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Ch.16 Evolution of Populations
How Common is Genetic Variation?
• Genetic variation is studied within populations.
• Because members within populations interbreed, they share a
common group of genes called a gene pool.
• A gene pool is made up of all of the genes (including all of the
alleles) that are present in a population.
• The relative frequency of an allele is the number of times that
allele shows up, compared to the number of times other alleles for
that same gene occur.
• In genetic terms, evolution is any change in the relative frequency
of alleles in a population.
• (change from 10-20% allele frequency= evolution)
Evolution Versus Genetic Equilibrium
• Genetic Equilibrium is when allele frequencies remain constant
(there is no evolution or change in the population)
• The Hardy –Weinberg Principle states that allele frequencies in a
population will remain constant as long as the following five
conditions are met:
•
•
•
•
•
Random mating- no preference in mate selection
Large population size- so that small changes will not be
significant
No migration- no gene flow: no new alleles brought into
the population
No mutations- no new alleles added to the population
No Natural selection- all organisms are reproductively
successful therefore no genes are favored
• If any of the above are not met, then the population will evolve
The Process of Speciation
• Recall that a species is defined as a group of
organisms that look alike and can interbreed to
produce fertile offspring in nature.
• The evolution of new species is called speciation.
• Reproductive isolation occurs when formerly
interbreeding organisms can no longer mate and
produce fertile offspring.
• There are three types of reproductive isolation:
behavioral, temporal, and geographic.
Types of Reproductive
Isolation
Behavioral Isolation
• Two organisms are capable of interbreeding,
but they have different courtship rituals.
Ex: different mating songs of birds eastern
and western meadowlarks
Temporal Isolation
• Two or more species reproduce at different
times. Ex: Pollen released at different times
from orchids.
Geographic Isolation
• Geographic isolation occurs whenever a
physical barrier divides a population . Ex:
tree frogs
Geographic Isolation
• When geographic isolation divides a population of tree frogs, the individuals no
longer mate across populations.
• Tree frogs are a single population.
• The formation of a river may divide the frogs into two populations.
• Over time, the divided populations may become two species that may no longer
interbreed, even if reunited.
Reproductive Isolation Flowchart
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species
Testing Natural Selection in Nature
•
•
Peter and Rosemary Grant have worked for the past twenty
years to show that Darwin’s hypothesis is correct.
They realized that Darwin relied on two assumptions:
1. In order for beak size and shape to evolve in finches, there
must have been many varieties in those traits for natural
selection to work on.
2. Differences in beak size and shape must produce
differences in fitness that cause natural selection to occur.
Variation
• The Grants found that the finches of the Galapagos Islands had a great variety
of heritable traits.
• Many of the characteristics appeared in a bell-shaped distributions typical of
polygenic traits.
• This change in beak size is
an example of directional
selection operating on an
anatomical trait.
Rapid Evolution
• The Grants found that
natural selection takes
place frequently—and
sometimes very rapidly.
• Changes in the food supply
caused measurable
fluctuations in the finch
populations over a period
of only decades.
• This is very different from
the slow, gradual evolution
that Darwin envisioned.
Speciation in Darwin’s Finches
Founders Arrive
•
A few finches travel from South America to
one of the islands, there they survive and
reproduce.
Separation of populations
• Some birds from species A cross to a second
island. The two populations no longer share a
gene pool.
Changes in the gene pool
• Seed sizes on the second island favor birds with larger beaks.
The population on the second island evolves into a population,
B, with larger beaks. Eventually, population A and B will
evolve into a separate species.
Reproductive Isolation
• Even if a few birds from population B move back to the first
island, they will not breed with the birds from population A.
• The differences in beak size and mating behavior will lead to
reproductive selection.
Ecological Competition
• The two species will compete with each other for seeds when
they live together.
Continued Evolution
• The process of isolation on different islands, genetic change,
and reproductive isolation will continue to repeat itself.
• Thirteen different species of finches live on the Galapagos
islands today.
Studying Evolution since Darwin
Why is understanding evolution so important?
• Evolution continues today.
• For example, bacteria and viruses are evolving resistance
to drugs.
• Insects are evolving resistance to pesticides.
• Evolutionary theory can help us understand and respond
to these changes in ways that can improve human life.
There are still many unanswered questions that need to be
addressed.
Chapter 17 The History of Life
Origins: The Early Idea
• In the past, the ideas that decaying
meat produced maggots, mud
produced fishes, and grain
produced mice were reasonable
explanations for what people
observed occurring in their
environment.
• Such observations led people to
believe in spontaneous
generation—the idea that nonliving
material can produce life.
Spontaneous generation is disproved
• In 1668, an Italian physician, Francesco Redi, disproved a
commonly held belief at the time—the idea that decaying
meat produced maggots, which are immature flies.
• However, during Redi’s time, scientists began to use the
latest tool in biology—the microscope.
• Although Redi had disproved
the spontaneous generation of Control group
large organisms, many
scientists thought that
microorganisms were so
numerous and widespread that
they must arise spontaneouslyprobably from a vital force in
the air.
Time
Time
Experimental group
Spontaneous generation is disproved
• In the mid-1800s, Louis Pasteur designed an experiment that
disproved the spontaneous generation of microorganisms.
• Pasteur set up an experiment in which air, but no microorganisms,
was allowed to contact a broth that contained nutrients.
• Pasteur’s experiment showed that microorganisms do not simply
arise in broth, even in the presence of air.
• From that time on, biogenesis, the idea that living organisms come
only from other living organisms, became a cornerstone of biology.
• The cell theory states that cells can arise from preexisting cells.
Broth is
boiled.
Broth is free of
microorganisms
for a year.
Curved neck
is removed.
Broth is
teeming with
microorganisms.
The Origin of Life
• Francesco Redi and Louis Pasteur designed controlled
experiments to disprove spontaneous generation.
• Their experiments and others like them convinced scientists to
accept biogenesis.
• But if life comes only from life, then how did life on Earth first
begin?
• No one has yet proven scientifically how life on Earth began.
• However, scientists have
developed theories about the
origin of life on Earth from
testing scientific hypotheses
about conditions on early Earth.
Origins: The Modern Ideas
1.
Early Earth was hot; atmosphere contained poisonous gases.
– Some scientists suggest that it was probably very hot. The
energy from colliding meteorites could have heated its surface,
while both the compression of minerals and the decay of
radioactive materials heated its interior.
– Volcanoes might have frequently spewed lava and gases,
relieving some of the pressure in Earth’s hot interior. These
gases helped form Earth’s early atmosphere. Earth’s early
atmosphere probably contained hydrogen cyanide, carbon
dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and
water.
Origins: The Modern Ideas
2. Earth cooled and oceans condensed.
• About 4.4 billion years ago, Earth might have cooled
enough for the water in its atmosphere to condense. This
might have led to millions of years of rainstorms with
lightning—enough rain to fill depressions that became
Earth’s oceans.
Origins: The Modern Ideas
3. Simple organic molecules
may have formed in the
oceans.
– In the 1930s, a Russian scientist,
Alexander Oparin, hypothesized
that life began in the oceans that
formed on early Earth.
– He suggested that energy from the
sun, lightning, and Earth’s heat
triggered chemical reactions to
produce small organic molecules
from the substances present in the
atmosphere
– Then, rain probably washed the
molecules into the oceans to form
what is often called a primordial
soup.
– In 1953, two American scientists,
Stanley Miller and Harold Urey,
tested Oparin’s hypothesis by
simulating the conditions of early
Earth in the laboratory
Origins: The Modern Ideas
4. Small sequences of RNA may have formed and replicated.
• Scientists hypothesize that two developments must have preceded
the appearance of life on Earth.
– First, simple organic molecules, or molecules that contain
carbon, must have formed.
– Then these molecules must have become organized into
complex organic molecules such as proteins, carbohydrates,
and nucleic acids that are essential to life.
5. First prokaryotes may have formed when RNA or DNA
was enclosed in microspheres.
• The first forms of life may have been prokaryotic forms that evolved
from a protocell. A protocell is a large, ordered structure, enclosed by a
membrane, that carries out some life activities, such as growth and
division.
– Because Earth’s atmosphere lacked oxygen, these organisms were most
likely anaerobic.
– For food, the first prokaryotes probably used some of the organic molecules
in oceans.
– Over time, these heterotrophs would have used up the food supply.
• However, organisms that could make food had probably evolved by the
time the food was gone.
– These first autotrophs were probably similar to present-day archaebacteria.
– Archaebacteria are prokaryotic and live in harsh environments, such as
deep-sea vents and hot springs.
– The earliest autotrophs probably made glucose by chemosynthesis rather
than by photosynthesis.
– In chemosynthesis, autotrophs release the energy of inorganic compounds,
such as sulfur compounds, in their environment to make their food.
• 6. Later prokaryotes were photosynthetic and produced oxygen.
– Photosynthesizing prokaryotes might have been the next type
of organism to evolve.
– As the first photosynthetic organisms increased in number, the
concentration of oxygen in Earth’s atmosphere began to
increase.
– Organisms that could respire aerobically would have evolved
and thrived.
– The presence of oxygen in Earth’s atmosphere probably
affected life on Earth in another important way.
• 7. An oxygenated atmosphere capped by the ozone layer protected
Earth.
– The sun’s rays would have converted much of the oxygen into
ozone molecules that would then have formed a layer that
contained more ozone than the rest of the atmosphere.
• 8. First eukaryotes may have been communities of prokaryotes.
– Complex eukaryotic cells probably evolved from prokaryotic cells.
– The endosymbiont theory, proposed by American biologist Lynn Margulis
explains how eukaryotic cells may have arisen. The theory proposes that
eukaryotes evolved through a symbiotic relationship between ancient
prokaryotes.
• New evidence from scientific research supports this theory and has shown
that chloroplasts and mitochondria have their own ribosomes that are
similar to the ribosomes in prokaryotes.
• In addition, both chloroplasts and mitochondria reproduce independently
of the cells that contain them.
• The fact that some modern prokaryotes live in close association with
eukaryotes also supports the theory.
Chloroplast
Aerobic
bacteria
Plants and
plantlike
protists
Ancient Prokaryotes
Nuclear
envelope
evolving
Photosynthetic
bacteria
Mitochondrion
Primitive Photosynthetic Eukaryote
Animals, fungi, and
non-plantlike protists
Ancient Anaerobic
Prokaryote
Primitive Aerobic Eukaryote
Origins: The Modern Ideas
9. Sexual reproduction increased genetic variability, hastening
evolution.
• increase in genetic variation greatly increases the chances
of evolutionary change in a species due to natural
selection
10. Multicellular eukaryotes evolved.
• cells increased in diversity more rapidly
Evolution of Life Review
Evolution of Life
Early Earth was hot; atmosphere contained poisonous gases.
Earth cooled and oceans condensed.
Simple organic molecules may have formed in the oceans..
Small sequences of RNA may have formed and replicated.
First prokaryotes may have formed when RNA or DNA was enclosed in microspheres.
Later prokaryotes were photosynthetic and produced oxygen.
An oxygenated atmosphere capped by the ozone layer protected Earth.
First eukaryotes may have been communities of prokaryotes.
Sexual reproduction increased genetic variability, hastening evolution.
Multicellular eukaryotes evolved.
Patterns of Evolution
• The history of life is the story of increasing complexity and
diversity, but what does this history reveal about the process of
evolution?
• Biologists have observed different patterns of evolution that occur
throughout the world in different natural environments.
• Macroevolution refers to the large-scale evolutionary changes that
take place over long periods of time.
• Six important patterns of macroevolution are
–
–
–
–
–
–
mass extinctions
adaptive radiation
convergent evolution
coevolution
punctuated equilibrium
changes in developmental genes
• These patterns support the
idea that natural selection
is an important agent for
evolution.
Mass Extinctions
•
•
•
•
•
•
The term used to describe a species that has died out is extinct.
More than 99 percent of all species that ever lived are now extinct.
Huge numbers of species have disappeared in mass extinctions.
The disappearance of so many species left many habitats open.
For the survivors, there was a new world of ecological opportunity.
Often, the result was a burst of evolution that produced an abundance of new
species.
We are the last Dodo birds on this
planet, so I have put all of our eggs
safely into this basket.
Divergent Evolution
• Divergent evolution occurs when populations change as they
adapt to different environmental conditions, eventually resulting
in new species. Species that were once similar to an ancestral
species diverge, or become increasingly distinct.
Extinct
mamo
Amakihi
Possible
Ancestral
Lasan finch
Crested
honeycreeper
Kauai
Niihau
• One type of
divergent
evolution is
adaptive
radiation.
• Ex:
Darwin’s
Finches
Molokai
Oahu
Maui
Lanai
Akialoa
Kahoolawe
Akepa
Akiapolaau
Akikiki
Liwi
Hawaii
Apapane
Maui
parrotbill
Palila
Ou
Grosbeak
finch
Adaptive Radiation
• When an ancestral species evolves into an array of species to fit a
number of diverse habitats, the result is called adaptive radiation.
• Adaptive radiation in both plants and animals has occurred and
continues to occur throughout the world and is common on
islands.
Coevolution
• The process by which two species evolve in response to
changes in each other over time is called coevolution.
• The trees have evolved in response to their seed
predators, we can observe geographic differences
in pinecones.
– Where there are squirrels, the pinecones are heavier
with fewer seeds, but have thinner scales, like the
pinecone on the left.
– Where there are only crossbills, pinecones are
lighter with more seeds, but have thick scales, like
the one on the right.
• The crossbills have evolved in response to the pine
trees, we can observe geographic differences in
birds.
– Where the pinecones have thick scales, birds should
have deeper, less curved bills (below right) than
where the pinecones have thin scales (below left).
Convergent Evolution
• A pattern of evolution in which distantly related organisms evolve
similar traits is called convergent evolution.
• Convergent evolution occurs when unrelated species occupy
similar environments in different parts of the world.
• Each of these animals has a streamlined body and various
appendages that enable it to move rapidly through water. Yet, the
shark is a fish, the penguin is a bird, and the dolphin is a
mammal.
• They have evolved similar traits because they share similar
environmental pressures.
Gradualism vs. Punctuated Equilibrium
• Scientists once argued that evolution occurs at a slow, steady rate, with small,
adaptive changes gradually accumulating over time in populations.
• In 1972, scientists proposed a different hypothesis known as punctuated
equilibrium, which argues that speciation occurs relatively quickly, in rapid
bursts, with long periods of genetic equilibrium in between.
• Environmental
changes, such as
higher
temperatures or
the introduction of
a competitive
species, lead to
rapid changes in a
small population’s
gene pool that is
reproductively
isolated from the
main population.
• Speciation happens
quickly—in about
10,000 years or
less.
Loxodonta
africana
Elephas
maximus
0
1
2
Elephas
3
4
Mammuthus
primigenius
Loxodonta
Mammuthus
5
Primelephas
6
Ancestral species
about 55 million years ago
Gradualism vs. Punctuated
Equilibrium
• Biologists generally agree that both gradualism and punctuated
equilibrium can result in speciation, depending on the
circumstances.
• Gradualism involves a
slow, steady change in
a particular line of
descent.
• Punctuated
equilibrium involves
stable periods
interrupted by rapid
changes involving
many different lines of
descent.
Developmental Genes and Body
Plans
• Hox genes are called “master control
genes,” because they control growth
as an embryo develops.
• Some hox genes determine which
parts become front, rear, top, and
bottom.
• Others control the growth of body
parts such as arms, legs, and wings.
• The timing of genetic control during
embryonic development can make
the difference between various traits.
Patterns of Evolution Flow Chart
Species
that are
Unrelated
form
Related
in
under
under
in
in
Inter-relationships
Similar
environments
Intense
environmental
pressure
Small
populations
Different
environments
can undergo
can undergo
can undergo
can undergo
can undergo
Co evolution
Convergent
evolution
Extinction
Punctuated
equilibrium
Adaptive
radiation