6 Convergent evolution has resulted in (a) sharks and (b) dolphins
having similar body shapes.
Convergent evolution can result in similar features evolving in very
distantly related organisms. The selective advantage of detecting and
responding to light, for example, resulted in the evolution of a range of lightdetecting organs. Protists have a simple eye spots, while arthopods, mollusks,
and vertebrates have complex and varied eyes (Figure 7, next page).
[End page]
Investigation X.X.X 25
[Start page]
[formatter: place three figures across text measure, with corresponding photo
below each]
[catch C08-F09a-OB11USB; Size C3; New. Diagram showing a Simple lens
eye.]
[CATCH C08-P019a-OB11USB; Size C1; Research. Photo of a vertebrate’s
eyes]
[CATCH C08-P019b-OB11USB; Size C1; Research. Photo of a spider’s eyes]
[catch C08-F09b-OB11USB; Size C3; New. Diagram showing a Simple corneal
eye.]
[CATCH C08-P019c-OB11USB; Size C1; Research. Photo of a mollusk’s eyes]
[catch C08-F09c-OB11USB; Size C3; New. Diagram showing a Compound
eye.]
[CATCH C08-P019d-OB11USB; Size C1; Research. Photo of insect eyes]
Figure 7 to come
Just as the patterns of divergent evolution can be predicted, so can the
outcomes of convergent evolution. We can predict two common outcomes:
• Natural selection will favour the evolution of similar traits in similar
environments.
• While some traits will converge in form or function, each species will retain
other features that provide evidence of their distinct evolutionary past.
Research This:
Convergent Evolution Down Under
Skills: Researching, Analyzing, Communicating
Australia is home to many unique species and is famous for its diversity of
marsupials. These unusual mammals include the well-known kangaroos and
the iconic koala. Why do so many marsupials live in Australia, and how do they
compare to mammals in other parts of the world?
1. Research the marsupials of Australia, considering the following issues:
• the anatomical differences between a marsupial and placental mammals
• the relationship between the separation of Australia from the Gondwanaland
mass, and the evolution of marsupial and placental mammals
• how convergent evolution influenced the marsupial mammals living in
Australia and the placental mammals in the rest of the world
• which introduced placental mammals have become invasive species in
Australia
A. Explain how the isolation of Australia led to the evolution of its unique
collection of marsupial mammals. [catch web link icon] [T/I]
B. Compare the physical appearance and ecological niches of several
marsupials to their “matching” placental mammals. For example, compare
Tasmanian wolves and grey wolves or sugar gliders and flying phalangers.
[catch web link icon] [T/I]
C. Briefly outline the current status of invasive mammal species in Australia.
Which species are of greatest concern? What is being done to try and mitigate
the situation? [catch web link icon] [T/I] [A]
[catch web banner icon]
Coevolution
coevolution a process in which
one species evolves in
response to the evolution of
A species experiences coevolution when its evolutionary success is closely
linked to that of another species. For example, certain plants have evolved
hard protective shells to protect their seeds, while some seed-eating mammals
have evolved powerful jaws and teeth for chewing through hard shells (Figure
Investigation X.X.X 26
another species
[CATCH C08-P020a-OB11USB;
Size D; Research. Photo of a
brazil nut fruit that has been
chewed open to show seeds]
[CATCH C08-P020b-OB11USB;
Size D; Research. Photo of an
agouti]
Figure 8 (a) Brazil nut trees
have evolved extremely hard
protective shells. (b) The agouti
is the only mammal with jaws
and teeth strong enough to bite
open the shell.
[End page]
8). Any seeds surrounded by a hard shell might be better protected from
herbivores and better able to survive than seeds with thin shells. Similarly, any
herbivore born with a slightly more powerful jaw might be able to acquire
more food than a herbivore born with a less powerful jaw. This result is
sometimes called an “evolutionary arms race.”
[End page]
[Start page]
Coevolution is most pronounced in symbiotic relationships. Certain orchid
species, for example, are completely dependent on moths to pollinate their
flowers. The moths, in turn, depend on the orchid nectar for food so they can
reproduce. Over time, the flowers of some orchid species have evolved
extremely long tubes, called flower spurs, which contain the nectar. Biologists
hypothesize that natural selection has favoured longer spurs because
obtaining nectar from a longer spur requires moths to expend more time and
effort, making them more likely to pick up pollen. For the moths, natural
selection favoured individuals with slighter longer tongues that could reach
the nectar at the bottom of the longest spurs. The ultimate result has been the
evolution of a most extreme pair. The Madagascar long-spurred orchid has
nectar at the end of a 30 cm long spur (Figure 9). Its only pollinator, a hawk
moth, has a tongue the same length!
As species coevolve, one or both species may become increasingly
dependent on the other. In these situations, threats to one species may
indirectly be a threat to the other, in extreme cases with the extinction of one
species leading to the extinction of the other.
[CATCH C08-P021a-OB11USB; Size C1; Research. Photo of a Madagascar
long-spurred orchid]
[CATCH C08-P021b-OB11USB; Size C1; Research. Photo of a hawk moth
showing its long tongue]
[CATCH C08-P021c-OB11USB; Size C1; Research. Close-up photo of the
moth with its tongue coiled]
Figure 9 (a) The Madagascar long-spurred orchid is pollinated by (b and c) a hawk moth whose
tongue is about 30 cm long.
8.3 Summary
- Adaptive radiation increases biodiversity, as a single species evolves into
many new species filling a number of different ecological niches.
- Adaptive radiation occurs rapidly when a species is able to exploit a wide
variety of new resources with little or no competition from other species.
- Divergent evolution increases biodiversity and leads to large-scale
predictable patterns of evolution as major ecological roles are filled by a
variety of species—each with their own specializations.
- Convergent evolution occurs when different species or groups evolved
similar adaptations under similar conditions.
- Coevolution occurs when the evolution of two species becomes linked.
Coevolution often strengthens symbiotic relationships.
8.3 Questions
1. Explain why a species is most likely to undergo adaptive radiation when
there is very little competition for a variety of resources. [K/U]
2. The Hawaiian Islands are home to about 30 species of very closely related
plants called silverswords. Some are treelike, while others are dwarf shrubs.
They are found nowhere else on Earth. Use your understanding of adaptive
Investigation X.X.X 27
radiation to describe their likely evolutionary past. [K/U] [T/I]
3. Compare and contrast divergent and convergent evolution. Include
examples to illustrate the similarities and differences. [K/U]
4. Many species of fish and waterfowl (like loons and ducks) are darker on
their upper surface and lighter coloured below. [T/I]
(a) What pattern of evolution is most likely at work?
(b) Suggest possible selective advantages for this coloration.
5. Most remote oceanic islands have at least one unique species of flightless
bird that shows little or no fear of humans or other large predators. Account for
this observation. [T/I] [A]
6. As dolphins swim, they arch their backs forward and back to generate
power and up-and-down motion of their tail flukes. Land mammals also use a
similar arching motion as they run. This is quite noticeable in the motion of
horses and dogs. Fish, however, use a side-to-side motion to move their tails.
Do online research to find out how amphibians and reptiles flex their backbones
as they move. [catch web link icon] [T/I]
7. Snakes are not the only legless terrestrial vertebrates. Caecilians (Figure
10) are a group of amphibians that also lack legs. Is this an example of
convergent or divergent evolution? Explain your reasoning.
[CATCH C08-P057-OB11USB; Size C; Research. Photo of a caecilian]
Figure 10
8. When Europeans first arrived in the Americas, they carried with them a
number of human diseases that were devastating to the local indigenous
peoples. How does an understanding of coevolution help to explain indigenous
peoples’ very low resistance to these new diseases? [T/I] [A]
[catch web banner]
Investigation X.X.X 28
8.4
Explore an Issue in Evolution [2
page]
SKILLS MENU
Researching
Identifying Alternatives
Analyzing
Communicating
[CATCH C08-P037-OB11USB;
Size D; Research. Photo of a
drawing of a Dodo bird]
Figure 1 The dodo was a large flightless
bird that evolved on the island of
Mauritius in the Indian Ocean. It went
extinct in the seventeenth century. Pigs,
monkeys, cats, dogs, and rats were all
introduced to the island by humans and
fed on the dodo eggs and young.
Humans destroyed their habitat and
hunted them for food.
[CATCH C08-P038-OB11USB;
Size D; Research. Photo of
coral reef bleaching]
Avoiding Extinctions
New species evolve, and living species go extinct (Figure 1). The rates at which
these changes take place vary. Over the entire history of Earth, the rate of
species formation has been, on average, greater that the rate of extinction. The
result is that over millions and billions of years the long-term trend has been a
gradual increase in the number of different species on Earth. This general
trend, however, has been sharply reversed on at least five occasions, when
mass extinction events have taken place. Mass extinction events occur on a
global scale and are biologically traumatic. The diversity of life on Earth
plummets.
Each past mass extinction event has been followed by the recovery of
species diversity over a period of millions of years. While such recoveries
might be considered rapid from the perspective of geological time, they are
extremely slow from the perspective of a human lifetime.
Past mass extinction events were caused by actions that altered Earth’s
biosphere in a profound way. An asteroid impact or a series of large volcanic
eruptions can cause sudden and profound changes in the chemical
composition of the oceans and atmosphere. The can lead to rapid climatic
changes that wipe out species before they are able to adapt.
Today, biologists around the world are deeply concerned about the
increasing rate of species extinction. The primary threats to species are habitat
loss and degradation, the introduction of invasive species, overharvesting,
pollution, and climate change—all caused by humans. The situation has
reached critical levels for many species. Some biologists estimate that without
concerted action and international cooperation, more than half of all plant and
animal species on Earth could be extinct within 100 years. The rate of
extinction is on a par with extinctions in Earth’s past. Consider some statistics
from the International Union for the Conservation of Nature (IUCN) on
threatened species:
• Primary forests are being lost at a rate of 6 million hectares per year.
• Present extinction rates are estimated at 1000 times the natural rate.
• About 70 % of the world’s coral reefs are threatened or severely damaged.
• 1895 species of amphibians are in danger of extinction.
• 17 291 species out of 47 677 that have been assessed to date are in danger of
extinction.
The Issue
A number of human activities are causing the rapid acceleration in the rate of
species extinction (Figure 2). These activities pose a serious threat to the
biodiversity of life on Earth and the sustainability of natural ecosystems.
Role
Figure 2 Corals are threatened by
pollution, warming waters, and changes
in ocean acidity. It is extremely difficult
for a species to adapt to multiple
environmental stresses that occur
quickly.
Your group’s goal is to investigate this issue from the perspective of an
evolutionary biologist. You will examine the influence that human activities are
having on the ability of species to adapt to change by natural evolutionary
processes.
Investigation X.X.X 29
Audience
Your audience will be members of the IUCN, whose mandate is to help the
world find pragmatic solutions to environmental challenges. The IUCN also
maintains the Red List of Threatened Species.
[End page]
[Start page]
Goal
Your group is to investigation the ways in which environmental changes
resulting from human activity directly undermine the ability of species to
evolve and adapt. You will assess how such impacts increase the likelihood
that species will go extinct. You will then present your assessment of these
impacts, including a set of recommendations to avoid or mitigate them. You are
to focus your investigation on the following human-caused environmental
changes: habitat loss and fragmentation, the introduction of exotic species,
modern agricultural practices, and climate change.
Research
As you research each type of environmental change, consider each of the
following relationships to the species of concern:
- How does the pace of change compare to the speed at which threatened
species are able to evolve?
- What influence is the human action having on the genetic diversity of the
threatened species? Is there a risk of causing a genetic bottleneck?
- How do new conditions compare to the conditions under which threatened
species evolved?
- What actions can be taken to reduce or eliminate negative impacts?
Identify Solutions
• Consider ways of halting or reversing the human actions that are responsible
for threatening species.
• Consider alternatives that do not reduce a species’ ability to evolve.
• Identify actions that might enhance the ability of a threatened species to
adapt to new environmental conditions.
Make a Decision
In your group, decide on a set of recommendations that you will present to the
IUCN panel.
Communicate
In your presentation, include specific examples of human impacts and possible
evolutionary consequences that have the potential to lead to the extinction of
species and loss in biodiversity. In presenting your recommendations to the
panel, explain how your recommendations will specifically enhance the
evolutionary potential of species to adapt to change.
PLAN FOR ACTION—EXTINCTION IS FOREVER
Chose a species that is currently threatened with extinction. It may be a local
species or a species in a different part of the world. Research the specific
human actions that are responsible for its current status.
Investigation X.X.X 30
[CATCH C08-P039-OB11USB;
Size D; Research. Photo of a
bat box]
Figure 3 Building and installing a bat
box is a simple and effective way to
increase habitat for these threatened
species.
Investigate the ways in which an individual could help this species adapt to
environmental change. Consider your set of recommendations from this Explore
an Issue activity as well as others specific to your chosen species.
Prepare a Plan of Action that an individual could follow in order to benefit this
species.
In preparing your Plan of Action, include:
- specific physical actions such as habitat improvements that an individual could
carry out (Figure 3)
- a list of organizations involved in conservation initiatives in support of the
species
- a list of government agencies and/or officials that could be contacted in order
to voice your concerns
- sources of information about this species that could be shared with friends,
family, and the public
Investigation X.X.X 31
8.5
macroevolution large-scale
evolutionary changes including
the formation of new species
and new taxa
Macro Evolution <Catch: 6 pages>
The history of life on Earth has been one of continual evolutionary change.
From simple beginnings, life on Earth has become more diverse and more
complex over time. The original single-celled ancestors of all living things have
given rise to the millions of species that are alive today, as well as to the many
millions that have gone extinct. In this section you will explore factors that
influence these large-scale processes of macroevolution in more detail and
learn how biologists use evidence to infer the evolutionary relationships
between different species and groups.
The Tree of Life
abiogenesis the origin of life
from non-living matter
WEB LINK
To learn more about the current
theories and evidence regarding
abiogenesis,
[catch Nelson web link banner]
http://exploringorigins.org/timelin
e.html GO TO NELSON
SCIENCE
The simplified tree of life depicts the evolution pathways of some of the major
branches of living organisms (Figure 1). The diagram raises an obvious
question: how did life begin? Or, alternatively, how did the first cell originate?
The study of abiogenesis, the formation of life from non-living matter, is being
actively researched. There are many fascinating and competing theories. It is
known, for instance, that many of the key building blocks of life, such as amino
acids, hydrocarbon chains, and other simple organic molecules, can form under
natural conditions. Some even occur in space and are compounds within
comets. It is also known that some RNA molecules are capable of replicating
themselves, independent of any other cell components. RNA molecules are
strong candidates for the first self-replicating precursors to living cells. The
challenge to scientists who study abiogenesis is to conceive of all the physical
and chemical situations that may have existed on Earth billions of years ago.
<web link icon here >
[catch C08-F10-OB11USB; Size B; MPU. Modifiued pickup of C01-F16OB11USB.]
Figure 1 Simple single-celled life has existed on Earth for at least 3.5 billion
years. Eukaryote evolution occurred much later and endosymbiosis.
Eukaryote evolution has given rise to the great diversity of complex multicellular
organisms. (edit to come from Doug)
There are many competing theories about the origin of the very first cells.
Scientists, however, have no doubt that life has existed on Earth for more than
3.5 billion years and has been evolving ever since. It took more than 2 billion
years for eukaryotic organisms to evolve and another several hundred million
years for multicellular life forms to evolve. While single-celled organisms are
very small and simple, multicellular organisms quickly evolved into a great
Investigation X.X.X 32
diversity of forms.
[End page]
[Start page]
Diversification and Mass Extinction
At one time dinosaurs dominated Earth’s terrestrial ecosystems. These
reptiles, some of which were enormous, first evolved some 250 million years
ago and began to diversify and flourish about 200 million years ago. For more
that 100 million years, dinosaurs were the dominant vertebrate herbivores
and carnivores on land. Despite all their success, however, the dinosaurs’ reign
ended abruptly with a now famous mass extinction event 65 million years ago
(Figure 2). The only surviving descendents of the dinosaurs are birds.
[catch C08-F11-OB11USB; Size B; New. Diagram showing the
radiation/diversification of the dinosaurs along with the correct timelines and the
survival of the birds.]
[CATCH C08-P022-OB11USB;
Size D; Research. Artist’s image
of the formation of Chicxulub
crater]
Figure 3 The extinction of most
species of dinosaurs 65 million
years ago is thought to been
caused by a large meteorite
impact. Recent finding suggest
multiple large impacts may have
occurred over a period of
several thousand years.
[catch C08-F12-OB11USB; Size
D; New. Diagram showing
Figure 2 The early dinosaurs branched into two major clades—the Ornithischia
and the Saurischia. The Ornithischia include the stegosaurs, triceratops, and
duck-billed dinosaurs, while the Saurischia include the massive sauropods—the
largest land animals to have ever lived, the famous Tyrannosaurus rex, and the
birds.
While the dinosaurs themselves were a truly remarkable group of
animals, their evolutionary history of successful diversification followed by
mass extinction is by no means unique. As you learned in Section 8.3, groups of
organisms often undergo a period of divergent evolution as they evolve to fill
ecological niches. In the case of the dinosaurs, this led to the evolution of more
than 300 known species and perhaps many more. Although individual species
may go extinct for a variety of reasons, what could cause the sudden
Investigation X.X.X 33
Earth’s life history and the
relationship between the
diversity of life and a number of
mass extinction events.]
Figure 4 The history of life on
Earth is characterized by a
general trend toward increasing
diversity, interrupted by a
number of sudden mass
extinction events, five of which
were particularly dramatic.
Cambrian explosion the rapid
evolution of most major animal
phyla that took place over a
period of approximately 40
million years during the
Cambrian period
[End page]
[catch C08-F13-OB11USB; Size
D; New. Cladogram showing the
phylogeny of major groups of
vertebrates]
Figure 5 Cladograms are
branching diagrams used to
show the evolutionary
relationships between different
groups. Here, letters at
branching points represent the
most recent common ancestor
disappearance of so many otherwise successful species?
The strongest evidence for the cause of this mass extinction is the asteroid
that formed a crater located on the edge of the Yucatan peninsula (Figure 3).
The crater is almost 10 km deep and 200 km in diameter. Some theorize that
the asteroid would have been moving at about 160 000 km/h and would have
blasted 200 000 km3 of vaporized debris and dust into Earth’s atmosphere.
The energy released by the impact would have produced a wave of superheated air capable of killing all life on land for thousands of kilometres in all
directions. Tsunamis 120 m high would have inundated coastlines around the
world, and smoke and dust would have blocked most sunlight for months. The
cold temperatures would have had devastating consequences for countless
species.
Earth’s history is divided into five eras, each of which is further
subdivided into periods and, in some cases, epochs. These time intervals are
based on patterns in the fossil record, and dramatic changes in the fossil
record are used to mark the intervals’ boundaries. Notice the trend of everincreasing diversity interrupted by sudden extinction events (Figure 4). The
Palaeozoic era, for instance, begins with the Cambrian explosion and ends with
the most massive extinction event in Earth’s history. The Cambrian explosion
is so called because it was the time during which most major groups of animals
first evolved and underwent rapid diversification. Around 245 million years
ago, a series of cataclysmic events eradicated more than 90 % of known
marine species. Although uncertainty remains about the cause of the extinction
event, many scientists suspect that massive tectonic movements, accompanied
by volcanoes and rapid climate change. played a primary role.
[End page]
[Start page]
A cataclysmic event is not needed to cause a species go extinct. Perhaps
surprisingly, even the five major mass extinction events since the Cambrian
explosion account for about only 4 % of all extinctions that have taken place
during this time.
As you are aware, the current rate of species extinction, due almost
entirely to the actions of humans, is very high. Many biologists predict that
more than half of all living animal species may be extinct in less than 100
years. You will learn more about this critical issue in the last section of this
unit.
Cladistics and Phylogeny
Cladograms are used to illustrate the evolutionary relationships, or phylogeny,
of different groups of species of organisms. The cladogram in Figure 5, for
example, shows the phylogeny of some major groups of vertebrates. By
examining a cladogram, one can infer which groups are more closely related
and the general sequence of events that gave rise to each group. In this
example, Species A is the most recent common ancestor shared by all groups,
while Species B is a common ancestor to all groups except the ray-finned fish.
Species C gives rise to the mammals and to a clade that includes all living and
extinct reptiles. The cladogram also indicates that birds and crocodiles are
closely related, sharing the most recent common ancestor of any two groups
(Species E).
Phylogenies are based on a careful evaluation of a wide range of evidence,
including the fossil record, morphology, and genetics. The most widely
accepted method of applying this evidence is called cladistics. Cladistics uses
the presence or absence of recently evolved traits, or derived traits, as the key
to determining how closely two groups are related. Two groups that share a
Investigation X.X.X 34
of all groups that arise beyond
that point.
cladistics a method of
determining evolutionary
relationships bases on the
presence or absence of recently
evolved traits
derived trait a trait that has
evolved relatively recently with
respect to the species or groups
being discussed
synapomorphy a derived trait
shared by two or more species
or groups
[catch C08-F14-OB11USB; Size
D; New. Cladogram of a
pathogen]
recently evolved trait, a synapomorphy, are thought to be more closely
related to each other than to groups that do not share the trait. For example, all
birds have feathers and are more closely related to each other than to reptiles
without feathers. Only the recently evolved form of a trait is useful for
grouping. Consider, for example, the presence or absence of a tail as a feature
for grouping vertebrates. Salamanders and howler monkeys have long tails,
but apes, such as chimpanzees and gorillas, do not. It would be an error to
think that because howler monkeys have tails they are more closely related to
salamanders than to apes. Instead, the evolutionary loss of a tail is the more
recently derived trait and therefore can be used as evidence that chimpanzees
and gorillas are more closely related to each other than to monkeys or
salamanders.
The key to cladistic analysis lies in making inferences based on
synapomorphies. Unfortunately, evolutionary changes can make this
challenging. Some suspected synapomorphies may be lost, while others may
turn out to be false. All mammals, for example, have evolved from ancestors
with hair, but whales have lost their hair. So the presence of hair is an ideal
synapomorphy for distinguishing most but not all mammals from other
vertebrates. Humans and birds both walk on two legs, but this trait evolved
independently, both among our ancestors and those of birds. In this case we
must consider these two instances of bipedalism as separate traits.
Biologists are able to apply the science of cladistics to large numbers of
related organisms and determine their phylogenetic relationships, based on
synapomorphies. They use advanced software programs and large data sets
from many sources that include a wealth of genetic information (Figure 6).
This science is extremely valuable in understanding the evolution of new
strains of disease causing viruses and micro-organisms.
[End page]
[Start page]
Figure 6 Detailed cladistic
analysis of human
immunodeficiency viruses (HIV)
reveal that closely related but
distinct strains have evolved
from an original SIV (simian
immunodeficiency virus) and
have jumped from chimpanzees
and monkey to humans on five
separate occasions.
TUTORIAL 1: CONSTRUCTING CLADOGRAMS [FORMATTER: SET AS 2
COLUMN, FULL PAGE WIDTH]
Sample Problem 1: Creating a Cladogram
Use the morphological evidence presented in Table 1 to construct a cladogram.
Based on the cladogram, describe the phylogeny of the organisms.
Table 1 Morphological Data
Animal
Characteristics
Digits
Skin
Forelimbs
Tail
surface
lemur
five digits hair
grasping hands
present
deer
two digits hair
non-grasping
present
cow
two digits hair
non-grasping
present
chimpanzee five digits hair
grasping hands
absent
human
five digits hair
grasping hands
absent
lizard*
five digits scales
non-grasping
present
*Note that an additional group must be included as an “outgroup.” An outgroup
is a group that is not closely related to the groups of interest and therefore
unlikely to share any recent traits with other groups. In this case a lizard was
chosen as a distantly related four-limbed vertebrate.
Solution:
Step 1. Consider each characteristic and judge which trait is the more recently
derived trait. This can usually be done by comparing the traits with the
outgroup. Biologists also use other sources of evidence such as the fossil
record to determine which traits are more recently derived.
In this case we make the following inferences:
- Two digits on each foot is a derived trait (having five digits is the primitive
Investigation X.X.X 35
condition).
- Having hair is a derived trait (having scales like reptiles and fish is a primitive
condition).
- Having grasping hands is a derived trait (having four non-grasping “feet” is the
primitive condition).
- The lack of a tail is a derived trait (having a tail is the primitive condition).
Step 2. Create a table of synapomorphies (shared derived traits) (Table 2).
Table 2 Synapomorphies
[CATCH C08-F33-OB11USB; Size C; New. Table of synapomorphies]
Step 3. Draw a “V,” with the outgroup at the upper left (Figure 7(a)). The base
of the V represents the common ancestor to all animals.
Step 4. All the animals except the lizard share the feature of having hair.
Therefore we can indicate the evolution of hair on the right branch leading away
from the lizard (Figure 7(b)).
Step 5. The remaining animals fall into two groups—those with two digits and
those with grasping hands. We therefore split the right branch into two and
locate the evolution of these traits above the split (Figure 7(c)). We can divide
the deer/cow branch in two and place the names of animals at the end of each
branch.
Step 6. The chimpanzee and human both lack a tail, so we create a new branch
and locate this derived trait above the split. (Figure 7(d)). Label the ends of the
remaining branches.
Notice that when you split a branch, the choice of left or right branch for
positioning the groups is arbitrary. We could have chosen to place the deer/cow
lineage on the right rather than on the left.
[formatter; place the next 4 images across the page]
[catch C08-F15a-OB11USB; Size D; New. Diagram of a wide V shape]
[catch C08-F15b-OB11USB; Size D; New. Diagram of a wide V shape with one
dot on the right hand side of the V.]
[catch C08-F15c-OB11USB; Size D; New. Diagram of a wide V shape with two
lines extending out of the right hand side, running parallel to the left hand side
of the V.]
[catch C08-F15d-OB11USB; Size D; New. Diagram of a wide V shape with two
lines extending out of the right hand side.]
Figure 7 Development of a cladogram from Table 2
Based on the completed phylogeny, we can infer that the cow and deer are
more closely related to each other than to other groups. Similarly, humans and
chimpanzees are more closely related to each other than to other groups. We
can also conclude that lemurs are more closely related to chimps and humans
than to cows and deer.
Practice:
1. Use the morphological evidence presented in Table 3 to construct a
cladogram. Based on the cladogram, describe the phylogeny of the organisms.
Table 3 Morphological Data
Animal
Characteristics
Mouth
Skin
Respiratory Bony
opening
surface
organ
limbs
lungfish
jaw
scales
lungs
absent
turtle
jaw
scales
lungs
present
robin
jaw
feathers lungs
present
pike
jaw
scales
gills
absent
lamprey*
no jaw
scales
gills
absent
*The lamprey has been chosen as the outgroup. [END Tutorial]
Investigation X.X.X 36
Gradualism and Punctuated Equilibrium
Another topic of great interest to evolutionary biologists is the pace of
evolution. How quickly do new species and entirely new groups evolve? How
long, for example, did it take birds to evolve from their reptile ancestor? And
how quickly can existing species adapt to changes in their environment?
Answers to these questions have significant implications. Knowing the
answers might allow us to judge how species will respond to climate change
and other human-influenced impacts on the environment.
Biologists know that at the level of individual species, some evolutionary
changes can be quite sudden. For example, a single mutation causing
polyploidy can give rise to a new species. Alternatively, other changes, such as
the evolution of the giraffes’ long neck, have occurred gradually over a period
of millions of years. Biologists have proposed two theories to explain the
patterns of evolution that take place over very long periods of time.
[End page]
[Start page]
theory of gradualism a theory that
attributes large evolutionary
changes in species to the
accumulation of many small and
ongoing changes and processes
theory of punctuated
equilibrium a theory that
attributes most evolutionary
changes to relatively rapid
spurts of change followed by
long periods of little or no
change
INVESTIGATION 8.5.1
Looking for SINEs of Evolution
In this observational study you will use
genetic data to reveal the evolutionary
relationships of whales.
The theory of gradualism states that as new species evolve, they appear
very similar to the original species and only gradually become more
distinctive. Over long periods of time small changes accumulate, resulting in
dramatically different organisms. If this theory holds true for all or most
species, we would expect to find this pattern in the fossil record, with many
fossil species representing changing transitional forms. The fossil evidence of
whale and horse evolution, for example, illustrates this pattern of gradual
change over millions of years. In many other cases, however, this is not the
case. Instead, the fossil record often shows new species appearing quite
suddenly and then remaining little changed over long periods of time. A theory
to account for this pattern was proposed by Niles Eldredge of the American
Museum of Natural History and Stephen Jay Gould of Harvard University. Their
alterative theory of punctuated equilibrium suggests that evolution usually
happens in rapid bursts and is then “punctuated” by periods of little change.
Figure 8 contrasts the two patterns predicted by these theories.
[catch C08-F16a-OB11USB; Size B1; New. Diagram illustrating the concept of
gradualism in birds]
[catch C08-F16b-OB11USB; Size B1; New. Diagram illustrating the concept of
punctuated equilibrium in birds]
Figure 8 (a) The theory of gradualism suggests that most evolutionary changes
are gradual while the theory of punctuated evolution (b) proposes that most
evolutionary changes are abrupt.
The theory of punctuated equilibrium consists of three main assertions:
• New species evolve rapidly in evolutionary time.
• Speciation usually occurs in small isolated populations and therefore leaves
behind few transitional fossils.
• After the initial burst of evolution, additional changes are very slow.
It is now widely accepted that both gradualism and punctuated
Investigation X.X.X 37
equilibrium play a significant role in evolution. In situations where the
environment changes slowly, evolutionary changes would likely be gradual. In
contrast, when a species is exposed to new or rapidly changing environmental
conditions, we can expect rapid evolution. After a mass extinction event, for
example, species that do survive enter an environment with far fewer
competitors.
Gaps and Missing Links?
LEARNING TIP
Theories and Gaps
Missing information or a lack of
understanding does not undermine
knowledge and theories that are
supported by other evidence. Historians,
for example, will never know all of the
events that took place leading up to and
during the war of 1812, but they do
know that the war took place and have
great confidence in their knowledge of
many of the details. The same is true in
science.
Our scientific understanding of the world around us is incomplete. There are
many significant gaps in our knowledge of biology, chemistry, and physics. If
this were not the case, there would be no need for future scientific research. If
we knew everything about chemistry, research chemists would be out of work.
If we knew everything about disease, medical research would be unnecessary.
But this is not the case. In fact, scientific research is more active than it has
ever been in human history. As our scientific knowledge has grown, so has the
number of research scientists looking for answers to new questions.
A commonly held misconception is that a gap in our scientific
understanding reflects scientific uncertainty over the underlying theory. The
misconception is often expressed in this form: “if scientists can’t explain how
“X” happened, or have still have not discovered “Y,” then their theory must be
weak or flawed.” Such gaps, however, should not cause uncertainty in a
scientific theory. Chemists had great confidence in atomic theory and the
validity of the periodic table of the elements long before it was complete. Our
understanding of atomic theory, evolution, and quantum theories are
incomplete but are not in any scientific doubt. These theories account for much
of what we do know, and they are in agreement with an extraordinary wealth
of evidence.
In evolutionary biology, a key source of evidence is the fossil record. The
fossil record, however, is not complete—there are many gaps. Species with
delicate bodies do not fossilize readily, and many species do not live in
environments where the conditions for fossilization occur. In Darwin’s day,
evolutionary biologists had a very limited fossil record. There were few fossils
of transitional forms—organisms intermediate in form between their
modern forms and their ancient relatives. These gaps in the fossil record were
referred to as “missing links.” For example, there were no fossils of early land
animals to offer direct evidence of life invading the land. There were no fossils
of early birds showing the beginnings of feathers and flight. Darwin knew that
a lack of evidence was not evidence against his theory. He also knew that an
understanding of evolution would enable biologists to make predictions about
these gaps and transitional forms.
[End page]
[Start page]
transitional form a fossil or
species intermediate in form
between two other species in a
direct line of descent
Today there is a wealth of fossil evidence, and many of the initial missing
links between major groups of organisms have been filled. The first and most
famous fossil of a transitional species was that of Archaeopteryx (Figure 9).
This species has features of both birds and more primitive reptiles. It had a
bony jaw with teeth and a long bony tail, but also feathered wings. Many more
ancestral bird fossils are being unearthed in China. With these fossils, we are
learning about the evolution of flight. Another gap has been filled with a series
of fossils of early whales, found in Pakistan (Figure 10).
[CATCH C08-P024a-OB11USB; Size C1; Research. Image of a Pakicetus skull.
This will be accompanied by C08-F018-OB11USB, an arrow pointing at the
nostrils at the front of the skull]
[catch C08-F17-OB11USB; Size E; New. Arrow pointing at the nostrils at the
front of the Pakicetus skull]
Investigation X.X.X 38
[CATCH C08-P023-OB11USB;
Size D; Research. Photo of an
Archaeopteryx fossil]
[CATCH C08-P024b-OB11USB; Size C1; Research. Image of an Aetiocetus
skull. This will be accompanied by C08-F019-OB11USB, an arrow pointing at
the nostrils at the middle of the skull]
[catch C08-F18-OB11USB; Size E; New. Arrow pointing at the nostrils at the
middle of the Aetiocetus skull ]
[CATCH C08-P024c-OB11USB; Size C1; Research. Image of a modern Beluga
Whale skull. This will be accompanied by C08-F020-OB11USB, an arrow
pointing at the nostrils at the top of the skull]
Figure 9 The fossil of
Archaeopteryx shows features
that are clearly transitional
between those of a reptile and a
bird.
[catch C08-F19-OB11USB; Size E; New. Arrow pointing at the nostrils at the
top of the modern Beluga Whale skull ]
Figure 10 The discovery of the fossilized skull of Aetiocetus filled a missing link
between the early ancestors of whales—with nostrils on the end of their snout,
and modern whales—with nostrils on the top of their head. The nostrils of
Aetiocetus are “half way” in between and provide an excellent example of a
transitional fossil.
8.5 Summary
- Abiogenesis is a topic of intense scientific interest and one of active research.
- The history of life on Earth follows a general trend toward increasing
diversity marked by rare mass extinction events.
- Cladistics uses the occurrence of shared derived traits to infer evolutionary
relationships.
- The theory of gradualism proposes that most evolutionary changes occur
over long periods of time.
- The theory of punctuated equilibrium proposes that major evolutionary
changes happen relatively rapidly and are then followed by long periods of
little change.
- All scientific understandings are incomplete. Scientific investigations
continue to fill these gaps.
8.5 Questions
1. Many scientists believe that at one time conditions on Mars may have been
suitable for the evolution of life. Do online research to learn about any evidence
that has been found that supports the possibility of past or present life on Mars.
[catch web link icon] [K/U] [T/I]
(a) Define abiogenesis.
(b) Why is abiogenesis considered to be distinct from evolution?
(c) Suggest ways in which the same principles of natural selection might
have influenced the formation of chemicals and the very first cell-like structures.
2. In what way was endosymbiosis critical for the evolution of animals, plants,
and fungi? [K/U]
3. You hear a scientist describe the history of life on Earth as one of both
increasing diversity and mass extinction. Explain this statement using a diagram
to illustrate the relationship between these opposing processes. [T/I] [A]
4. Some scientists suggest that without the mass extinction of the dinosaurs,
mammals would not have been able to undergo adaptive radiation. Use your
understanding of competition for resources to support or refute this suggestion.
[T/I] [A]
5. Birds are the only group of dinosaurs that survived the mass extinction of
65 million years ago. Speculate on how their ability to fly and endothermy
(being warm-blooded) may have been keys to their survival. [T/I] [A]
6. Both salamanders and dogs have long tails, while bears do not. However,
Investigation X.X.X 39
both bears and dogs have hair, while salamanders do not. Explain why having a
long tail is not evidence that dogs are more closely related to salamanders than
they are to bears. Explain why having hair is good evidence that dogs and
bears are more closely related than dogs and salamanders. [K/U] [T/I] [A]
7. Do online research to find out about secondary endosymbiosis. Use a
simple sketch to show how this occurs. What organisms exhibit evidence of
secondary endosymbiosis? [T/I] [A]
8. Use the cladogram in Figure 11 to answer the following questions.
Assume that each number represents the evolution of a new feature and that
each letter represents a species alive today. [K/U] [T/I] [C]
[catch C08-F31-OB11USB; Size C; New. Cladogram showing five branches. ]
Figure 11
(a) In your notebook, draw a table similar to Table 4 below. Complete the
table of derived traits, using the relationships shown in Figure 11
Table 4
Species
Derived trait
1
2
3
4
5
A
B
C
D
E
(b) Which two species are most closely related?
(c) List the synapomorphies shared by Species C and E.
(d) To which species is/are Species C most closely related?
(e) Is species B more closely related to Species A or E? Explain your
reasoning.
(f) Which number(s) represents new features that were not needed to
draw this cladogram?
9. Many scientists do not use the term “missing link” and consider it
misleading. They suggest that is gives a false impression that evidence “should”
have been found. Instead, they counter that in all science, newly discovered
evidence simply adds to our understanding—and that it was never “missing.”
Do you think the term “missing link” is misleading? Why or why not? [A] [C]
10. Examine the fossil hind limbs in Figure 12. Scientists believe this animal
was a transitional species between land mammals and modern whales. Do you
think this animal spent all, some, or none of its time on land? Explain your
answer. [T/I]
[CATCH C08-P058-OB11USB; Size C; Research. Photo of fossil hind limbs.
Author will supply.]
Figure 12
Investigation X.X.X 40
8.6
Biology Journal: Tiktaalik—
Triumph of a Theory
[FORMATTER: RUN ABSTRACT IN ONE COLUMN, AS PER
DESIGN]
ABSTRACT
Among the most significant events in evolution was the invasion of
the land by terrestrial vertebrates. Paleontologist Neil Shubin used
the theory of evolution to correctly predict the location of fossil
remains of a transitional species in Canada’s high Arctic. The species,
Tiktaalik roseae, has many features of both fish and four-limbed land
vertebrates. Shubin and his colleagues published their dramatic
findings in the prestigious journal Nature.
Investigation X.X.X 41
[Format in 2 columns as per Product Profile]
Introduction
Fossils of the now famous Tiktaalik roseae were first discovered on Ellesmere Island in the Canadian Arctic in 2004
(Figure 1). Tiktaalik has an odd mix of features. It has fins and scales, a neck and wrist bones, and an unusual flattened
head. As a result it was nicknamed “fishapod.” The discovery not only produced a very important transitional fossil, but
also served as a perfect example of how scientists use and test theories.
[formatter: place two images side by side]
[CATCH C08-P025a-OB11USB; Size C3; Research. Image of a Tiktaalik fossil]
[CATCH C08-F28-OB11USB; Size C3; New. Map showing the location of Ellesmere island in the Canadian arctic.]
Figure 1 (a) Fossils of Tiktaalik were discovered (b) within 1000 km of the North Pole on Ellesmere island in the
Canadian arctic.
The Power to Predict
Neil Shubin is a palaeontologist and professor at the University of Chicago. He is keenly interested in major
evolutionary steps, including the evolution of the first land vertebrates. According to the theory of evolution, the first
land vertebrates evolved from fish that made the transition onto land. Therefore, the theory predicts that in the past
there must have been species that were transitional between lobe-finned fish and the first simple land vertebrates.
Lobe-finned fish have bones extending part way into their fins and were therefore considered the most likely ancestors
of the first animals to walk on all fours on land. When he began his search in the 1990s, Shubin knew that no
transitional fossil species between land vertebrates and lobe-finned fish had been found. He hoped that by using the
theory he could predict where to find such fossils.
Shubin began his research by gathering information on all the fossil finds of both primitive land vertebrates and
lobe-finned fish. He noted several key points:
- The first fossils of lobe-finned fish appeared 390 million years ago.
- The earliest fossils of land vertebrates appeared 360 million years ago.
- The earliest land vertebrate fossils were found in freshwater sedimentary deposits.
- Fossils of lobe-finned fish were also associated with freshwater sedimentary deposits.
Shubin used these facts to make three inferences:
1. The transition from sea to land occurred sometime between 360 and 390 million years ago (Figure 2).
2. The transition occurred in freshwater ecosystems.
3. Fossils of transitional species would be found in freshwater sedimentary deposits from about 375 million years ago.
[catch C08-F20-OB11USB; Size C2; New. Diagram showing the theory that transitional species should have lived
between 360 and 390 million years ago]
Figure 2 Theory predicted that transitional species would have lived between 390 and 360 million years ago.
Shubin now knew what he was looking for and set out to find it in the map room of the local library. There, Shubin
examined geological maps of the world looking for sedimentary rocks that had been deposited in freshwater and were
approximately 375 million years old. They also had to be at Earth’s surface. Rocks buried a half kilometre underground
would be inaccessible. Of the three suitable deposits in North America that Shubin found, only one had not been
explored in the past.
Investigation X.X.X 42
Field Work Begins
In 1999, Shubin and his team organized their first expedition to Ellesmere Island. At first their explorations were
unsuccessful. Their first year’s dig site turned out to be of marine deposits from an ancient ocean. In 2000, they moved
their exploration site to the east and began again. That year they unearthed a rich fossil deposit containing many
freshwater fish species. In the following years they returned to the same site to keep digging. In 2004, they unearthed
Tiktaalik and knew immediately that their predictions had been confirmed.
The “Fishapod”
The 10 fossil specimens that Shubin and his team found are extremely well preserved. They range in size from just
under 1 m in length to almost 3 m! Tiktaalik really does look like a cross between a fish and a four-legged land animal.
Like fish, Tiktaalik had webbed fins supported with thin bones, gills, and scales. However, like four-limbed vertebrates,
Tiktaalik had a neck and shoulders, thick ribs, and sturdy wrist bones (Figure 3). The skull of Tiktaalik was flattened,
with eyes on top and two notches that are closer in size to those of land vertebrates than of fish. In early land
vertebrates the notches function as primitive ears.
[catch C08-F21-OB11USB; Size C2; New. Image showing that Tiktaalik had features of both fish and land vertebrates]
Figure 3 Tiktaalik had features of both fish and land vertebrates.
Tiktaalik was not able to walk, but its limb bones would have allowed it to prop itself up in a “push-up” position.
Based on the overall shape of its head and body, Tiktaalik likely lived in shallow freshwater.
Scientific Recognition
Shubin and his fellow researchers knew that their findings were of great scientific significance. They reported their
initial findings in two scientific papers that they submitted to the prestigious journal Nature in October 2005. After
being peer reviewed, the articles were accepted for publication in February 2006 and published in April of that year.
The Tiktaalik discovery was the cover story (Figure 4).
[CATCH C08-P026-OB11USB; Size C2; Research. Image of the cover of nature in October 2005]
Figure 4 The cover of the journal Nature in April 2006
Further Reading
Clark, J.A. (2005, November) Getting a leg up on land. Scientific American.
Daeschler, E.B., Shubin, N.H. & Jenkins, F.A. (2006). A Devonian tetrapod-like fish and the evolution of the tetrapod
body plan. Nature, 440 (7085), 757–763.
Daeschler, E.B., Shubin, N.H. & Jenkins, F.A. (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod
limb. Nature, 440 (7085), 764–771.
Shubin, N.H. (2008). Your inner fish: a journey into the 3.5-billion-year history of the human body. New York: Pantheon.
Books
8.6 Questions
1. How did an understanding of the theory of evolution enable Shubin to predict the age and type of sedimentary rock
in which he would find Tiktaalik? [K/U]
2.
Why did Shubin decide to look for fossils in Canada’s extreme north? [K/U]
3. It took five years of field work to find Tiktaalik. What does this suggest about the nature of palaeontology? What
challenges do you think Shubin’s team faced? [A]
4.(d) In 2010 scientists announced the discovery of 395- million- year- old fossil footprints they believe were made by
land vertebrates. If confirmed this would mean that Tiktaalik was not the direct ancestor of the earliest land vertebrates.
Investigation X.X.X 43
Do you think such a discovery detracts from Shubin’s accomplishments? Explain your reasoning. [T/I] [A]
5.(e) Some fish have primitive lungs. Make a prediction— - do you think Tiktaalik had lungs in addition to gills? Why or
why not? Do online research to check your prediction. [catch web link icon] [A]
6.(f) Many scientists think the term “missing link” is misleading. They feel it implies that they “should” be able to find
fossils to fill every gap in the fossil record. Scientists point out that the fossil record will always by incomplete because
many species do not fossilize and even those that do may never be found. Do you agree? [A]
7.(g) Articles submitted to the science journal Nature undergo a rigorous peer review process before being accepted.
What is the benefit of such a process? Why would scientists not prefer to publish their findings in journals that do not
require a review? [T/I] [A]
[catch web banner icon]
Investigation X.X.X 44
8.7
Human Evolution [8 pages]
Like every other species, humans have evolved a combination of
characteristics that have enabled them to survive and flourish. In this section
we examine what characteristics are responsible for human success and how
and when humans evolved.
Human Biological Characteristics
[CATCH C08-P027-OB11USB;
Size D; Research. Photo of a
human baby demonstrating fine
motor skills and coordination.
For example, baby can be
stacking blocks on top of each
other]
Figure 1 Even young children
can manipulate objects in their
environment.
primate a group of relatively
large-brained, mostly arboreal
mammals that include the
prosimians, monkeys, apes, and
humans
What makes humans unique as a species? Among the most obvious and
important human characteristics are our large brains, our dexterous hands,
upright walking, our ability to communicate, and making and using tools. But
dolphins have large brains, many animals can communicate, and apes and
some birds make and use tools. What is different in humans is the degree to
which these and other biological characteristics have evolved.
Human success can be attributed in large part to
- the ability to perform complex reasoning, coupled with an exceptional ability
to learn
- the ability to make and use sophisticated tools
- the ability to communicate using complex symbolic language
These human abilities also required the evolution of at least three distinct
physical characteristics:
- a very large brain relative to body size
- hands capable of fine manipulation and coordination (Figure 1)
- upright walking (bipedalism) that frees hands for using tools
Human Phylogeny
Homo sapiens are primates. Primates are a relatively small group of mammals
characterized by large brains relative to body size, forward-directed eyes,
flexible hands and feet, and arms that can rotate fully. Many primates also have
opposable thumbs that can touch their fingers and enable them to hold and
manipulate objects. Most primates have tails (Figure 2).
[catch C08-F22-OB11USB; Size B; New. Diagram showing the early primates
and the groupings that have evolved from this.]
Figure 2 t/k
[End page 1]
Investigation X.X.X 45
[Start page]
prosimian a term used for a
number of groups of primate
mammals closely related to the
monkeys and apes; the
prosimians include lemurs,
lorises, and tarsiers
anthropoid the group of
primate mammals that includes
all monkeys, apes, and humans
INVESTIGATION 8.7.1
Human Chimpanzee
Chromosome Comparison
In this investigation you will have a
chance to compare and analyze human
and chimpanzee chromosomes.
All primates share a common ancestor dating from about 60 to 70 million
years ago. The tree branches early on, giving rise to the prosimians, relatively
small nocturnal species, and the anthropoids. The anthropoid group then split
into two distinct groups: the monkeys and the hominoids, or apes. During this
time, continental drift was separating South America (the new world) from the
land mass that would form Africa and Eurasia (the old world). With the split,
the early ancestors of monkeys divided into new world and old world
populations. The apes, easily distinguished by their lack of a tail, branched into
the lesser apes, the gibbons, and what are referred to as the great apes and
humans.
Genetic sequences of the entire human and chimpanzee genomes have
now been completed. Detailed analysis of the genomes confirms that
chimpanzees are our closest living relatives. Humans and chimpanzees share
approximately 98.8 % of their DNA. Humans and chimpanzees both differ from
gorillas by about 1.6 %. The most recent common ancestor we share with
chimpanzees lived in Africa more than 6 million years ago. The branch ending
with modern humans is a clade of many different species, including our direct
ancestors. Members of this clade are called hominids.
The Hominid Fossil Record
hominid all species descended
from the most recent common
ancestor of chimpanzees and
humans that are on the human
side of the lineage
Hominid fossils record information about the sequence of steps in the
evolution of humans. Fossils of hip bones, feet, leg bones, and footsteps
provide information about whether or not a species walked upright. Fossils of
the skull can be measured and track trends in brain size (Figure 3). Remains of
stone tools and burial sites inform scientist about tool use and early human
culture.
[formatter: place 3 photos across text measure]
[CATCH C08-P029a-OB11USB; Size D; Research. Photo of Australopithecus
africanus skull]
[CATCH C08-P029b-OB11USB; Size D; Research. Photo of Homo habilis skull]
[CATCH C08-P029c-OB11USB; Size D; Research. Photo of Homo
neanderthalensis skull]
Figure 3 Fossil skulls of (a) Australopithecus africanus, (b) Homo habilis, and
(c) Homo neanderthalensis show a progression in brain size. Note that
bipedalism evolved in Australopithecus before the subsequent evolution of large
brains.
LEARNING TIP
Becoming Human
Hominid evolution happened gradually
over millions of years. Over time, an
Australopithecus species changed
enough to be considered the first Homo
species. Although the process is
gradual, scientists must still decide on a
specific point to switch naming from one
Between 6 and 7 million years ago, Sahelanthropus was beginning to
occasionally walk upright (Figure 4(a), next page). Recently discovered fossils,
and the first evidence of stone tools used by Australopithecus afarensis, date to
3.4 million years ago. By 2 million years ago, the first members of our own
genus, Homo gautengensis, and Homo habilis had evolved. Homo habilis, often
called the handy man, was making stone axes and large cutting tools. Tools for
hunting permitted hominids to dramatically increase the amount of meat in
their diet, providing a rich source of protein and fats. By this time, hominid
brains were significantly larger than the chimpanzee-sized brains of
australopithecines.
The use of hearths for cooking dates to at least 790 000 years ago, and fire
may be have been used for cooking as early as 1.5 million years ago. Cooking
food may have reduced disease and increased the variety of foods that could be
Investigation X.X.X 46
genus to another. This is analogous to
becoming an adult. Even though the
process is gradual, you gain adult status
“instantly” on your eighteenth birthday.
[CATCH C08-P028-OB11USB;
Size B1; Research. Photo of the
famous Laetoli footprints]
consumed. Perhaps most notably, hominid brain size increased relatively
rapidly from about 800 000 years ago to 200 000 years ago. During that time,
Homo heidelbergensis evolved and may have given rise to both Homo
neanderthalensis and Homo sapiens. The first essentially modern humans had
evolved by no later than 100 000 years ago in East Africa. Figure 4(b)
illustrates a simplified version of a widely accepted but tentative cladogram of
the Homo genus.
[End page 2 – break in previous paragraph where necessary]
[Start page]
[catch C08-F23-OB11USB; Size B1; New. Chart illustrating the time frames
during which early hominid species were living]
[catch C08-F24-OB11USB; Size D; New. Simplified cladogram showing the
probable relationships within the Homo genus.]
Figure 4 (a) Time frames during which early hominid species were living. Two
very recently discovered hominid species, Homo gautengensis and
Australopithecus sediba, are not included in this chart. (b) This simplified
cladogram depicts the probable relationships within the Homo genus. Fossil
evidence of the new species Homo gautengenis suggests it may have been an
ancestor of Homo habilis. [the dark bars are the hypothesised “lineages” while
the coloured bars represent the “known” time span for the existance of each
species - eg H. erectus lived from about 1.8 - 0.2 MYA Add A.g to (b) in pages]
Today the hominid fossil record of more than 20 species consists of fossils
ranging in size from small bone fragments to almost complete skeletons. They
include the spectacular Australopithecus afarensis fossil finds at Laetoli,
Tanzania, which include a set of 69 footprints dated to 3.7 million years ago
(Figure 5). These footprints show that human ancestors evolved the ability to
walk upright long before they had large brains. Although the precise
Investigation X.X.X 47
Figure 5 The famous Laetoli
footprint fossils are clear
evidence of upright walking
dating to about 3.6 million years
ago.
CAREER LINK
Anthropologist
Anthropologists study the
origins of humanity. To learn
more about becoming an
anthropologist, [catch Nelson
Science] GO TO NELSON
SCIENCE
relationships among the many early hominid species remain unclear, an early
branch probably gave rise to a number of robust Paranthropus species with
heavy jaws and relatively small brains, while another ultimately gave rise to
the genus Homo.
[End page 3]
[Start page]
[SET AS TWO COLUMNS] Mini Investigation
Following Footsteps in TIME
Mary Leakey’s team discovered a variety of small animal footprints in Laetoli,
Tanzania in 1976. Two years later while working on the site they uncovered a
set of 69 hominid tracks. The 3.6-million-yer-old fossils were the earliest
evidence of bipedal motion in a hominid ancestor. Paleontologists can use
measurements of the tracks to make inferences about the individuals who made
them. In this mini investigation you will do a correlational study to look for
relationships between foot length, stride length, and height. You will then use
your findings to predict the height of two of the individuals who made the Laetoli
footprints. [catch career link]
Skills: Researching, Predicting, Planning, Performing, Observing, Analyzing,
Evaluating, Communicating
Materials and Equipment: measuring tape
1. Make predictions about possible correlations between foot length, stride
length, and height.
2. Measure the height and right foot length of at least 10 people. Ideally choose
individuals with a wide range in height.
3. Have each individual walk at a normal leisurely pace; measure the distance
they travel in 10 steps. Use this value to calculate their average stride length.
4. Plot two graphs—one of height versus foot length and one of height versus
stride length. Draw a line of best fit through both sets of data.
5. Conduct online research on the Laetoli and human footprints.
A. Did you find a correlation between foot length and/or stride length and
height? Describe the correlation(s). [T/I]
B. Use the data in Table 1 and your own graphs to estimate the height of the
two individuals. [T/I] [A]
C. Ask your teacher for the heights of the two individuals as calculated by
paleontologists. How close were your height estimates to those of
paleontologists? Suggest possible reasons for any differences. [T/I]
D. What role did volcanic eruptions and rainfall play in the formation of the
fossilized tracks? [T/I]
E. Describe the comical events surrounding the discovery of the Laetoli fossils.
[catch web link icon] [T/I]
F. How do the Laetoli and human footprints differ from those of chimpanzees
when they walk on two feet? [catch web link icon] [A]
Table 1 Laetoli Footprint Data
Characte Individu
Individu
ristic
al 1
al 2
foot
18.5 cm 21.5 cm
length
stride
28.7 cm 47.2 cm
length
[catch web banner icon] [END Mini Investigation]
Out of Africa
Fossils record the history of the distribution of species in both time and space.
Fossil evidence tells the story of when and where our ancestors evolved and
how they spread out across and around the world.
WEB LINK
Investigation X.X.X 48
To learn more about migration patterns
of Homo species, [catch go to Nelson
Science] GO TO NELSON SCIENCE
All early hominids evolved and lived in Africa (Figure 6). The first species
to spread beyond Africa was Homo erectus, about 1.9 million years ago. The H.
erectus population that left Africa spread out across much of Eurasia and
survived until at least 100 000 years ago. The next species to spread beyond
Africa, some 500 000 to 300 000 years ago, were the ancestors of the
Neanderthals. Homo neanderthalensis populated parts of Europe. Relatively
shortly after the earliest modern humans evolved, they too began to spread out
of Africa and into Europe and Asia, eventually reaching the Americas. [catch
web link icon]
[catch C08-F29-OB11USB; Size B; New. World map showing the spread of
Homo erectus and later species out of Africa into Europe, Asia, and the
Americas.]
Figure 6 t/k
Fossils and DNA
In what can only be described as a stunning technological achievement,
scientists have been able to extract enough DNA from Neanderthal bones to
sequence the entire Neanderthal genome. This has allowed geneticists to
compare the DNA of Neanderthals with that of modern humans. The most
striking discovery is that some small sequences of the Neanderthal genome are
also found in humans of Asian and European descent but not in any humans of
African descent. These genetic remnants no longer have any function in
humans, but they do suggest that some interbreeding may have occurred when
early Homo sapiens made contact with Neanderthals.
[End page 4]
[Start page]
Cultural Evolution
[CATCH C08-P031-OB11USB;
Size D; Research. Photo of a
chimpanzee making some
gesture (either facial or hand)]
The first biologists to compare human and chimpanzee DNA were so struck by
their similarity that they joked that perhaps the only differences between
humans and chimpanzees were cultural. Their joke was not without some
grain of truth. While our genetic differences are less than 2 %, our cultural
differences are enormous.
Like humans, chimpanzees engage in some ritualized behaviours and use
different forms of symbolic gestures to communicate (Figure 7). They also
have complex social organization. However, neither chimpanzees nor any
other animal has developed anything comparable to the extraordinary
richness of human culture. More than 6000 human languages have been
spoken and human societies have engaged in countless artistic endeavours in
music, dance, and the fine arts. Humans admire and cherish the talents of
others. We have athletic heroes and movie stars. Human societies have
different rituals, customs, and belief systems.
It is fascinating to consider how biological evolution might have
influenced the development of culture and vice versa. Evidence suggests that
Investigation X.X.X 49
Figure 7 Chimpanzees use both facial
expressions and simple hand gestures to
communicate.
[CATCH C08-P032-OB11USB;
Size D; Research. Photo of a
perforated shell that was worn
as a neck pendant from a
Neanderthal site]
Figure 8 Perforated shell worn as a neck pendant
along with pigments that might have been used
as cosmetics—from a Neanderthal site
from the time of our common ancestor with the chimpanzee, our ancestors
lived as hunter–gatherers for more than 300 000 generations. In only the last
1000 generations or less have humans domesticated plants and animals,
developed agricultural systems, and begun to live in large population centres.
Only during the last 10 generations has our population size skyrocketed.
There is evidence in the fossil record that our own species, Homo sapiens,
and Homo neanderthalensis both performed burials and made body ornaments
more than 50 000 years ago (Figure 8). While some believe that both species
evolved these behaviours independently, others suggest that the Neanderthals
began copying the activities of humans shortly after the two species came in
contact.
A vital component of human culture is language and communication.
Without the development of a rich spoken language, humans could not have
realized our current success as a species. Evolutionary biologists are just
beginning to understand the evolution of speech. We now know, for example,
that a gene called FoxP2 codes for a protein that regulates a number of other
genes and is vital for human speech. Individuals with even one defective copy
of the gene have a severe speech and language disorder. The gene is found in
all mammals and is highly conserved—meaning it varies little from species to
species. The human and Neanderthal version of the FoxP2 gene differs from
that of the chimpanzee in just two bases out of more than 2100. Genetic
analysis also provides evidence that these two mutations have been strongly
favoured by natural selection. It could be that one or both of these mutations
provided our early ancestors with an enhanced ability to communicate.
Cultural evolution influences biological evolution
An interesting example of how cultural evolution influences biological
evolution is evident in the recent evolution of lactose tolerance. Lactose
tolerance evolved as human populations began to domesticate goats, cattle,
and camels, and consume their milk. In these populations natural selection
favoured those rare individuals who were more capable of digesting lactose.
Mathematical modelling suggests that tolerant individuals had a 4 % to10 %
enhanced reproductive success in these populations. In this way, it was the
cultural choice to domesticate livestock and consume their milk that created
the new selective pressure for the evolution of lactose tolerance.
[End page 5 – break middle of previous paragraph if necessary]
[Start page]
Human culture has influenced the evolution of many other species but
none more so than domesticated species. Indeed, the domestication of plants
and animals by artificial selection and breeding is nothing less than “directed”
evolution by humans. It is interesting to consider that after 300 000
generations of our own biological evolution, the human population numbered
no more than a few million. It has been our ability to direct the evolution of
other species that has allowed us to mass produce foods and feed a population
that numbers in the billions (Figure 9).
[CATCH C08-F30-OB11USB; Size D; New. Map of the Near East indicating the
Fertile Crescent and areas of domestication of pig, cattle, sheet, and goats with
dates of initial domestication in calibrated years B.P.]
Investigation X.X.X 50
Figure 9 Humans have directly influenced the evolution of many species, including all the species
we have domesticated through artificial selection.
Human Races
Detailed comparisons of human populations from around the world have
conclusively shown that from a biological perspective human “races” do not
exist. Traits that we associate with races, most notably skin colour, are visually
obvious but genetically minimal. There is far more diversity within so called
races than there is between races. The classification of humans by race is
therefore a cultural choice on a par with classifying people based the language
they speak, their blood type, or their religious beliefs (Figure 10).
[CATCH C08-P035-OB11USB;
Size D; Research. Photo
showing faces of teens of
different “races”. Photo should
show skin colour differences]
Figure 10 Although humans have used skin
colour as a way of categorizing people into races,
the underlying genetic differences are very slight
and there are no biologically distinct races of
humans.
[CATCH C08-P036-OB11USB;
Size D; Research. Photo
showing a teenager eating ice
cream]
Figure 11 t/k
WEB LINK
To learn more about Darwinian
medicine, [catch Nelson
Science icon.>GO TO NELSON
SCIENCE
Are Humans Still Evolving?
Is our evolutionary path of any consequence today? Does it matter if we
evolved from a common ancestor of apes and are if we still evolving today?
Understanding our evolutionary past and how evolutionary processes
work today has many benefits. For example, it can provide insights into
healthcare problems. We know for instance that in our past, rich food sources
were scarce and natural selection favoured individuals who could detect
sweet-tasting and fatty foods and could gain weight during times when food
was readily available. Our sense of taste evolved to “let us know” that these
were valuable foods. In our modern world, we still relish these foods but now
it easy for us to overindulge. This evolutionary preference for ice cream and
similar foods is therefore partly responsible for the problems of obesity and
heart disease (Figure 11).
[End page 6]
[Start page]
Darwinian medicine, the use of evolutionary theory to understand
medicine, is providing many important insights into the cause and spread of
diseases. An evolutionary perspective can be very important. For example,
humans often get a fever with they have a serious infection. The first question
an evolutionary biologist might ask is whether or not a fever is an evolved
response of the body to help fight off the infection? If this is the case, then
taking a drug to lower your body temperature might be unwise. Similarly, is
coughing an evolved mechanism to expel disease-causing organisms from your
lungs? If so, would taking a medication that suppresses a cough be helpful? In
modern society vitamin D deficiency is widespread. This is not at all surprising
from an evolutionary perspective. For millions of years, ancestral and modern
humans spent time outdoors every day and evolved the ability to synthesize
vitamin D when exposed to sunlight. Today many people spend little or no
time outside, and when they do they are told to use sunscreen. Researchers are
investigating the best way to balance the need to protect ourselves from
damaging UV radiation and the risk of skin cancer, with our evolved
requirement to synthesize vitamin D.
In serious infectious diseases, biologists can track the evolution of the
disease-causing agents as well as human evolutionary responses to these
diseases. For example, a newly discovered allele called CCR5-D32 provides
very strong protection against HIV/AIDS infections and may be under the
influence of strong selective pressure in some populations. <CATCH: web link>
Research This
Investigation X.X.X 51
Evolution on the Track [FORMATTER SET IN 1 COLUMNS, TEXT WIDTH]
Skills: Researching; Analyzing
Evolutionary biology is providing insights on the running track. Humans evolved
as barefoot walkers and runners. Although humans are not very fast compared
to most other large mammals, evidence suggests that humans did evolve to be
very efficient at running long distances. Today barefoot walking and running is
rare in modern society. Instead, most of us wear shoes to walk and specially
designed sports shoes for running or other athletic activities. In this activity you
will conduct online research to investigate the advantages and disadvantages of
barefoot running.
1. Research the differences in how shoed runners and barefoot runners land on
their feet as they run.
2. Investigate the current trend in barefoot running and the claim by Daniel
Lieberman, a professor of human evolutionary biology at Harvard University,
that it is possible to run barefoot on the hardest of surfaces.
A. Which running method produces more impact stress on the runner? [catch
web link icon] [T/I]
B. How might natural selection have influenced the running mechanics of
humans? Would you expect our natural running style to cause undue stress on
our bodies? [catch web link icon] [T/I] [A]
C. Is it possible to just switch from running with shoes to running barefoot?
Explain. [catch web link icon] [A]
D. Describe the key advantages and disadvantages of running barefoot. [catch
web link icon] [T/I]
E. Abebe Bikila and Zola Budd won Olympic gold medals running barefoot. In
what events did they compete? [catch web link icon] [K/U]
F. Would you ever consider routine barefoot running? (Figure 12) Why or why
not? [A]
[CATCH C08-P059-OB11USB; Size C3; Research. Photo of a group of runners,
one of two of which are barefoot]
Figure 12 Have you considered running in bare feet?
[catch web banner icon] [End Research This]
A Human Legacy?
Evolution can inform us about where we have come from but not where we are
going. We have evolved a brain and body capable of great achievements in the
arts and in understanding the world around us. We have discovered our own
biological origins and have the tools and freedom to direct our own future. Yet
humans also behave in ways that threaten our own future and the future of
other species.
There is no doubt that humans are biologically limited by our
evolutionary history. We are not going to sprout wings and fly, but there is
nothing in our genes that prevents us from making intelligent choices about
our own future. We have certainly made much progress—legalized slavery and
routine sacrifice were once widespread, accepted practices. Today, in most
countries women and men have equal rights under the law, and forced child
labour has been criminalized. We have evolved a brain capable of knowing
which actions are needed to ensure a secure and sustainable future for
ourselves and our descendents as well as for the biodiversity of life on Earth as
a whole. How human society decides to act remains an unknown.
[End page 7]
[Start page]
Investigation X.X.X 52
8.7 Summary
• Key evolutionary characteristics of humans include bipedalism, large brain
size, and hands capable of fine manipulation.
• Chimpanzees are our most closely related living species.
• There is a rich fossil record of human ancestors consisting of more than 20
different hominid species.
• Homo sapiens first evolved in Africa and began spreading out from Africa
about 50 000 years ago.
• Biological and cultural evolution influence each other.
• An understanding of human evolution has many applications.
8.7 Questions
1. What selective advantage does each of the following traits provide to
humans? [K/U]
(a) a large brain
(b) upright walking
(c) complex finger movements
(d) complex language
2. Most ground-dwelling mammals have eyes that look to the side, giving
them a wide field of view to help them avoid predators. Many tree-dwelling
mammals have eyes that are directed forward, giving them better 3-D vision.
Suggest an evolutionary explanation for why ground-dwelling humans have
forward-directed eyes? [T/U]
3. Look at your feet. What feature of the arrangement of your toes provides
evidence of an evolutionary history that involved a tree-dwelling lifestyle? [T/I]
[A]
4. What evidence suggests that human ancestors walked upright before they
evolved large brains? [T/I] [A]
5. Human are born with an extraordinary ability to learn new behaviours. In
contrast, most behaviours of most species are instinctive. This suggests that
there are many advantages and disadvantages of learned behaviour. For each
of the following examples, decide how the learned behaviour is advantageous
and/or disadvantageous: [T/I]
(a)
young humans learn to walk at about age one year, while deer
fawns walk on their own when they are a few hours old.
(b) young humans learn their parents’ spoken language, while most frogs
and birds sing their mating calls instinctively.
(c) humans learn to adapt to many different environmental conditions,
while most species use instinctive behaviours well suited to a very specific type
of environment.
6. New hominid fossils continue to be found. In 2003, Australian and
Indonesian scientists discovered the fossil remains of a hominid species 1 m tall
with a small brain. They called the species Homo floresiensis, and it is
affectionately known as “the hobbit” (Figure 13). Do online research to learn
more about this recent discovery. [catch web link icon] [T/I]
(a) Are all scientists convinced these fossils represent a new species?
(b) What alternative hypothesis was presented to explain the small brain
size of the individual?
(c) What is the current status of Homo floresiensis? Which hypothesis is
more widely accepted?
[CATCH C08-P060-OB11USB; Size C1; Research. Photo of Homo floresiensis.
Figure 13 An almost the complete skeleton of Homo floresiensis was found in 2003.
7. Compare modern humans and Neanderthals. What evidence suggests we
are very closely related species? [K/U] [T/I]
8. Differences in skin colour, spoken languages, religious beliefs, or
geographic location are often used to “classify” people into different groups.
Investigation X.X.X 53
Explain why biologists do not use such characteristics as a basis for
classification. [T/I] [A]
9. Humans were living as small groups of hunter–gatherers during most of
the last 100 000 years. Our food as hunter–gatherers consisted of some
animals and many plants. Sweet foods, high-energy fatty foods, and salts were
relatively rare. Today the favourite foods of most people fit into these same
categories. [T/I] [A]
(a) Make a list of 10 of your favourite foods. How many of them are sweet,
high in fat, or salty?
(b) Suggest a way that natural selection might have caused us to evolve
this love for these foods.
(c) How has the evolutionary love of these foods influenced our societal
problems related to obesity and poor eating habits?
10. Medical researchers are using sea urchins to gain a better understanding
of human diseases. Sea urchins and humans have 7000 genes in common. In
humans, some of these genes play a role in muscular dystrophy and
Alzheimer’s, Parkinson’s, and Huntington’s disease. [catch web link icon] [T/U]
(a) How does the theory of evolution account for these two species sharing
so many genes?
(b) Do online research to find out why medical researchers might choose to
study sea urchins.
11. It is thought that the initial population of Homo sapiens that left Africa and
expanded into the rest of the world was relatively small, while the population
that remained in Africa was quite large. Use this information to predict whether
genetic variability with be higher among human populations in Africa or in other
parts of the world. Do online research to check your prediction. [catch web link
icon] [T/I] [C]
12. Homo sapiens is a young species, having existed for only a few hundred
thousand years. Most of the recent success of humans can be attributed to
cultural and technological advances. [T/I] [A]
(a) Do you think humans will continue to evolve as a species? Support your
ideas.
(b) Brainstorm some of the selective pressures that you think humans may
be experiencing now and may experience in the future.
(c) Hypothesize about potential human adaptations that could result.
[catch web banner]
Investigation X.X.X 54
8.1.1
Correlational Study
Bird Monogamy and Sexual
Dimorphism
Bird species exhibit a variety of lifelong mating patterns. Some are
monogamous—they mate for life, while others are polygamous—
choosing a different mate, or multiple mates, each year. Bird species
also vary in the appearances of males compared to females. Some
bird species exhibit sexual dimorphism, with noticeable differences
between males and females. In other species the sexes are similar. In
most dimorphic species the males are the more brightly coloured sex.
This characteristic helps males attract females but makes them more
vulnerable to predators. In this investigation you will explore
possible correlations between these variables.
SKILLS MENU
Questioning
Researching
Hypothesizing
Predicting
Planning
Controlling Variables
Performing
Observing
Analyzing
Evaluating
Communicating
Investigation X.X.X 55
Figure 1
Purpose
To look for correlations between lifelong mating
patterns and the occurrence of sexual dimorphism
Variables
In this correlational study the variables you will
consider are (1) mating pattern of the species and (2)
appearance of the sexes.
For this investigation mating patterns should be
classified as either monogamous or polygamous and the
appearance of the sexes as either similar or dimorphic.
Note that in a correlational study the scientist
looks for relationships between variables but does not
control or manipulate any variables. Therefore, the
variables are not classified as independent and
dependent. Keep in mind that correlations may be
positive or negative. In a positive correlation the
presence of, or increase in, one variable is associated
with an increased likelihood of the presence of, or
increase in, another variable. In a negative correlation
the presence of, or increase in, one variable is
associated with the absence of, or decrease in, another
variable.
Procedure
1. Use the Internet and or print sources to compile data
for a variety of bird species. Your data is unlikely to
result in equal numbers of monogamous and
polygamous species, but you should try to include at
least 10 species of each. The larger the number of bird
species you consider, the better.
2. Plot your data as histograms using a format similar
to Figure 1. The pairs of columns should represent
numbers of species that are dimorphic and have
similar-looking sexes.
[catch C08-F25-OB11USB; Size C2; New. Chart
showing Number of species on the Y axis and
Monogamous and Polygamous columns on the X axis.]
Analyze and Evaluate
(a) Were there any correlations between the
variables? If so, describe the correlation(s). [K/U]
(b) Were monogamous species more or less likely to be
dimorphic than polygamous species? [K/U]
(c) Were dimorphic species more or less likely to be
polygamous or monogamous? [K/U]
(d) Use your understanding of the theory of evolution
to offer a possible explanation for any correlations you
found. What do these results suggest about the
different selective pressures at work in bird species?
[T/I]
(e) How might the benefit of one trait have influenced
the selective advantage of another? [T/I]
Apply and Extend
(f) Male red capped manikins are brightly coloured and
perform a complex “moon-walk” mating dance, while
male and female penguins look very similar. Predict
whether or not these species are monogamous or
polygamous. Do online research to test your
predictions. [catch web icon] [T/I] [A]
(g) Male and female ruffed grouse look similar, except
during courtship when the males raise their neck
feathers and fan out their tails to create a visual display
(Figure 2). These grouse spend much of their time on
the ground. How might natural selection have favoured
this particular adaptation? [A]
[CATCH C08-P040-OB11USB; Size C2; Research.
Photo of a male roughed grouse with raised neck
feathers and a fanned out tail]
Figure 2 Male ruffed grouse
(h) Few other land vertebrate species have evolved
bright coloration of males. Suggest a possible
explanation for this difference between birds and most
other vertebrates. T/I] [A]
[CATCH WEB BANNER]
Investigation X.X.X 56
8.5.1
Observational Study
Looking for SINEs of Evolution
Whales have undergone very large physical changes in their recent
evolutionary past. This makes comparisons with other mammals
difficult and understanding their evolutionary kinships particularly
challenging. However, genetic sequences are now providing
biologists with powerful new tools for revealing evolutionary
relationships. One potentially ideal type of genetic sequence is
known as a SINE (short interspersed elements). In this investigation
you will use the presence of SINEs to construct a cladogram and infer
the evolutionary relationship of whales to some land mammals.
SKILLS MENU
Questioning
Researching
Hypothesizing
Predicting
Planning
Controlling Variables
Performing
Observing
Analyzing
Evaluating
Communicating
Investigation X.X.X 57
To use the presence of SINEs to investigate the
evolutionary kinship of whales with cows, giraffe,
hippopotamus, and pigs
2. Your cladogram should include the letters A, B, C,
and D at positions indicating when each of these SINE
insertion events may have taken place.
3. Also include the label MRCA to indicate the position
of the most recent common ancestor of whales on their
most closely related species.
Predictions:
Analyze and Evaluate
Purpose
Make the following two predictions:
• Which of these species do you suspect whales are
most closely related to?
• Which species to you think whales are least closely
related to?
Procedure
Background: SINEs are sequences of DNA (more than
100 bases long) that have become inserted into a
chromosome at random during a viral infection.
Because the DNA molecules in chromosomes contain
millions of bases, the chances of two identical SINEs
getting inserted into the same location in many
different individuals is extremely remote. If all African
elephants, for example, have the same SINE at the same
position on chromosome #9, we can conclude that all
African elephants inherited this SINE from a common
ancestor. By extension, if every Asian elephant also has
the identical SINE in the same position on their
chromosome #9, then we can assume that the SINE
was found in an individual that was an ancestor to both
Asian and African elephants.
(a) Were your predictions correct? To what species
are whales most closely related? To what species
(ignoring the outgroup) are they most distantly
related? [K/U]
(b) Which SINE insertion occurred first—A or B?
Explain your reasoning. [K/U]
(c) Are pigs more closely related to camels or whales?
How do you know? [K/U]
(d) What SINE(s) would you expect to find in all
giraffes? What SINE(s) would you never expect to find
in a giraffes? [K/U]
Apply and Extend
(e) Imagine a geneticist discovers a new matching
SINE in DNA samples from a hippo and a cow. Predict
the results of looking for this same SINE in DNA
samples from each of the other mammals. [T/I]
(f) A new SINE is discovered in the pygmy
hippopotamus but not in the closely related common
hippopotamus. Do you think it would be possible to
find this same SINE in whales? Explain your reasoning.
[T/I]
1. Use the information in Table 1 to construct a
cladogram showing the phylogenetic relationships
between these five species. Note: each of these SINEs
can be considered a recently derived trait.
Table 1
Species
cow
pig
fin whale
giraffe
hippopotamus
camel*
SINEs
A B
+ +
–
–
–
+
+ +
–
+
–
–
C
–
–
+
–
+
–
D
+
+
+
+
+
–
*The camel is the outgroup.
Investigation X.X.X 58
8.7.1
Observational Study
Human and Chimpanzee
Chromosome Comparison
Chimpanzees are thought to be the mostly closely related living
species to humans. Biologists believe our most recent common
ancestor lived between 5 and 7 million years ago. Genetic
information provides compelling evidence for this close kinship.
Humans have 22 sets of autosomal chromosomes and 2 sex
chromosomes, while chimpanzees have 23 sets of autosomal
chromosomes and 2 sex chromosomes. In this investigation you will
compare the banding patterns in human and chimpanzee
chromosomes, looking for evidence of a shared ancestry. You will
also examine detailed DNA evidence to account for a significant
difference between the genomes of these species.
SKILLS MENU
Questioning
Researching
Hypothesizing
Predicting
Planning
Controlling Variables
Performing
Observing
Analyzing
Evaluating
Communicating
Investigation X.X.X 59
Purpose
To compare human and chimpanzee chromosomes,
looking for evidence of shared ancestry
Equipment and Materials
•
•
•
•
•
•
complete set of chimpanzee karyograms
complete set of human karyograms
scissors and glue stick
telomere worksheet
human chromosome 2 DNA sequence (partial)
highlighter marker
Procedure
Part A: Comparing banding patterns
Background: Geneticists use specialized stains to
reveal distinctive and detailed banding patterns within
chromosomes. The sizes and locations of the bands are
carefully measured and displayed in diagrams called
karyograms (Figure 1). These banding patterns are
variable and can be used to compare species. All
members of the same species share virtually identical
chromosome banding patterns. Very closely related
species have similar banding patterns, while species
that are not closely related have no matching patterns.
[catch C08-F26-OB11USB; Size C2; New. Karyogram
showing detailed banding patterns]
Figure 1 Karyograms show detailed banding patterns
1. Obtain the handouts of the human and
chimpanzee karyograms. Use a pair of scissors to cut
out the chimpanzee chromosomes.
2. Take each chimpanzee chromosome and find its
human homologue—the human chromosome that most
closely matches the banding pattern.
3. Align and glue each chimpanzee chromosome next
to its human homologue.
4. Note that human chromosome #5 and its
chimpanzee homologue have, on either side of the
centromere, a matching but reversed banding pattern
(Figure 2). These matching but “flipped” patterns are
evidence of past inversion events. Carefully examine
each human/chimpanzee pair. Attempt to locate the
eight additional cases of chromosome inversions.
Highlight the portions that represent inverted
segments.
[catch C08-F27-OB11USB; Size C2; New. 2
chromosomes with identical banding patterns but also
with an inversion event that has flipped a portion of one
of the chromosomes]
Figure 2 These chromosomes had identical banding
patterns, but an inversion event has flipped a portion of
one of the chromosomes.
Part B: Evidence of a Fusion Event
Background: Chimpanzees have an extra autosomal
chromosome. As you may have discovered, two small
chimpanzee chromosomes are homologous with the
single large human chromosome #2. If humans and
chimpanzees share a recent common ancestor, then
some event must have caused this difference. Because
gorillas and orangutans also have 23 sets of autosomal
chromosomes, we can assume that the change took
place in the human lineage. The most likely explanation
is that a fusion event took place in which two smaller
chromosomes became attached “end to end” to form
the larger chromosome #2. If this did occur, it would
explain the matching banding patterns.
We can test this hypothesis by a more detailed
analysis of human chromosome #2. The ends of
chromosomes have special DNA regions called
telomeres. Telomeres are sections of repetitive DNA
base sequences that serve a number of functions. In all
primates the repetitive DNA sequences are
recognizable and usually found nowhere else. If two
telomeres joined, we would expect to find these
recognizable sequences at the location of a fusion site.
5. Obtain the telomere worksheet and the copy of
the partial DNA sequence from human chromosome
#2.
6. Read through the telomere worksheet and identify
the short DNA base repeat sequence that you would
expect to find at an end-to-end (telomere-to-telomere)
fusion site.
7. Search for this repeat sequence within the human
DNA sequence. Record the beginning and ending
positions of telomere sequences within chromosome
#2.
Analyze and Evaluate
(a) Were the banding patterns more or less similar
than you expected? What does the similarity suggest
about the kinship of humans and chimpanzees? [T/I]
(b) Compare the human and chimpanzee
chromosome banding patterns. Estimate the percent
similarity between the two species. Consider
inversions as “matches,” as long as the same banding
patterns occur within the inverted segments. [T/I]
(c) Assume that chimpanzees and humans share a
recent common ancestor that lived 7 million years ago.
If each inversion represents a single mutation event,
Investigation X.X.X 60
calculate the average number of years between these
mutation events. Would you describe this mutation
rate as rapid or slow? [T/I]
(d) One function of telomeres is to protect the ends of
the chromosomes in a way that is analogous to the
front and back covers of a book. Use this analogy to
describe the kind of evidence one would expect to find
if two books became joined together by a fusion event.
[T/I] [A]
Apply and Extend
(e) When two chromosomes fuse, the new
chromosome will have a second centromere. Over time
one of these centromeres becomes non-functional.
Label the location where you would predict to find this
non-functional centromere on human chromosome
#2—assuming it resulted from a fusion event. [T/I]
(f) Geneticists are able to recognize non-functional
centromeres by their DNA sequence. Do online
research to find out if such a sequence has been located
in human chromosomes #2. [catch web link icon] [T/I]
(g) Do online research and find out if it is possible for
individuals with fused chromosomes to breed
successfully with individuals with separate
chromosomes. Report on your findings. [catch web link
icon] [T/I] [C]
[catch web banner]
Investigation X.X.X 61
[new page]
[Formatter: format this page as per Design]
Chapter 8 Summary
Summary Questions
1. Create a study guide based on the Key Concepts listed at the beginning of the chapter, on page XXX. For each point,
create three or four subpoints that provide further information, relevant examples, explanatory diagrams, or general
equations.
2. Return to the Starting Points questions at the beginning of the chapter, on page XXX. Answer these questions using
what you have learned in this chapter. Compare your answers with those that you gave at the beginning of the chapter.
How has your understanding changed? What new knowledge and skills do you have?
Career Pathways
Grade 11 Biology can lead to a wide range of careers. Some require a college diploma or a B.Sc. degree. Others require
specialized or postgraduate degrees. This graphic organizer shows a few pathways to careers mentioned in this
chapter.
1. Select two careers related to Evolution that you find interesting. Research the educational pathways that you would
need to follow to pursue these careers. What is involved in the required educational programs? Prepare a brief report
of your findings.
2. For one of the two careers that you chose above, describe the career, main duties and responsibilities, working
conditions, and setting. Also outline how the career benefits society and the environment.
[CATCH C08-F32-OB11USB; Size A; MPU. MPU C04-F21-OB11USB.]
[Format Vocab in columns, as per Design]
Vocabulary
directional selection (p. xxx)
stabilizing selection (p. xxx)
disruptive selection (p. xxx)
sexual selection (p. xxx)
genetic drift (p. xxx)
genetic bottleneck (p. xxx)
founder effect (p. xxx)
Hardy–Weinberg principle (p. xxx)
microevolution (p. xxx)
speciation (p. xxx)
reproductive isolating mechanism (p. xxx)
prezygotic mechanism (p. xxx)
postzygotic mechanism (p. xxx)
allopatric speciation (p. xxx)
sympatric speciation (p. xxx)
adaptive radiation (p. xxx)
divergent evolution (p. xxx)
convergent evolution (p. xxx)
coevolution (p. xxx)
macroevolution (p. xxx)
abiogenesis (p. xxx)
Cambrian explosion (p. xxx)
cladistics (p. xxx)
Investigation X.X.X 62
derived trait (p. xxx)
synapomorphy (p. xxx)
theory of gradualism (p. xxx)
theory of punctuated equilibrium (p. xxx)
transitional form (p. xxx)
primate (p. xxx)
prosimian (p. xxx)
anthropoid (p. xxx)
hominid (p. xxx)
[end page]
Investigation X.X.X 63
[new page]
Chapter 8 Self-Quiz
[QUESTIONS TO COME]
[end page]
[new page – 6 pages begin]
Chapter 8 Review
[QUESTIONS TO COME]
[end 6 pages]
Investigation X.X.X 64
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Unit 3
Unit Task
[TO COME]
[end 2 page spread]
Investigation X.X.X 65
[new page – 2pp spread begins]
Unit 3
Self-Quiz
[TO COME]
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Investigation X.X.X 66
[new page – 8pp total]
Unit 3
Review
[TO COME]
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Investigation X.X.X 67