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Evolution and Diversity
Evolution of Life
VI
part
27
All living things are descended from the first cell(s)
and are adapted to their environment. 549
Microbiology
28
Some viruses, bacteria, protists, and fungi cause human
diseases, but most bacteria, protists, and fungi are free-living and
perform environmental services. 575
Plants
29
Plants are photosynthetic organisms adapted to live on land. Reproduction in
seed plants does not require a watery medium. 607
Animals: Part I
30
Animals are heterotrophic organisms that must take in organic food. Animals
evolved in the water, and only certain forms live on land. 625
Animals: Part II
31
The more complex animals are divided into two major groups according to the
way they develop. Arthropods in one group and vertebrates in the other have
jointed skeletons suitable for locomotion on land. 649
548
mad86751_ch27_548-574.indd 548
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We tend to think of evolution
as happening over long
timescales. However, we have recently
27
chapter
learned that human activities can
accelerate the process of evolution quite
rapidly—even over a few years! For
example, farmers are continuously
Evolution of Life
challenged by the fact that insects
evolve pesticide resistance (see the
Health Focus at the end of this
chapter). That is, when new pesticides
are sprayed, a certain (usually small)
proportion of insect populations are
resistant to the pesticide. These
individuals survive and reproduce,
while those that are susceptible die.
Thus, over time, the proportion of
resistant individuals increases to the
point that the pesticide is no longer
effective and crop damage
increases. Then, new pesticides
have to be used or developed.
C h a p t e r
C o n c e p t s
27.1 Origin of Life
■ What type of evolution preceded biological evolution? Explain. 550–551
■ What conditions were needed for a true cell to come into being? 551–552
27.2 Evidence of Evolution
■ What types of evidence show that common descent with modification
has occurred? 552–558
27.3 The Process of Evolution
■ What type of change occurs when a population evolves? Do individual
organisms evolve? 559–562
■ What five agents lead to evolutionary changes? 562–566
■ Which agent allows populations to become better adapted to their
environment? 564
27.4 Speciation
Similar problems have arisen with
■ How does speciation occur? 566–569
antibiotics; much in the same way,
■ What is meant by the phrase “the pace of speciation”? 567
bacteria evolve resistance to
antibiotics, making some
ineffective for treating
certain infections.
The good news is that our
understanding of evolutionary
27.5 Classification
■ What are the basic categories of classification? 569
■ What does evolution have to do with classification of organisms? 569
■ What are the five kingdoms and the three domains? What kingdoms are in
Domain Eukarya? 569–570
■ How do evolutionary trees of traditionalists differ from cladograms done
by phylogeneticists? 571
biology has helped change human
behavior to deal with the evolution
of resistance in pests and bacteria.
For example, many farmers now
use “integrated pest management,”
which involves rotating pesticide
types as well as using natural
enemies (predators) to fight crop
pests. Similarly, doctors no longer
prescribe antibiotics unless they are
Colorado potato beetles,
Leptinotarsa decimlineata
relatively certain a patient has a
bacterial infection, and they tend to
prescribe the minimum strength
antibiotic to do the job. These are
examples of why evolution is important
in people’s everyday lives.
In this chapter, you will learn about
evidence that indicates evolution has
occurred and about the way the
evolutionary process works.
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550
27.1
Part Six Evolution and Diversity
Origin of Life
In order to understand evolution, it is first important to
understand how life began. The common ancestor for all living things was the first cell or cells. The planet Earth was in
existence a long time before the first cell arose—over a billion
years, in fact. Earth is 4.6 billion years old, and the earliest
fossils of prokaryotes are 3.5 billion years old. A billion years
is about 13 million human life spans, assuming humans live
75 years. The origin of the first cell is an event of low probability, because a complex series of events would have had
to occur—but this length of time is long enough for an event
of low probability to have occurred.
Today we do not believe that life arises spontaneously from nonlife, and we say that “life comes only from
life.” However, the very first living thing had to have come
from nonliving chemicals. Under the conditions of early
Earth, it is possible that a chemical reaction produced the
Biological Evolution
photosynthesis
cellular respiration
cell
DNA
RNA
origin of
genetic code
first cell(s). A particular mix of inorganic chemicals could
have reacted to produce small organic molecules such as
glucose, amino acids, and nucleotides. Then these would
have polymerized into macromolecules. Once a plasma
membrane formed, a structure called a protocell could
have come into existence (Fig. 27.1).
Evolution of Small Organic Molecules
Most chemical reactions take place in water, and the first
protocell undoubtedly arose in the ocean. But where? One
possibility is that the protocell formed on the surface of the
seas and in seaside pools where much energy was available.
Ultraviolet radiation was intense in these areas because
there was no ozone shield, the layer of O3 that today blocks
much of the ultraviolet radiation coming from the sun.
Oxygen molecules later reacted with one another to produce the ozone shield.
In 1953, Stanley Miller and Harold Urey performed
an experiment (known as the Miller-Urey experiment) that
supports the hypothesis that small organic molecules were
formed at the ocean’s surface. In the early Earth, volcanoes
erupted constantly, and the first atmospheric gases would
have consequently contained methane (CH 4 ), ammonia
(NH3), and hydrogen (H2). These gases could then have been
washed into the ocean by the first rains. Fierce lightning and
unabated ultraviolet radiation would have allowed them to
react and produce the first organic molecules.
protocell
electrode
aggregation
macromolecules
plasma membrane
Chemical Evolution
polymerization
small organic molecules
energy
capture
stopcock for
adding gases
stopcock for
withdrawing liquid
abiotic
synthesis
electric
spark
CH4
NH3
H2
H2O
condenser
gases
hot water out
cool water in
liquid droplets
inorganic chemicals
boiler
cooling
early Earth
heat
small organic molecules
Figure 27.1 Origin of the first cell(s).
Figure 27.2 Miller and Urey’s apparatus and
A chemical evolution may have produced the first cell. First, inorganic
chemicals reacted to produce small organic molecules, which polymerized
to form macromolecules. With the origination of the plasma membrane,
the first primitive cell (a protocell) evolved, and once this cell could
replicate, life began.
Gases that were thought to be present in the early Earth’s atmosphere were
admitted to the apparatus, circulated past an energy source (electric spark),
and cooled to produce a liquid that could be withdrawn. Upon chemical
analysis, the liquid was found to contain various small organic molecules.
mad86751_ch27_548-574.indd 550
experiment.
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551
Chapter Twenty–Seven Evolution of Life
Macromolecules
plume of hot water
rich in iron-nickel sulfides
hydrothermal
vent
Figure 27.3 Chemical evolution at
hydrothermal vents.
Minerals that form at deep-sea hydrothermal vents like this one can
catalyze the formation of ammonia and even organic molecules.
To test the hypothesis of chemical evolution, Miller
placed the inorganic chemicals mentioned in a closed system, heated the mixture, and circulated it past an electric
spark (simulating lightning). After a week, the solution contained a variety of amino acids and organic acids (Fig. 27.2).
This and other similar experiments support the hypothesis that inorganic chemicals in the absence of oxygen (O2)
and in the presence of a strong energy source can result in
organic molecules.
Is there any other place in the oceans where life could
have evolved? Scientists have discovered mid-oceanic
ridges within the depths of the sea. Hydrothermal (hot
water) vents (openings) occur in the region of mid-oceanic
ridges. A vent can be huge, measuring 10–15 meters wide
with sides about 15–20 meters high. Hot water spewing
out of these vents contains a mix of iron-nickel sulfides.
Amazingly, scientists have discovered communities of
organisms, including tube worms and giant clams, living
in the regions of hydrothermal vents. It is possible that in
the past, the right combination of conditions occurred at
these vents to initiate life (Fig. 27.3).
The formation of small organic molecules is thought to be
the first step toward the origination of a cell, from which
other life-forms evolved.
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Once formed, the first small organic molecules gave rise
to still larger molecules and then macromolecules. There
are three primary hypotheses concerning this stage in the
origin of life. One is the RNA-first hypothesis, which suggests that only the macromolecule RNA (ribonucleic acid)
was needed at this time to progress toward formation of
the first cell or cells. Scientists formulated this hypothesis
after discovering that RNA can sometimes be both a genetic
substrate and an enzyme. Such RNA molecules are called
ribozymes. The first genes and enzymes could thus both
have been composed of RNA, since we now know that ribozymes exist. Scientists who support this hypothesis say it
was an “RNA world” some 4 billion years ago.
Another hypothesis is termed the protein-first
hypothesis. Sidney Fox has shown that amino acids polymerize abiotically (without life) when exposed to dry heat.
He suggests that amino acids collected in shallow puddles along the rocky shore, and the heat of the sun caused
them to form proteinoids, small polypeptides that have
some catalytic properties. When proteinoids are returned
to water, they form microspheres, structures composed
only of protein that have many of the properties of a cell.
Some of these proteins could have had enzymatic properties. This hypothesis assumes that DNA genes came after
protein enzymes arose.
The third hypothesis is put forth by Graham CairnsSmith. He believes that clay was especially helpful in causing the polymerization of both proteins and nucleic acids
at the same time. Clay attracts small organic molecules and
contains iron and zinc, which may have served as inorganic
catalysts for polypeptide formation. In addition, clay tends
to collect energy from radioactive decay and then discharge
it when the temperature or humidity changes, possibly providing a source of energy for polymerization. Cairns-Smith
suggests that RNA nucleotides and amino acids became
associated in such a way that polypeptides were ordered
by, and helped synthesize, RNA.
Chemical reactions likely produced the macromolecules
we associate with living things.
The Protocell
After macromolecules formed, something akin to a modern
plasma membrane was needed to separate them from the
environment. Thus, before the first true cell arose, there
would likely have been a protocell, a structure that had
a lipid-protein membrane and carried on energy metabolism. Fox has shown that if lipids are made available to
microspheres, the two tend to become associated, producing a lipid-protein membrane (Fig. 27.4a).
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Part Six Evolution and Diversity
Figure 27.4 Protocell components.
a. Microspheres, which are composed only of protein,
have a number of cellular characteristics and could
have evolved into the protocell. b. Liposomes form
automatically when phospholipid molecules are put into
water. Plasma membranes may have evolved similarly.
a.
Aleksandr Oparin, a Soviet biochemist, showed that under
appropriate conditions of temperature, ionic composition, and
pH, concentrated mixtures of macromolecules can give rise to
complex units called coacervate droplets. Coacervate droplets
have a tendency to absorb and incorporate various substances
from the surrounding solution. Eventually, a semipermeable
boundary may form around the droplet. In a liquid environment, phospholipid molecules automatically form droplets
called liposomes (Fig. 27.4b). Perhaps the first plasma-like membrane formed in this manner. However it happened, development of the plasma membrane was key because it separated the
genetic material from the outside environment.
The Heterotroph Hypothesis
It has been suggested that the protocell likely was a heterotroph, an organism that takes in preformed food. During
the early evolution of life, the ocean contained abundant
nutrition in the form of small organic molecules. This suggests that heterotrophs preceded autotrophs, organisms
that make their own food.
At first, the protocell may have used preformed ATP,
but as this supply dwindled, cells that could extract energy
from carbohydrates to transform ADP to ATP were favored.
Glycolysis is a common metabolic pathway in living things,
and this testifies to its early evolution in the history of life.
Since there was no free oxygen, we can assume that the
protocell carried on a form of fermentation. It seems logical
that the protocell at first had limited ability to break down
organic molecules and that it took millions of years for glycolysis to evolve completely.
b.
called reverse transcriptase that uses RNA as a template to
form DNA, which then undergoes protein formation. Perhaps,
with time, reverse transcription gave rise to DNA genes. Once
DNA genes existed, they may have specified proteins.
According to the protein-first hypothesis, proteins,
or at least polypeptides, were the first of the three molecules (i.e., DNA, RNA, and protein) to arise. Only after the
protocell developed sophisticated enzymes did it have the
ability to synthesize DNA and RNA from small molecules
provided by the ocean. Researchers point out that because
nucleic acids are very complicated molecules, the likelihood
that RNA arose on its own is minimal.
Cairns-Smith proposes that polypeptides and RNA
evolved simultaneously. Therefore, the first true cell would
have contained RNA genes that could have replicated
because of the presence of proteins. This eliminates the baffling chicken-and-egg paradox: which came first, proteins
or RNA? It does mean, however, that two unlikely events
would have had to happen at the same time.
Once the protocells acquired genes that could replicate, they became cells capable of reproducing, and biological evolution began.
The hypothesis that the origin of life followed a transition from small organic molecules to macromolecules to
protocells to true cells is currently widely favored by scientists. However, recently some scientists have argued that
early cells may have come from asteroids from Mars that
hit the Earth. Recent expeditions to Mars have shown that
water possibly existed there in the past. Nonetheless, it is
thought that cells, the basic building block of life, arose at
some point from nonliving matter.
The True Cell
A true cell is a membrane-bounded structure that can carry
on protein synthesis to produce the enzymes that allow
DNA to replicate. The central concept of genetics states
that DNA directs protein synthesis and that information
flows from DNA to RNA to protein. It is possible that this
sequence developed in stages.
According to the RNA-first hypothesis, RNA would
have been the first genetic material to evolve, and the first
true cell would have had RNA genes. These genes would have
directed and enzymatically carried out protein synthesis, as in
ribozymes. Also, today we know that some viruses have RNA
as their genetic material. These viruses have a protein enzyme
mad86751_ch27_548-574.indd 552
Once the protocell was capable of reproduction, it became
a true cell, and biological evolution began.
27.2
Evidence of Evolution
Evolution is all the changes that have occurred in living
things since the beginning of life due to differential reproductive success. That is, some individuals reproduce more
than others because they are better “fit” to their environment. Table 27.1 indicates that Earth is about 4.6 billion
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553
Chapter Twenty–Seven Evolution of Life
years old and that prokaryotes, probably the first living
organisms, evolved about 3.5 billion years ago. The eukaryotic cell arose about 2.1 billion years ago, but multicellularity didn’t begin until perhaps 700 million years ago. This
means that only unicellular organisms were present for 80%
of the time that life has existed on Earth. Most evolutionary
events we will be discussing in future chapters occurred in
less than 20% of the history of life!
Evolution is defined as “common descent.” Because of
descent with modification, all living things share the same
fundamental characteristics: they are made of cells, take
chemicals and energy from the environment, respond to
external stimuli, and reproduce. Living things are diverse
because individual organisms exist in the many environments throughout the Earth, and the features that enable
them to survive in those environments are quite diverse.
Many fields of biology provide evidence that evolution through descent with modification occurred in the
past and is still occurring. Let us look at the various types
of evidence for evolution.
wing
feathers
feet
a.
Figure 27.5 Transitional fossils.
teeth
tail with vertebrae
a. Archaeopteryx was a transitional link between reptiles and birds. Fossils
indicate it had feathers and wing claws. Most likely, it was a poor flier. Perhaps
it ran over the ground on strong legs and climbed into trees with the assistance
of these claws. b. Archaeopteryx also had a feather-covered, reptilian tail
that shows up well in this artist’s representation. (Orange labels=reptilian
characteristics; green label=bird characteristic.)
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Fossils are the remains and traces of past life or any other
direct evidence of past life. Most fossils consist only of hard
parts of organisms, such as shells, bones, or teeth, because
these are usually preserved after death. The soft parts of a
dead organism are often consumed by scavengers or decomposed by bacteria. Occasionally, however, an organism is
buried quickly and in such a way that decomposition is never
completed or is completed so slowly that the soft parts leave
an imprint of their structure. Traces include trails, footprints,
burrows, worm casts, or even preserved droppings.
The great majority of fossils are found embedded in
sedimentary rock. Sedimentation, a process that has been
going on since Earth formed, can take place on land or in
bodies of water. The weathering and erosion of rocks produces particles that vary in size and are called sediment.
As such particles accumulate, sediment becomes a stratum
(pl., strata), a recognizable layer of rock. Any given stratum
is older than the one above it and younger than the one
immediately below it, so that the relative age of fossils can
be determined based on their depth.
Paleontologists are biologists who study the fossil
record and from it draw conclusions about the history of life.
Particularly interesting are the fossils that serve as transitional links between groups. For example, the famous fossils
of Archaeopteryx are intermediate between reptiles and birds
(Fig. 27.5). The dinosaur-like skeleton of this fossil has reptilian
features, including jaws with teeth and a long, jointed tail, but
Archaeopteryx also had feathers and wings, all suggesting
that reptiles evolved from birds. Other transitional
links among fossil vertebrates suggest that fishes
evolved before amphibians, which evolved before
reptiles, which evolved before both birds and
mammals in the history of life.
wing
head
tail
Fossil Evidence
claws
b.
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Part Six Evolution and Diversity
TABLE 27.1
Era
The Geological Timescale: Major Divisions of Geological Time and Some of the Major Evolutionary
Events of Each Time Period
Period
Epoch
Millions of
Years Ago
Plant Life
Animal Life
Holocene
0.01
Human influence on plant life
Age of Homo sapiens
Significant Mammalian Extinction
Quaternary
Cenozoic*
Tertiary
Pleistocene
Herbaceous plants spread and
diversify.
Presence of ice age mammals.
Modern humans appear.
First hominids appear.
Pliocene
(5–1.8)
Herbaceous angiosperms flourish.
Miocene
(23–25)
Grasslands spread as forests
contract.
Oligocene
(36–23)
Many modern families of flowering
plants evolve.
Browsing mammals and
monkeylike primates appear.
Eocene
(57–36)
Subtropical forests with heavy rainfall thrive.
All modern orders of
mammals are represented.
Paleocene
(65–57)
Flowering plants
continue to diversify.
Primitive primates, herbivores,
carnivores, and insectivores appear.
Apelike mammals and grazing
mammals flourish; insects flourish.
Mass Extinction: Dinosaurs and Most Reptiles
Mesozoic
Cretaceous
(144–65)
Flowering plants spread; conifers
persist.
Placental mammals appear; modern
insect groups appear.
Jurassic
(231–144)
Flowering plants appear.
Dinosaurs flourish;
birds appear.
Mass Extinction
Triassic
(248–118)
Forests of conifers
and cycads dominate.
First mammals appear; first dinosaurs
appear; corals and molluscs dominate seas.
Mass Extinction
Permian
(280–248)
Gymnosperms diversify.
Reptiles diversify; amphibians
decline.
Carboniferous
(360–280)
Age of great coal-forming forests:
ferns, club mosses, and horsetails
flourish.
Amphibians diversify;
first reptiles appear; first great
radiation of insects.
Mass Extinction
Paleozoic
Devonian
(408–360)
First seed plants appear. Seedless
vascular plants diversify.
Jawed fishes diversify and
dominate the seas; first insects and
first amphibians appear.
Silurian
(438–408)
Seedless vascular plants appear.
First jawed fishes appear.
Mass Extinction
Precambrian Time
Ordovician
488.3
Nonvascular land plants appear.
Marine algae flourish.
Invertebrates spread and diversify;
jawless fishes (first
vertebrates) appear.
Cambrian
542
First plants appear on land. Marine
algae flourish.
All invertebrate phyla present;
first chordates appear.
600
Oldest soft-bodied invertebrate fossils
1,400–700
Protists evolve and diversify.
2,200
Oldest eukaryotic fossils
2,700
O2 accumulates in atmosphere.
3,500
Oldest known fossils (prokaryotes)
4,600
Earth forms.
*
Many authorities divide the Cenozoic era into the Paleogene period (contains the Paleocene, Eocene, and Oligocene epochs) and the Neogene period (contains the Miocene, Pliocene, Pleistocene, and Holocene epochs).
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Figure 27.6 Dinosaurs of the late Cretaceous period.
Parasaurolophus walkeri, although not as large as other dinosaurs, was one of the largest plant-eaters of the late Cretaceous period. The crest atop its head was about 2
meters long and was used to make booming calls. Also living at this time were the rhino-like dinosaurs represented here by Triceratops (left), another herbivore.
Geological Timescale
As a result of studying strata, scientists have divided Earth’s
history into eras, and then periods and epochs (Table 27.1).
The fossil record has helped determine the dates given in the
table. There are two ways to date fossils. The relative dating
method determines the relative order of fossils and strata
depending on the layer of rock in which they were found, but
it does not determine the actual date they were formed.
The absolute dating method relies on radioactive dating
techniques to assign an actual date to a fossil. All radioactive
isotopes have a particular half-life, the length of time it takes
for half of the radioactive isotope to change into another stable element. Carbon 14 (14C) is the only radioactive isotope in
organic matter. Assuming a fossil contains organic matter, half
of the 14C will have changed to nitrogen 14 (14N) in 5,730 years.
To estimate how much 14C was in the organism to begin with,
it is assumed that organic matter always begins with the same
amount of 14C. Scientists compare the 14C radioactivity of the
fossil to that of a modern sample of organic matter. For example, if a fossil has one-fourth the amount of radioactive 14C as
a modern sample, then the fossil is approximately 11,460 years
old (2 half-lives). After 50,000 years, however, the amount of
14
C radioactivity is so low that it cannot be used to measure the
age of a fossil accurately. In that event, certain other ratios of
isotopes with longer half-lives can be used to date rocks even
billions of years old, and then the age of a fossil contained in
the rock can be inferred in a similar way. Using both relative
and absolute dating methods, we can learn from fossils about
the various organisms and environments that existed across
the planet during any time period.
Mass Extinctions
Extinction is the death of every member of a species. During
mass extinctions, a large percentage of species become
extinct within a relatively short period of time. So far, there
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have been five major mass extinctions. These occurred at
the ends of the Ordovician, Devonian, Permian, Triassic,
and Cretaceous periods (see Table 27.1), and a sixth is likely
occurring now, probably as a result of human activities
(discussed in Chapter 36). Following mass extinctions, the
remaining groups of organisms are likely to spread out and
fill the habitats vacated by those that have become extinct.
It was proposed in 1977 that the Cretaceous extinction
(or “Cretaceous crisis”) was due to an asteroid that exploded,
producing meteorites that fell to Earth. A large meteorite
striking Earth could have produced a cloud of dust that
mushroomed into the atmosphere, blocking out the sun and
causing plants to freeze and die. A huge crater that could have
been caused by a meteorite involved in the Cretaceous extinction was found in the Caribbean–Gulf of Mexico region on
the Yucatán Peninsula. During the Cretaceous period, great
herds of dinosaurs roamed the plains, as did Parasaurolophus
walkeri and Triceratops (Fig. 27.6), but all dinosaur species
went extinct near the end of the Cretaceous period.
In 1984, paleontologists found that marine animals
have a mass extinction about every 26 million years, and
surprisingly, astronomers can offer an explanation. Our sun
moves up and down as it orbits in the Milky Way, a starry
galaxy. Astronomers predict that when this vertical movement causes our solar system to approach certain other
members of the Milky Way, an unstable situation develops
that could lead to a meteorite striking Earth. This evidence
suggests that mass extinctions can be associated with extraterrestrial events, but these events are not necessarily the
only cause of mass extinctions.
Fossils allowed scientists to construct the geological timescale
that traces the history of life. Several mass extinctions have
occurred in the past, possibly due to extraterrestrial events.
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Part Six Evolution and Diversity
Laurasia
Laurasia
a
e
ga
n
Pa
Go
nd
Go
n
wa
n
a
Permian period
~250 million years ago
Figure 27.7 Continental drift.
During the Permian period, all the continents
were joined into a supercontinent called
Pangaea. During the Triassic period, the joined
continents of Pangaea began moving apart,
forming two large continents called Laurasia
and Gondwana. Then all the continents began
to separate. This process is continuing today.
North America and Europe are presently
drifting apart at a rate of about 2 cm per year.
Eurasia
North
America
Eurasia
Africa
South
America
Africa
India
India
South
America
Australia
Australia
Antarctica
Cretaceous period
65 million years ago
Biogeographical Evidence
Another type of evidence that supports evolution through
descent with modification is found in the field of biogeography, the study of the distribution of species throughout the
world. The world’s six biogeographical regions each have
their own distinctive mix of living things. For example, the
mammals and flowering plants of North America are different from those in Africa, even though parts of the two
continents have similar environmental conditions. If you
want to see zebras and lions, you have to go to Africa, not
to the midwestern United States. Similarly, cactuses flourish in the deserts of North America, but euphorbias, not
cactuses, occupy similar arid habitats of Africa. What is the
best explanation for this phenomenon? Different mammals
and flowering plants evolved separately in each biogeographical region, and barriers such as mountain ranges and
oceans prevented them from migrating to other regions.
Many of these barriers arose through a process called
continental drift. That is, the continents have never been
fixed; rather, their positions and the positions of the oceans
have changed over time (Fig. 27.7). During the Permian
period, all the present landmasses belonged to one continent
and then later drifted apart. As evidence of this, fossils of
one species of seed fern (Glossopteris) have been found on all
the southern continents separated by oceans. This species’
presence on Antarctica is evidence that this continent was
not always frozen. In contrast, many Australian species are
restricted to that continent, including the majority of marsupials (pouched mammals such as the kangaroo). What is the
mad86751_ch27_548-574.indd 556
Jurassic period
144 million years ago
Triassic period
~220 million years ago
North
America
dw
an
a
Antarctica
Present day
explanation for these distributions? Some organisms must
have evolved and spread out before the continents broke
up; then they became extinct.
The distribution of many organisms on Earth is
explainable by knowing when they evolved, either
before or after the continents moved apart.
Anatomical Evidence
The fact that anatomical similarities exist among organisms
provides further support for evolution via descent with modification. Vertebrate forelimbs are used for flight (birds and
bats), orientation during swimming (whales and seals), running (horses), climbing (arboreal lizards), or swinging from
tree branches (monkeys). Yet all vertebrate forelimbs contain
the same sets of bones organized in similar ways, despite
their dissimilar functions. The most plausible explanation
for this unity is that the basic forelimb plan belonged to a
common ancestor, and then the plan was modified in the succeeding groups as each continued along its own evolutionary
pathway. Structures that are anatomically similar because
they are inherited from a common ancestor are called homologous structures (Fig. 27.8). In contrast, analogous structures
serve the same function, but are not constructed similarly,
nor do they share a common ancestry. The wings of birds
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Chapter Twenty–Seven Evolution of Life
and insects and the eyes of octopi and humans are analogous
structures and are similar due to a common environment, not
common ancestry. The presence of homology, not analogy, is
evidence that organisms are related.
Vestigial structures are anatomical features that
are fully developed in one group of organisms but that
are reduced and may have no function in similar groups.
Most birds, for example, have well-developed wings for
flight. However, some bird species (e.g., ostrich) have
greatly reduced wings and do not fly. Similarly, snakes
have no use for hindlimbs, and yet some have remnants
of hindlimbs in a pelvic girdle and legs. The presence of
vestigial structures can be explained by common descent.
Vestigial structures occur because organisms inherit their
anatomy from their ancestors; they are traces of an organism’s evolutionary history.
The homology shared by vertebrates extends to their
embryological development (Fig. 27.9). At some time during
development, all vertebrates have a postanal tail and exhibit
paired pharyngeal pouches. In fishes and amphibian larvae,
these pouches develop into functioning gills. In humans, the
first pair of pouches becomes the cavity of the middle ear and
the auditory tube. The second pair becomes the tonsils, while
the third and fourth pairs become the thymus and parathyroid glands. Why should terrestrial vertebrates develop and
then modify structures like pharyngeal pouches that have
lost their original function? The most likely explanation is
that fishes are ancestral to other vertebrate groups.
bird
humerous
ulna
radius
metacarpals
phalanges
557
In 1859, Charles Darwin (See Science Focus, p. 560)
speculated that whales evolved from a land mammal. His
hypothesis has now been substantiated. In recent years the
fossil record has yielded an incredible parade of fossils that
link modern whales and dolphins to land ancestors (Fig.
27.10). The presence of a vestigial pelvic girdle and legs in
modern whales is also significant evidence.
Organisms that share homologous structures are
closely related and have a common ancestry. Studies of
comparative anatomy and embryological development
reveal homologous structures.
fish
salamander
tortoise
bat
chick
whale
cat
horse
human
pharyngeal
pouches
human
postanal
tail
Figure 27.8 Significance of homologous structures.
Figure 27.9 Significance of developmental similarities.
Although the specific design details of vertebrate forelimbs are different, the
same bones are present (note color-coding). Homologous structures provide
evidence of a common ancestor.
At these comparable developmental stages, vertebrate embryos have many
features in common, which suggests they evolved from a common ancestor.
(These embryos are not drawn to scale).
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Part Six Evolution and Diversity
Figure 27.10 Ancestor to whales.
Ambulocetus, an ancestor to modern whales, dated 50 MYA. The presence of
limbs is evidence that land-based mammals gave rise to whales.
Biochemical Evidence
Almost all living organisms use the same basic biochemical
molecules, including DNA, ATP (adenosine triphosphate),
and many identical or nearly identical enzymes. Further,
organisms use the same DNA triplet code for the same 20
amino acids in their proteins. Since the sequences of DNA
bases in the genomes of many organisms are now known,
it has become clear that humans share a large number of
genes with much simpler organisms. It appears that life’s
vast diversity has come about by only a slight difference in
many of the same genes. The result has been widely divergent types of bodies.
yeast
Number of Amino Acid Differences
Compared to Human Cytochrome c
0
moth
fish
turtle
When the degree of similarity in DNA nucleotide
sequences or the degree of similarity in amino acid
sequences of proteins is examined, the more similar the
DNA sequences are, generally the more closely related
the organisms are. For example, humans and chimpanzees are about 99% similar! Cytochrome c is a molecule that
is used in the electron transport chain of all the organisms
appearing in Figure 27.11. Data regarding differences in the
amino acid sequence of cytochrome c show that the sequence
in a human differs from that in a monkey by only one amino
acid, from that in a duck by 11 amino acids, and from that in a
yeast by 51 amino acids. These data are consistent with other
data regarding the anatomical similarities of these organisms
and, therefore, how closely they are related.
Evolution is no longer considered a hypothesis. It is
one of the great unifying theories of biology. In science,
the word theory is reserved for those conceptual schemes
that are supported by a large number of observations and
scientific experiments. The theory of evolution has the
same status in biology that the germ theory of disease has
in medicine.
Many lines of evidence support the theory of evolution by
descent with modification. Recently, biochemical evidence
has also been found to support evolution. A hypothesis is
strengthened when it is supported by many different lines
of evidence.
duck
pig
monkey
human
10
20
30
40
Cytochrome c is a small protein
that plays an important role
in the electron transport chain
within mitochondria of all cells.
50
Figure 27.11 Significance of biochemical differences.
The branch points in this diagram indicate the number of amino acids that differ between human cytochrome c and the organisms depicted. These
biochemical data are consistent with those provided by a study of the fossil record and comparative anatomy.
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Chapter Twenty–Seven Evolution of Life
27.3
The Process of Evolution
Some people have the misconception that individuals
evolve; however, evolution occurs at the population level.
As evolution takes place, genetic changes occur within a
population, and over generations, these lead to phenotypic
changes that are commonly seen in that population. In
this section we will consider a change in gene frequencies
within a population over time, defined as microevolution.
Microevolution can be studied using population genetics,
which investigates changes in gene frequencies.
Population Genetics
A population is all the members of a single species that
occupy a particular area at the same time and that interbreed
and exchange genes. A population could be all the green
frogs in a frog pond, all the field mice on a farm, or all the
English daisies on a hill. The members of a population reproduce with one another to produce the next generation.
Each member of a population is assumed to be free
to reproduce with any other member, and when reproduc-
p 2 +2 pq+q2
p 2=frequency of homozygous dominant individuals (AA)
p=frequency of dominant allele (A)
q 2=frequency of homozygous recessive individuals (aa)
q=frequency of recessive allele (a)
2 pq=frequency of heterozygous individuals (Aa)
Realize that
p+q=1 (There are only 2 alleles.)
p 2 +2 pq+q 2 =1 (These are the only genotypes so the
total frequency of the genotypes in a
population must add up to 1, or 100%.)
Example: An investigator has determined by inspection that 16%
of a human population has a recessive trait. Using this
information, we can complete all the genotype and allele
frequencies for this population.
q 2=16%=0.16 are homozygous recessive individuals
Given:
q= 0.16=0.4=frequency of recessive allele
p=1.0-0.4=0.6=frequency of dominant allele
p 2=(0.6)(0.6)=0.36=36% are homozygous
dominant individuals
2 pq=2(0.6)(0.4)=0.48=48% are heterozygous
individuals
or
2 pq=1.00-0.52=0.48
84% have
the dominant
phenotype
Therefore,
Figure 27.12 Calculating gene pool frequencies
using the Hardy-Weinberg equation.
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559
tion occurs, the genes of one generation are passed on in
the manner described by Mendel’s laws. Therefore, in this
so-called Mendelian population (as discussed in Ch. 23)
of sexually reproducing individuals, the total number of
alleles at all the gene loci in all the members make up a gene
pool for the population. It is customary to describe this gene
pool in terms of allele frequencies for the various genes.
Using this methodology, two investigators, G. H. Hardy, an
English mathematician, and W. Weinberg, a German physician, discovered a principle that now bears their names.
Hardy and Weinberg decided to use the binomial
equation p2+2pq+q2 to calculate the genotype and allele
frequencies of a population. Figure 27.12 shows how this
is done. Once you know the allele frequencies, you can calculate the ratio of genotypes in the next generation using a
Punnett square. The data from Figure 27.12 reveal that the
next generation will have exactly the same ratio of genotypes as before:
eggs
sperm
0.6 L
0.4 l
0.6 L
0.36 LL
0.24 Ll
0.4 l
0.24 Ll
0.16 ll
Genotype frequencies:
0.36 LL+0.48 Ll+0.16 ll=1
or
L2+2 Ll+l2
It is important to realize that the sperm and eggs represented in this Punnett square are actually the frequencies
of alleles L and l in an entire population, not gametes produced by individuals.
The Hardy-Weinberg Principle
The Hardy-Weinberg principle states that allele frequencies
in a gene pool will remain at equilibrium, and thus constant,
after one generation of random mating in a large, sexually
reproducing population as long as five conditions are met:
1. No mutations. Genetic mutations are an alteration
in an allele, due to a change in DNA composition.
Under Hardy-Weinberg assumptions, allele changes
do not occur, or changes in one direction are balanced by changes in the opposite direction.
2. No genetic drift. Genetic drift is random changes in
allele frequencies by chance. If a population is very
large, changes in allele frequencies due to chance
alone are insignificant.
3. No gene flow. Gene flow is the sharing of alleles between
two populations through interbreeding. If there is no
gene flow, migration of individuals, and therefore their
genes, into or out of the population does not occur.
4. Random mating. Random mating occurs when individuals pair by chance, not according to their genotypes
or phenotypes.
5. No selection. Often, the environment selects certain
phenotypes to reproduce and have more offspring
than other phenotypes. If selection does not occur, no
phenotype is favored over another to reproduce.
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a.
b.
Figure 27.13 Microevolution.
Both dark-colored and light-colored individuals occur in populations of the peppered moth, Biston betularia. a. When tree trunks are light, dark-colored moths
are seen and eaten by predatory birds, and the light-colored moths increase in number. b. When tree trunks are dark due to pollution, light-colored moths are
seen and eaten by predatory birds, and the dark-colored moths increase in number.
In real life, these conditions are rarely, if ever, met,
and allele frequencies in the gene pool of a population do
change from one generation to the next. Because a change
in allele frequencies is our definition of microevolution,
then evolution has occurred. A significance of the HardyWeinberg principle is that microevolution can be detected
by noting deviations from a Hardy-Weinberg equilibrium of
allele frequencies in the gene pool of a population.
Such deviations suggest that one or more of the five
conditions is occurring in a population. Figure 27.13 gives an
example of microevolution due to selection in a population of
peppered moths. Peppered moths can be dark colored or light
colored, and the percentage of each in the population can vary.
Predatory birds are the selective agent that causes the makeup
of the population to vary. When dark-colored moths rest on
light trunks in a nonpolluted area, they are seen and eaten
by these birds. With pollution, the trunks of trees darken, so
light-colored moths stand out and are eaten more than darkcolored moths. We know that evolution has occurred in Figure
27.13 because the population changes from 10% dark-colored
phenotype to 80% dark-colored phenotype over time. In this
example, evolution has occurred because a selective force
(predatory birds) favored one genotype over another.
The Hardy-Weinberg principle predicts that allele
frequencies in a population will remain constant
generation after generation, and this provides a baseline
by which to judge whether evolution has occurred.
A change of allele frequencies in the gene pool of a
population signifies that evolution has occurred.
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Five Agents of Evolutionary Change
The list of conditions for genetic equilibrium stated previously implies that the opposite conditions can cause evolutionary change. These conditions are mutations, genetic
drift, gene flow, nonrandom mating, and natural selection.
Mutations
Mutations are genetic changes that provide the raw material for evolutionary change; mutations create new alleles.
For example, a mutation can result in a nucleotide change in
a gene. A mutation can be “silent” if the nucleotide change
does not result in an amino acid change or if it is recessive and masked by a dominant allele in a diploid organism. If a mutation does, however, affect protein function, it
can be harmful to an organism. Mutations are random and
are most often thought to result in no change or a negative
effect on an individual’s reproductive success.
In a changing environment, however, even a seemingly
harmful mutation that results in a phenotypic change can be
a source of an adaptive variation. For example, the water flea
Daphnia ordinarily thrives at temperatures around 20°C, but
there is a mutation that requires Daphnia to live at temperatures between 25°C and 30°C. The adaptive value of this mutation is entirely dependent on environmental conditions.
Genetic Drift
Genetic drift refers to changes in the allele frequencies of
a gene pool due to chance, as illustrated by the green and
brown frogs in Figure 27.14. As you can imagine, genetic
drift has greater effects in smaller populations. For exam-
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Chapter Twenty–Seven Evolution of Life
ple, the chance death of one individual in a population of a
million will not have an appreciable effect on allele frequencies, but the chance death of one individual in a population
of ten could change the frequency of an allele by 10% or
even cause its loss altogether (if that individual was the
only one with that allele). In nature, two situations, called
founder effect and bottleneck effect, lead to small populations whereby genetic drift can drastically affect allele
frequencies in a gene pool.
The founder effect occurs when a few individuals
form a new colony, and only a fraction of the total genetic
diversity of the original gene pool is represented in these
individuals. The particular alleles carried by the founders is dictated by chance alone. The Amish population
of Lancaster, Pennsylvania, is an isolated religious sect
descended from a few German founders. Today, as many as
one in 14 individuals in this group carries a recessive allele
that causes an unusual form of dwarfism (it affects only the
lower arms and legs) and polydactylism (extra fingers) (Fig.
27.15). Genetic drift has caused this proportion to be much
higher in the Amish than the non-Amish, where the allele
is found in only one in 1,000 people.
Sometimes a population is subjected to near extinction
because of a natural disaster (e.g., earthquake or fire) or
because of human interference. The disaster acts as a bottleneck, preventing the majority of genotypes from breeding
to form the next generation. For example, a large genetic
similarity found in cheetahs is believed to be due to a bottleneck effect. In a skin grafting study, most cheetahs failed
to reject skin grafts from unrelated cheetahs because they
were so genetically similar.
death
Gene Flow
Gene flow is the movement of alleles between populations,
as occurs when individuals migrate from one population to
another and breed in that new population. For example, adult
plants are not able to migrate, but their gametes are often
either blown by the wind or carried by insects. The wind, in
particular, can carry pollen for long distances and can therefore be a factor in gene flow among plant populations.
Gene flow among populations keeps their gene pools similar. It also prevents close adaptation to a local environment.
Nonrandom Mating
Nonrandom mating occurs when individuals pair up, not
by chance, but according to their genotypes or phenotypes.
Inbreeding, or mating between relatives to a greater extent
than by chance, is an example of nonrandom mating.
Inbreeding decreases the proportion of heterozygotes and
increases the proportions of homozygotes at all gene loci.
In a human population, inbreeding increases the frequency
of recessive abnormalities (see Fig. 27.15).
genetic drift
Figure 27.14 Genetic drift.
Genetic drift occurs when by chance only certain members of a population
(in this case, green frogs) reproduce and pass on their alleles to the next
generation. The allele frequencies of the next generation’s gene pool may
be markedly different from those of the previous generation, particularly in
small populations.
mad86751_ch27_548-574.indd 561
561
Figure 27.15 Founder effect.
A member of the founding population of Amish in Pennsylvania had a
recessive allele for a rare kind of dwarfism linked with polydactylism. The
percentage of the Amish population now carrying this allele is much higher
compared to that of the general population.
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Charles Darwin’s Theory of Natural Selection
Although Charles Darwin is often credited
as the first to believe in descent with modification, biologists before him had slowly
begun to accept the idea of evolution. JeanBaptiste de Lamarck (1744–1829), a predecessor of Darwin, concluded after studying
the succession of life-forms in geological
strata, that more complex organisms are
descended from less complex organisms.
To explain the process of adaptation to the
environment, Lamarck proposed of inheritance of acquired characteristics—that the
environment can bring about inherited
change. One example he gave—and the
one for which he is most famous—is that
the long neck of a giraffe developed over
time because animals stretched their necks
to reach food high in trees and then passed
gradually longer necks to their offspring
(Fig. 27A).
This hypothesis for the inheritance
of acquired characteristics has never been
substantiated. The molecular mechanism
of inheritance explains why. Phenotypic
changes acquired during an organism’s lifetime do not result in genetic changes that
can be passed to subsequent generations.
As an example, consider tail cropping in
Doberman pincers. All Doberman puppies
are born with tails, even though their parents’ tails are most often cropped. That is,
tail cropping is a phenotypic change that is
not inherited in the DNA. We now know
that Lamarck’s ideas, although important
for advancing ideas about evolution, were
incorrect.
Charles Darwin (1809–1882) came to a
different conclusion from Lamarck’s after
going on a five-year trip as a naturalist
aboard the ship the HMS Beagle. He read
a book by Charles Lyell, a geologist who
suggested the world is very old and has
been undergoing gradual changes for
many many years. This meant that there
was time for evolution to occur.
Because the ship sailed in the tropics of the Southern Hemisphere, Darwin
encountered different living things that
were more abundant and varied than those
found in his native England. When Darwin
compared the animals of Africa to those of
South America, he noted that the African
Early giraffes probably had
short necks that they stretched
to reach food.
Their offspring had longer
necks that they stretched to
reach food.
Eventually, the continued
stretching of the neck resulted
in today’s giraffe.
Figure 27A Jean-Baptiste de
Lamarck’s proposal of acquired
characteristics.
ostrich and the South American rhea,
although similar in appearance, were actually different animals. He reasoned that
they had a different line of descent because
they were on different continents. When he
arrived at the Galápagos Islands, he began
to study the diversity of finches (see Fig.
27.20), whose adaptations could best be
explained by assuming they had diverged
from a common ancestor. He found such a
hypothetical ancestor on the mainland of
South America, supporting his theory. With
this type of evidence, Darwin concluded
that species evolve (change) with time.
When Darwin returned home, he spent
the next 20 years gathering data to support
the principle of biological evolution. His
most significant contribution was his theory
of natural selection, which explains how
populations of a species become adapted to
their environment. This theory is explained
in Figure 27B. Before formulating the theory, Darwin read an essay on human population growth written by Thomas Malthus.
Malthus observed that although humans
have a great reproductive potential, many
environmental variables, such as availability of food and living space, tend to keep
the human population in check with factors such as disease and famine. Darwin
applied these ideas to all populations of
organisms. A population is all the members of a species living in one particular
place. Darwin calculated that a single pair
of elephants could have 19 million descendants in 750 year. He realized that other
organisms have an even greater reproductive potential than this pair of elephants
yet, usually population sizes remain about
the same. Darwin decided there is a constant struggle for existence, whereby only
certain members of a population survive to
reproduce. Those individuals best adapted
to their environment produce the greatest
number of offspring, and it is their traits
that increase in frequency in a population
in successive generations. This so-called
“survival of the fittest” causes the next generation to be better adapted to the environment than the previous generation.
Darwin’s theory of natural selection
was nonteleological, meaning that there
562
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Chapter Twenty–Seven Evolution of Life
is no design or purpose in the works or
processes of nature. However, rather than
believing that organisms strive to adapt
themselves to the environment, Darwin
concluded that the environment acts on
individual phenotypes to select those individuals that are best adapted. These individuals
have been “naturally selected” to pass on
their characteristics to the next generation. In contrast, the Lamarckian explanation for the long neck of the giraffe was
incorrect because ancestors of the modern giraffe were “trying” to reach into
the trees to browse on high-growing vegetation. Lamarck’s proposal is teleological
because, according to him, the outcome
(longer necks) is predetermined. Darwin’s
theory of evolution, rather than being progressive or “forward looking” implies that
the changing environment does not move
toward any predetermined outcome.
The critical elements of Darwin’s theory are as follows:
Variations. Individual members
of a population vary in physical
characteristics. To be affected by
natural selection, physical variations
must be inherited from generation
to generation by reproduction rather
than being environmentally induced.
If there is no variation in a trait in a
population, natural selection cannot
act.
■ Overproduction and struggle for
existence. The members of all
populations compete with each other
for limited resources. Certain members
are able to capture or utilize these
resources better than others.
■ Survival of the fittest. Just as humans
carry on artificial breeding programs
to select which plants and animals
will reproduce, natural selection
by the environment determines
which members of a population
survive and reproduce. While
Darwin emphasized the importance
of survival, modern evolutionists
emphasize the importance of unequal
reproduction. That is, certain members
of the population produce more
offspring than others simply because
they happen to have a variation or
variations that make them better
suited to the environment. In a
biological sense, fitness is the number
of fertile offspring an individual
produces throughout its lifetime.
■ Adaptation. The result of natural
selection is that populations come
to resemble the “best types”—those
individuals that produce the most
offspring because they are better
adapted to the environment.
Early giraffes probably had
necks of various lengths.
■
mad86751_ch27_548-574.indd 563
Natural selection due to
competition led to survival of
the longer-necked giraffes and
their offspring.
Darwin was prompted to publish his
findings only after he received a letter from
another naturalist, Alfred Russel Wallace,
who had come to the same conclusions
about evolution. Although both scientists
subsequently presented their ideas at the
same meeting of the famed Royal Society
in London in 1858, only Darwin had outlined his reasoning for the theory in a draft
of The Origin of Species by Means of Natural
Selection, which he had completed 16 years
earlier and eventually published in 1859.
This book is still studied by many biologists today.
Can natural selection account for the
origin of new species and for the great
diversity of life? Yes, Darwinian selection
is the only accepted scientific theory for the
diversity of life.
Discussion Questions
1. Currently, a debate is in progress
regarding the teaching of intelligent
design alongside evolution as a theory
for the diversity of life. Explain
why the intelligent design idea is
teleological.
2. Explain why variation in a trait
must be present in order for natural
selection to operate.
3. Explain why Lamarck’s idea
of “inheritance of acquired
characteristics” is incorrect.
Eventually, only long-necked
giraffes survived the
competition.
Figure 27B Charles Darwin’s
theory of natural selection.
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Part Six Evolution and Diversity
Initial
Distribution
Survival of Young
564
After
Time
less than 4 eggs
4 to 5 eggs
more than 5 eggs
Survival of Young
Clutch Size
After
More Time
Survival of Young
Clutch Size
Clutch Size
Figure 27.16 Stabilizing selection.
Stabilizing selection occurs when natural selection favors the intermediate
phenotype over the extremes. For example, Swiss starlings that lay four to
five eggs (usual clutch size) have more surviving young than birds that lay
fewer than four eggs or more than five eggs.
Natural Selection
Natural selection is the process by which some individuals
produce more offspring than others. The Science Focus outlines how Charles Darwin explained evolution by natural
selection. Here, we restate these steps in the context of modern
evolutionary theory. Evolution by natural selection requires:
1. Individual variation. The members of a population
differ from one another.
2. Inheritance. Many of these differences are heritable
genetic differences.
3. Overproduction. Individuals in a population are
engaged in a struggle for existence because breeding
individuals in a population tend to produce more offspring than the environment can support.
4. Differential reproductive success. Individuals that
are better adapted to their environment produce
more offspring than those that are not as well
adapted, and consequently, their fertile offspring will
make up a greater proportion of the next generation.
In biology, the fitness of an individual is measured
by the number of fertile offspring produced throughout its
lifetime. Gene mutations are the ultimate source of variation because they provide new alleles. However, in sexually
reproducing organisms, genetic variation can also result from
crossing-over and independent assortment of chromosomes
during meiosis and also fertilization when gametes are com-
mad86751_ch27_548-574.indd 564
bined. A different combination of alleles can lead to a new
and different phenotype.
In this context, consider that most of the traits on
which natural selection acts are polygenic and thus controlled by more than one gene. Such traits have a range of
phenotypes that follow a bell-shaped curve.
The three main types of natural selection are stabilizing
selection, directional selection, and disruptive selection.
Stabilizing selection occurs when an intermediate
phenotype is favored. With stabilizing selection, extreme
phenotypes are selected against, and individuals near the
average are favored. Stabilizing selection can improve
adaptation of the population to those aspects of the
environment that remain constant. As an example,
consider that when Swiss starlings lay four to five
eggs, more young survive than when the female lays
more or less than this number (Fig. 27.16). Genes
determining physiological characteristics, such as the
production of yolk, and behavioral characteristics,
such as how long the female will mate, are involved
in determining clutch size.
Through the years, hospital data have shown that
human infants born with an intermediate birth weight (3–4
kg) have a better chance of survival than those born with an
extreme birth weight—either higher or lower than that range.
Stabilizing selection serves to reduce the variability in birth
weight in human populations.
Directional Selection Directional selection occurs when
an extreme phenotype is favored and the distribution curve
shifts in that direction (Fig. 27.17). This changes the average
phenotype in a population. Such a shift can occur when
a population is adapting to a changing environment. For
example, the gradual increase in the size of the modern
horse, Equus, can be correlated with a change in the environment from forest conditions to grassland conditions.
Hyracotherium, the ancestor of the modern horse, was about
the size of a dog and was adapted to the forestlike environment of the Eocene epoch of the Paleogene period. This
animal could have hidden among the trees for protection,
and its low-crowned teeth would have been appropriate
for browsing on leaves. Later, in the Miocene and Pliocene
epochs, grasslands began to replace the forests. Then the
ancestors of Equus were subject to selective pressure for
the development of strength, intelligence, speed, and
durable grinding teeth. A larger size provided the strength
needed for combat, elongated legs ending in hooves gave
speed for escaping from enemies, and the durable grinding teeth enabled the animals to feed efficiently on grasses.
Nevertheless, the evolution of the horse should not be
viewed as a straight line of descent; there were many side
branches that became extinct. The evolution of peppered
moths discussed previously is another good example of directional selection.
Disruptive Selection In disruptive selection, two or
more extreme phenotypes are favored over any intermediate phenotype (Fig. 27.18). For example, British land snails
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Chapter Twenty–Seven Evolution of Life
After More Time
Body Size
Number of
Individuals
After Time
Number of
Individuals
Number of
Individuals
Initial Distribution
565
Body Size
Body Size
a.
Hyracotherium
Merychippus
b.
Figure 27.17 Directional selection.
Equus
Initial
Distribution
Number of
Individuals
a. Directional selection occurs when natural selection favors one extreme phenotype, resulting in a shift in the distribution curve. b. For example, Equus, the
modern-day horse, which is adapted to a grassland habitat, is much larger than its ancestor, Hyracotherium, which was adapted to a forest habitat.
After
Time
Number of
Individuals
Banding Pattern
After
More Time
Number of
Individuals
Banding Pattern
Banding Pattern
a.
b.
Figure 27.18 Disruptive selection.
a. Disruptive selection favors two or more extreme phenotypes. b. Today, British land snails comprise mainly two different phenotypes, each adapted to a
different habitat. Snails with dark shells are more prevalent in forested areas, and light-banded snails are more prevalent in areas with low-lying vegetation.
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(Cepaeanemoralis) have a wide habitat range that includes
grass fields and hedgerows and forested areas. In areas with
low-lying vegetation, thrushes feed mainly on snails with
dark shells that lack light bands, and in forested areas, they
feed mainly on snails with light-banded shells. Therefore,
the two different habitats have resulted in two different
phenotypes in the population.
The agents of evolutionary change are mutations,
genetic drift, gene flow, nonrandom mating, and natural
selection. These processes cause changes in the allele
frequencies of a population. Of these, only natural
selection results in adaptation to the environment.
Maintenance of Variation
You might think that genetic variation, particularly due
to deleterious alleles, would eventually disappear because
natural selection tends to remove those alleles from a population. But sickle cell disease exemplifies how genetic variation is sometimes maintained in a population. Persons
homozygous for the allele that causes sickle cell disease
have sickle-shaped red blood cells, which can clog blood
vessels and deprive the body of oxygen. Therefore, you
would expect this condition to be selected against and eliminated from a population. However, heterozygotes for the
sickle cell allele have some sickle-shaped cells, and are also
resistant to malaria, a disease caused by a parasite that lives
in red blood cells. Malaria is a leading killer in many parts
of the world. As a result, the allele for sickle cell disease is
maintained in relatively high frequency in regions where
there is a high incidence of malaria. A study of the three
genotypes and phenotypes involved shows why:
Genotype
Phenotype
Result
Hb A Hb A
Normal
Dies due to
malarial infection
Hb A Hb S
Some sickle cells
Lives due to
protection from malaria
Hb S Hb S
Sickle cell disease1
Dies due to
sickle cell disease
1All
red blood cells sickle shaped
The frequency of the sickle cell allele in some parts of
Africa is 0.40, while among African Americans, it is only 0.05
due to lowered incidence of malaria in the United States. In
Africa, the favored heterozygote keeps the two homozygotes equally present in the population. Maintenance of the
same ratio of two or more phenotypes in each generation is
called balanced polymorphism.
mad86751_ch27_548-574.indd 566
Five agents of evolutionary change are: mutations, genetic
drift, gene flow, non-random mating and natural selection.
Variation can be maintained in populations through
balancing selection or through alleles that cause diseases
that may provide an advantage in the heterozygous form.
27.4
Speciation
Usually, a species occupies a certain geographical range,
within which several subpopulations exist. For our present
discussion, species is defined as a group of subpopulations
that are capable of interbreeding and are isolated reproductively from other species. The subpopulations of the same
species can exchange genes, but different species do not
exchange genes. Reproductive isolation of similar species
is accomplished by the isolating mechanisms listed in Table
27.2. Prezygotic isolating mechanisms are in place before
fertilization, and thus reproduction is never attempted.
Postzygotic isolating mechanisms are in place after fertilization, so reproduction may take place, but it does not
produce fertile offspring.
The Process of Speciation
Speciation has occurred when one species gives rise to two
species, each of which continues on its own evolutionary pathway. How can we recognize speciation? Whenever reproductive isolation develops between two formerly interbreeding
groups of populations, speciation has occurred. One type of
speciation, called allopatric speciation, usually occurs when
populations become separated by a geographic barrier and
gene flow is no longer possible. Figure 27.19 illustrates an
TABLE 27.2
Reproductive Isolating Mechanisms
Isolating Mechanism Example
Prezygotic
Habitat isolation
Species at same locale occupy different habitats
Temporal isolation
Species reproduce at different seasons or
different times of day
Behavioral isolation
In animals, courtship behavior differs,
or they respond to different songs, calls,
pheromones, or other signals
Mechanical isolation
Genitalia unsuitable for one another
Postzygotic
Gamete isolation
Sperm cannot reach or fertilize egg
Zygote mortality
Fertilization occurs, but zygote does not survive
Hybrid sterility
Hybrid survives but is sterile and cannot
reproduce
F2 fitness
Hybrid is fertile, but F2 hybrid has reduced
fitness
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Chapter Twenty–Seven Evolution of Life
1. Members of a northern ancestral population migrated southward.
Ensatina eschscholtzi picta
A
AD
EV
A N INS
RR TA
SIE OUN
M
GE
AN
S
LR
TA AIN
AS NT
CO MOU
Ensatina eschscholtzi
oregonensis
2. Subspecies are separated by
California’s Central Valley. Some
interbreeding between populations
does occur.
It is also possible that a single population could suddenly
divide into two reproductively isolated groups without being
geographically isolated. The best evidence for this type of speciation, called sympatric speciation, is found among plants,
where multiplication of the chromosome number in one plant
prevents it from successfully reproducing with others of its kind.
Self-reproduction can maintain such a new plant species.
Speciation is the origin of a new species. This usually requires
geographic isolation followed by reproductive isolation.
Adaptive Radiation
Ensatina eschscholtzi platensis
L
RA
NT Y
CE LLE
VA
Ensatina eschscholtzi
xanthoptica
Ensatina eschscholtzi
croceater
Ensatina eschscholtzi
eschscholtzii
3. Evolution has occurred, and in
the south two subspecies look
quite different from one another.
Ensatina eschscholtzi
klauberi
Figure 27.19 Allopatric speciation.
In this example of allopatric speciation, the Central Valley of California
is separating a range of populations descended from the same northern
ancestral species. Those to the west along the coastal mountains and those to
the east along the Sierra Nevada mountains experience gene flow, but gene
flow is limited between the eastern populations and the western populations.
Members of the most southerly eastern and western populations are quite
different in color pattern.
example of allopatric speciation that has been extensively studied in California. Apparently, members of an ancestral population of Ensatina salamanders existing in the Pacific Northwest
migrated southward, establishing a series of populations. Each
population was exposed to its own selective pressures along
the coastal and Sierra Nevada mountains. Due to the presence of the Central Valley of California, which is largely dry
and thus unsuitable habitat for amphibians, gene flow rarely
occurs between eastern and western populations of Ensatina.
Genetic differences also increased from north to south, resulting in two distinct forms of Ensatina salamanders in Southern
California that differ dramatically in color.
mad86751_ch27_548-574.indd 567
One of the best examples of “allopatric” speciation is provided by the finches on the Galápagos Islands, located
600 miles west of Ecuador, South America. The 13 species
of finches found there are often called Darwin’s finches
because Darwin first realized their significance as an example of how evolution works. These species are believed to be
descended from mainland finches that migrated to one of
the islands. We can imagine that after the original species
on a single island increased, some individuals dispersed to
other islands.
The islands are ecologically different enough to
have promoted divergent feeding habits. This is apparent because, although the birds physically resemble each
other in many respects, they have different beaks, each
adapted to gathering and eating a different type of food
(Fig. 27.20). There are seed-eating ground finches, cactus-eating ground finches, insect-eating tree finches, also
with different-sized beaks; and a warbler-type tree finch,
with a beak adapted to eating insects and gathering nectar. Among the tree finches is a woodpecker type, which
lacks the long tongue of a true woodpecker but makes
up for this by using a cactus spine or a twig to ferret
out insects. Remarkably, each of these types is found on
islands where its beak matches the abundant food type.
Therefore, Darwin’s finches are an example of adaptive
radiation, or the proliferation of a species by adaptation
to different ways of life.
The Pace of Speciation
Currently, there are two hypotheses about the pace of speciation and, therefore, evolution. One hypothesis is called
the phyletic gradualism model, and the other is called the
punctuated equilibrium model. Each model gives a different answer to the question of why so few transitional links
are found in the fossil record.
Traditionally, evolutionists have supported a model
called phyletic gradualism, which states that change is very
slow but steady within a lineage before and after a divergence
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Medium ground finch,
Geospiza fortis
Small tree finch,
Camarhynchus parvulus
Medium tree finch,
Camarhynchus pauper
Cactus finch,
Geospiza scandens
Sharp-beaked ground finch,
Geospiza difficilis
Large tree finch,
Camarhyncus psittacula
Vegetarian finch,
Platyspiza crassirostris
Small ground finch,
Geospiza fuliginosa
Mangrove finch,
Cactospiza heliobates
Woodpecker finch,
Cactospiza pallida
(holding a cactus spine)
Warbler finch,
Certhidea olivacea
Large cactus finch,
Geospiza conirostris
Large ground finch,
Geospiza magnirostris
Figure 27.20 The Galápagos finches.
Each of these finches is adapted to gathering and eating a different type of food. Note the different sizes and shapes of the beaks in the different species.
Tree finches have beaks largely adapted to eating insects and, at times, plants. The woodpecker finch, a tool-user, uses a cactus spine or twig to probe in
the bark of a tree for insects. Ground finches have beaks adapted to eating prickly-pear cactus or different-sized seeds.
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Chapter Twenty–Seven Evolution of Life
new
species 1
transitional link
ancestral
species
new
species 2
569
has occurred. Why? Because a new species comes about after
reproductive isolation, and reproductive isolation cannot
be detected in the fossil record! Only when a new species
evolves and displaces the existing species is the new species
likely to show up in the fossil record.
A model of evolution called punctuated equilibrium
has also been proposed (Fig. 27.21b). It says that long periods
of stasis, or no visible change, are followed by rapid periods
of speciation. With reference to the length of the fossil record
(about 3.5 billion years), speciation occurs relatively rapidly,
and this can explain why few transitional links are found.
Mass extinction events are often followed by rapid (relative
to the age of the Earth) periods of speciation.
Adaptive radiation is an example of allopatric speciation
that is easily observable. Whether speciation occurs slowly
or rapidly is being debated.
Time
a.
27.5
new
species 1
ancestral
species
ancestral
species
new
species 2
Time
b.
Figure 27.21 Phyletic gradualism compared to
punctuated equilibrium.
a. Supporters of the phyletic gradualism model believe that speciation takes
place gradually and many transitional links occur. b. Supporters of the
more recent, punctuated equilibrium model believe that speciation occurs
rapidly, with no transitional links.
(splitting of the line of descent) (Fig. 27.21a). Therefore, it is
not surprising that few transitional links such as Archaeopteryx
(see Fig. 27.5) have been found. Indeed, the fossil record, even
if it were complete, might be unable to show when speciation
mad86751_ch27_548-574.indd 569
Classification
Recall that each type of organism is given a scientific name
and the scientific name for modern humans is Homo sapiens.
The first word, Homo, is the genus, a classification category that
contains many species. The second word is the species name,
which may describe the organism. The word sapiens refers to
a large brain. When species are classified, they are placed in
a hierarchy of categories: species, genus, family, order, class,
phylum, and kingdom. This text uses one further classification
category, called a domain.
Phylogeny is the evolutionary relationship among organisms. Ideally, classification reflects phylogeny in that it tells how
organisms are related through evolution and common ancestry.
Species in the same genus are more closely related than species
in separate genera and so forth as we proceed from genus to
domain. Each higher classification category is more inclusive
than the one below it; therefore, there are many more species
within a kingdom than in a phylum, for example.
Five-Kingdom System
For many years, most biologists favored a five-kingdom classification system consisting of Plantae, Animalia, Fungi, Protista,
and Monera. Organisms were placed into these kingdoms on
the basis of type of cell (prokaryotic or eukaryotic), level of
organization (unicellular or multicellular), and mode of nutrition. In this system, the organisms in the kingdom Monera are
distinguished by their structure—they are prokaryotic (lack a
membrane-bounded nucleus)—whereas the organisms in the
other kingdoms are eukaryotic (have a membrane-bounded
nucleus). An evolutionary tree, also called a phylogenetic tree,
depicts the relationships among organisms based on how they
are classified. Figure 27.22 is an evolutionary tree depicting the
five-kingdom system of classification. The tree suggests that
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Part Six Evolution and Diversity
protists evolved from monerans and that fungi, plants, and
animals evolved from protists via three separate lines of evolution. In an evolutionary tree, two or more groups that separate
from the same juncture share the same common ancestor.
The five-kingdom system of classification suggests that fungi,
plants, and animals share the same ancestor, presumably an
extinct protist known only from the fossil record.
Three-Domain System
Within the past ten years, new information has called into
question the five-kingdom system of classification. The molecule rRNA probably changes slowly during evolution and,
indeed, may change when there is a major evolutionary
event. Molecular data based on the sequencing of rRNA
suggest that there are three domains: Bacteria, Archaea,
and Eukarya.
Cellular data also support the three-domain system.
Bacteria and archaea are both unicellular prokaryotes that lack a
membrane-bounded nucleus. However, bacteria and archaea are
distinguishable from each other on the basis of lipid and cell wall
biochemistry. The biochemical attributes of many archaea allow
them to live in very hostile environments, including anaerobic
swamps, salty bodies of water, and even hot, acidic environments,
such as hot springs and geysers. Molecular and cellular data also
suggest that the archaea and eukarya are more closely related
to each other than either is to the bacteria. The evolutionary tree
depicted in Figure 27.23 reflects this evolutionary relationship. It
shows that the archaea and the eukarya share a more recent common ancestor than do all three domains.
The kingdoms Protista, Fungi, Plantae, and Animalia,
as illustrated in Figure 1.5, are all placed in the domain
Eukarya. Exactly how these kingdoms are related is still
being determined.
Phylogenetics
Phylogenetics is the modern way in which organisms are
classified and arranged in evolutionary trees. Phylogeneticists
arrange species and higher classification categories into clades.
Clades may be represented on a diagram called a cladogram.
A clade contains a most recent common ancestor and all its
descendant species—the common ancestor is presumed and
not identified. Figure 27.24 depicts a cladogram for seven
groups of vertebrates. Only the lamprey, the so-called “outgroup,” lacks jaws, but the other six groups of vertebrates are
in the same clade because they all have jaws, a derived characteristic relative to their ancestors. On the other hand, the
vertebrates beyond the shark are all in the same clade because
they have lungs, and so forth.
Figure 27.24 is somewhat misleading because, although
single traits are noted on the tree, phylogeneticists use much
more data to arrange groups of organisms into clades.
Phylogeneticists are aided in their endeavor by the computer
and any and all available data, including morphological data
and DNA sequences. In making decisions, they are often guided
fungi
plants
Kingdom Plantae
Kingdom Animalia
EUKARYA
Kingdom Fungi
animals
protists
protists
cyanobacteria
Kingdom Protista
heterotrophic
bacteria
BACTERIA
ARCHAEA
Kingdom Monera
common ancestor
Figure 27.22 Five-kingdom system of classification.
Figure 27.23 The three-domain system of
All prokaryotes are in the kingdom Monera. The eukaryotes are in
kingdoms Protista, Fungi, Plantae, and Animalia. The evolutionary
tree shows the lines of descent. Note that the three domain system of
classification (Fig. 27.23) is currently preferred.
Representatives of each domain are depicted. The phylogenetic tree shows
that domain Archaea is more closely related to domain Eukarya than either
is to domain Bacteria.
mad86751_ch27_548-574.indd 570
classification.
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571
Figure 27.24 Cladogram.
lamprey
shark
salamander
lizard
A cladogram gives comparative information
gorilla
tiger
human about relationships. Organisms in the same
clade share the same derived characteristics.
Humans and all the other vertebrates
shown, except lampreys, are in the same
clade as sharks because they all have
jaws. However, humans are also alone
in a clade because only they are bipedal.
This cladogram is simplified because
phylogeneticists actually use a great deal
more data to construct cladograms.
bipedal
no tail
hair
amniotic
membrane
lungs
jaws
Class
Mammalia
mammals
Time
by the principle of parsimony, which states that the pattern that
requires the fewest evolutionary changes is the most likely.
Like phylogeneticists, another group of scientists called
“traditionalists” consider descent from a common ancestor
when grouping organisms, but they also consider the amount
of adaptive evolutionary change. For example, traditionalists
place crocodiles (class Reptilia) and birds (class Aves) in separate classes (Fig. 27.25a) because of the adaptive advantage of
feathers, despite the fact that they agree with phylogeneticists
that these groups share a recent common ancestor. However,
phylogeneticists place crocodiles and birds in the same group
(Archosaurs, Fig. 27.25b) because of the many traits they share.
Class
Reptilia
turtles
snakes
and lizards
Class
Aves
crocodiles
dinosaurs
birds
early reptiles
a. Traditional systematics
Organisms are classified into groups based on their
evolutionary relationships. Currently a three domain system
replaces the previously preferred five kingdom system of
classification. Phylogenetics is used to classify groups of closely
related organisms, called clades, into evolutionary trees.
Mammalia
mammals
Reptilia
Archosaurs
turtles
crocodiles
dinosaurs
birds
snakes
and lizards
Summarizing the Concepts
27.1 Origin of Life
Chemical reactions are believed to have led to the formation of the
first true cell(s). Inorganic chemicals, probably derived from the
primitive atmosphere, reacted to form small organic molecules.
These reactions occurred in the ocean, either on the surface or in
the region of hydrothermal vents deep within.
After small organic molecules such as glucose, amino acids, and
nucleotides arose, they polymerized to form the macromolecules.
Amino acids joined to form proteins, and nucleotides joined to form
nucleic acids. Perhaps RNA was the first nucleic acid. The RNAfirst hypothesis is supported by the discovery of ribozymes, RNA
enzymes. The protein-first hypothesis is supported by the observation
that amino acids polymerize abiotically when exposed to dry heat.
Once a plasma membrane developed, the protocell came into
being. Eventually, the DNADRNADprotein system evolved,
and a true cell came into being.
27.2 Evidence of Evolution
The fossil record and biogeography, as well as studies of comparative
anatomy, development, and biochemistry, all provide evidence of evo-
mad86751_ch27_548-574.indd 571
early reptiles
Time
b. Cladistic systematics
Figure 27.25 Traditional versus cladistic view of
reptilian phylogeny.
a. According to traditionalists, crocodiles and birds are in separate classes.
b. According to phylogeneticists, crocodiles and birds share a recent
common ancestor and should be in the same clade.
lution. The fossil record gives clues about the history of life in general
and allows us to trace the descent of a particular group. Biogeography
shows that the distribution of organisms on Earth can be influenced by
a combination of evolutionary and geological processes. Comparing
the anatomy and the development of organisms reveals homologous
12/21/06 7:36:14 PM
Evolution of Antibiotic Resistance
Through Darwin’s theory of natural selection, we have come to understand why bacteria become resistant to the antibiotics we use
to treat patients. Some people refer to the use
of antibiotics as “artificial selection,” because
humans are involved. Nonetheless, the process
is the same as natural selection—antibiotics
kill bacteria that are susceptible, but bacterial
populations within a single patient are usually
so large that resistant individuals are likely
to survive and reproduce. Through time, the
frequency of resistant individuals increases
in the population to the point that a certain
antibiotic may no longer be effective. Keep in
mind that a patient infected with a resistant
strain might require several antibiotics.
Ever since the introduction of antibiotics, resistance has often evolved soon after
(Table 27A). An extreme example is methicillin, an antibiotic that it only took one year
for bacteria to evolve resistance against! This
is the type of “accelerated evolution” you
learned about at the beginning of this chapter. Anitbacterial resistance creates the need
for even newer antibiotics. The development
of a single new antibiotic is estimated to cost
between $400 and $500 million. Antibiotic
resistance adds $30 billion to annual medical
costs in the United States alone!
Some strains of tuberculosis (or TB),
a disease caused by bacteria in the genus
Mycobacterium, are resistant to multiple antibiotics. TB is spread though the air from one
person to another, usually when an infected
person coughs or sneezes. TB is the most
common infectious disease today, infecting
about one-third of the world’s population,
or about 2 billion people. You may not realize it, but TB kills 2–3 million people worldwide each year—right up there with AIDS
and malaria! In extreme cases, infections can
spread through the lungs, causing lesions and
even holes, and eventually leading to death in
untreated patients.
When a patient tests positive for TB, he
or she is usually put on a six-month course of
the antibiotic isoniazid. Many patients feel
better after a few weeks on the drug, and
some discontinue its use. This selects for
resistant strains of TB. Our understanding
of evolutionary biology has helped doctors
treat patients with TB. In New York City, for
example, patients are treated with what is
called “direct observation therapy,” in which
a doctor or nurse actually watches a patient
take the medicine. In addition, the initial
diagnosis of TB includes testing whether the
strain is drug resistant. This way, doctors can
treat patients with an effective drug regimen,
instead of allowing their infection to persist
and spread to other people.
In patients with strains that are resistant
to isoniazid, four drugs are recommended
for treatment. Multidrug-resistant strains of
TB can be very difficult and costly to treat;
usually an 18-month course of multiple antibiotics is necessary, and treatment for strains
of TB resistant to methicillin alone can cost
$50,000 per person!
TABLE 27A
1. In New York City, state health officials
have the power to quarantine TB
patients who do not take their medicine.
That is, they can essentially lock them
up for as long as needed (often up to 18
months) to treat their illness. However,
some of the medications can have
serious side effects. What do you think
about this policy?
2. Many people in less-developed countries
die from TB, not because their disease is
incurable, but simply because they do not
have health insurance and cannot afford
the medications. Should we in the United
States pay more for our medications
so that pharmaceutical companies can
provide them to lower-income people at a
reduced cost or for free?
3. Antibiotics kill bacteria only, not viruses.
Knowing what you know about antibiotic
resistance and natural selection, when
would you prescribe antibiotics if you
were a doctor?
Dates of Antibiotic Discovery
and Resistance
Antibiotic
Discovery/
Introduction
Resistance
Penicillin
1928/1943
1946
Sulfonamides
1930s
1940s
Streptomycin
1943/1945
1959
Chloramphenicol
1947
1959
Tetracycline
1948
1953
Erythromycin
1952
1988
Vancomycin
1956
1988/1993
Methicillin
1960
1961
Ampicillin
1961
1972
Cefotaxime/ceftazidime
1981/1985
1983/1984/1988
structures among those that share common ancestry. All organisms
have certain biochemical molecules in common, and these chemical
similarities indicate the degree of relatedness.
27.3 The Process of Evolution
Discussion Questions
Microevolution is a process that involves a change in allele frequencies within the gene pool of a sexually reproducing population. The Hardy-Weinberg principle states that gene pool frequencies arrive at an equilibrium that is maintained generation after
generation unless disrupted by mutations, genetic drift, gene flow,
nonrandom mating, or natural selection. Any change from the
initial allele frequencies in the gene pool of a population signifies
that evolution has occurred.
27.4 Speciation
Speciation is the origin of new species. This usually requires geographic isolation, followed by reproductive isolation. The evolution of several species of finches on the Galápagos Islands is an
example of speciation caused by adaptive radiation because each
one has a different way of life.
Currently, there are two hypotheses about the pace of speciation. Traditionalists support phyletic gradualism—slow, steady
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Chapter Twenty–Seven Evolution of Life
change leading to speciation. In contrast, a more recent model,
called punctuated equilibrium, proposes that long periods of stasis
are interrupted by rapid speciation.
27.5 Classification
Classification involves assigning species to a hierarchy of categories: kingdom, phylum, class, order, family, genus, and species,
and in this text, domain. The five-kingdom system of classification recognizes these kingdoms: Monera (the bacteria), Protista
(algae, protozoans), Fungi, Plantae, and Animalia. The more recent
which is three-domain system (Bacteria, Archaea, and Eukarya),
based on molecular data, is currently preferred. Both bacteria
and archaea are prokaryotes. Members of the kingdoms Protista,
Fungi, Plantae, and Animalia are eukaryotes.
Phylogeneticists classify and diagram the evolutionary relationships among organisms. They use as many characteristics as
possible to put species in clades, which are represented on portions of a diagram called a cladogram. A clade contains a most
recent common ancestor and all its descendant species, which
share the same derived characteristics relative to their ancestors.
Testing Yourself
Choose the best answer for each question.
1. The atmosphere in which life arose lacked
a. carbon.
c. oxygen.
b. nitrogen.
d. hydrogen.
2. The RNA-first hypothesis for the origin of cells is supported
by the discovery of
a. ribozymes.
c. polypeptides.
b. proteinoids.
d. nucleic acid polymerization.
3. Protocells probably obtained energy as
a. photosynthetic autotrophs. c. heterotrophs.
b. chemoautotrophs.
d. None of these are correct.
4. All true cells are able to
a. replicate DNA.
c. absorb nutrients.
b. synthesize sugars.
d. export minerals.
5. DNA genes may have arisen from RNA genes via
a. DNA polymerase.
c. reverse transcriptase.
b. RNA polymerase.
d. DNA ligase.
6. Fossils that serve as transitional links allow scientists to
a. determine how prehistoric animals interacted with each other.
b. deduce the order in which various groups of animals arose.
c. relate climate change to evolutionary trends.
d. determine why evolutionary changes occur.
7. Carbon dating cannot be used to determine the age of
dinosaur fossils because
a. levels of atmospheric carbon were very low when
dinosaurs were alive.
b. dinosaurs contained very low levels of carbon.
c. dinosaur fossils contain very low levels of carbon.
d. the half-life of radioactive carbon is too short.
8. Marine animals experience mass extinction approximately
every 26 million years. Scientists believe this pattern is due
to meteorites that reach Earth because
a. our solar system shifts its location in the Milky Way in a
26-million-year pattern.
mad86751_ch27_548-574.indd 573
b. the sun moves far away from Earth every 26 million years.
c. Earth moves into an asteroid belt every 26 million years.
d. the moon moves close to Earth every 26 million years.
9. Which of the following groups of organisms might be found
on multiple continents? See Table 27.1, and remember that the
continents began to move apart about 250 million years ago.
a. primitive primates
b. reptiles
c. birds
d. Both b and c are correct.
e. All of these are correct.
10. The flipper of a dolphin and the fin of a tuna are
a. homologous structures.
b. homogeneous structures.
c. analogous structures.
d. reciprocal structures.
11. Which of the following is not an example of a vestigial structure?
a. human tailbone
b. ostrich wings
c. pelvic girdle in snakes
d. dog kidney
12. Which of the following is sure to be a population?
a. grizzly bears of the Rocky Mountains
b. mosquitoes of the United States
c. barracuda in the Caribbean Sea
d. sequoia grove in Sequoia National Park
e. dandelions of Pennsylvania
13. The frequency of a rare disorder expressed as an autosomal
recessive trait is 0.0064. Using the Hardy-Weinberg principle,
determine the frequency of carriers for the disease in this
population.
a. 0.147
d. 0.020
b. 0.080
e. 0.846
c. 0.920
14. Which of the following generally results in a gain in genetic
variability?
a. genetic drift
b. mutation
c. founder effect
d. bottleneck
For questions 15–19, match the description with the
appropriate term in the key.
Key:
a. mutation
d. bottleneck
b. natural selection
e. gene flow
c. founder effect
f. nonrandom mating
15. The Northern elephant seal went through a severe
population decline as a result of hunting in the late 1800s. As
a result of a hunting ban, the population has rebounded but
is now homozygous for nearly every gene studied.
16. A small, reproductively isolated religious sect called the
Dunkers was established by 27 families that came to the
United States from Germany 200 years ago. The frequencies
for blood group alleles in this population differ significantly
from those in the general U.S. population.
17. Turtles on a small island tend to mate with relatives more
often than turtles on the mainland.
18. Within a population, plants that produce an insect toxin are
more likely to survive and reproduce than plants that do not
produce the toxin.
19. The gene pool of a population of bighorn sheep in the
southwest U.S. is altered when several animals cross over a
mountain pass and join the population.
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Part Six Evolution and Diversity
20. People who are heterozygous for the cystic fibrosis gene are
more likely than others to survive a cholera epidemic. This
heterozygote advantage seems to explain why homozygotes
are maintained in the human population, and is an example of
a. disruptive selection.
c. high mutation rate.
b. balanced polymorphism.
d. nonrandom mating.
21. The creation of new species due to geographic barriers is called
a. isolation speciation.
d. sympatric speciation.
b. allopatric speciation.
e. symbiotic speciation.
c. allelomorphic speciation.
22. Which of the following models is supported by the observation
that few transitional links are found in the fossil record?
a. phyletic gradualism
c. Both a and b are correct.
b. punctuated equilibrium
d. None of these are correct.
23. Organisms have been placed in the five-kingdom
classification system based on
a. level of organization and mode of nutrition.
b. genetic diversity and mode of nutrition.
c. genetic diversity and level of organization.
d. reproductive traits and mode of nutrition.
e. reproductive traits and genetic diversity.
24. The three-domain classification system has recently been
developed based on
a.
b.
c.
d.
e.
mitochondrial biochemistry and plasma membrane structure.
cellular and rRNA sequence data.
plasma membrane and cell wall structure.
rRNA sequence data and plasma membrane structure.
nuclear and mitochondrial biochemistry.
Understanding the Terms
adaptive radiation 567
allopatric speciation 566
analogous structure 556
Archaea 570
autotroph 552
Bacteria 570
biogeography 556
bottleneck effect 563
clade 570
cladogram 570
class 569
common ancestor 570
continental drift 556
directional selection 564
disruptive selection 564
domain 569
Eukarya 570
evolution 552
family 569
fitness 564
fossil 553
founder effect 563
gene flow 563
mad86751_ch27_548-574.indd 574
gene pool 559
genetic drift 562
genus 569
heterotroph 552
homologous structure 556
kingdom 569
kingdom Animalia 570
kingdom Fungi 570
kingdom Plantae 570
kingdom Protista 570
liposome 552
microevolution 559
microsphere 551
mutation 562
natural selection 564
nonrandom mating 563
order 569
phyletic gradualism 567
phylogenetics 570
phylogeny 569
phylum 569
population 559
postzygotic isolating
mechanism 566
prezygotic isolating
mechanism 566
protein-first hypothesis 551
proteinoid 551
protocell 550
punctuated equilibrium 569
RNA-first hypothesis 551
speciation 566
species 566, 569
stabilizing selection 564
sympatric speciation 567
transitional link 553
vestigial structure 557
Match the terms to these definitions:
a._______________ Process by which populations become
adapted to their environment.
b._______________ Type of natural selection in which an
extreme phenotype is favored, usually in a changing
environment.
c._______________ An evolutionary model that proposes
periods of rapid change dependent on speciation
followed by long periods of stasis.
d._______________ Structure that is similar in two or more
species because of common ancestry.
e._______________ Movement of genes from one
population to another via sexual reproduction
between members of the populations.
Thinking Critically
1. Viruses such as HIV are rapidly replicated and have very high
mutation rates; thus, evolution of the virus can be observed
in a single infected person. Using what you have learned in
this chapter, explain why HIV is so hard to treat, even though
multiple drugs to treat HIV have been developed.
2. If the conditions for the Hardy-Weinberg principle are rarely, if
ever, met in nature, why is it such an important idea?
3. You observe a wasting disease in cattle that you know is
genetically caused and thus heritable. The disease is fatal in
young cattle. The allele frequency for the gene that causes the
disease is 0.05 in the United States, but 0.35 in South America.
Explain why such a difference in allele frequencies might exist.
4. Why are homologous structures, as opposed to analogous
ones, used to determine the evolutionary relationships of
species and to reconstruct phylogenies?
5. Why do scientists continue to devise experimental systems
that mimic the conditions of the early Earth, despite the fact
that it is difficult to do and there is no way to know for sure
what the conditions on Earth were billions of years ago? Are
you aware of any such experiments and their results?
ARIS, the Inquiry into Life Website
ARIS, the website for Inquiry into Life, provides a wealth of information organized and integrated by chapter. You will find practice quizzes, interactive activities, labeling exercises, flashcards,
and much more that will complement your learning and understanding of general biology.
www.aris.mhhe.com
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