1. Chapter
16
Evolution of Populations
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2. 16-1 Genes and Variation
• As Darwin developed his theory of evolution, he worked under a serious handicap
• He didn’t know how heredity worked
• This lack of knowledge left two big gaps in Darwin’s thinking
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2.
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• During the 1930’s Evolutionary biologists connected Mendel’s work to Darwin’s
• By then biologists understood that genes control heritable traits
How Common Is Genetic Variation?
• Many genes have at least ________ forms or alleles
• Animals such as horses, dogs, mice, and humans often have ____________________
alleles for traits such as body size or coat color
Variation and Gene Pools
• Genetic variation is studied in populations
Population - _____________________________________________________________
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• Because members of a population interbreed, they share a common group of genes
called a gene pool
Gene pool - ______________________________________________________________
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3. _______________________________________________________________________
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Relative frequency - _______________________________________________________
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• In genetic terms, evolution is any change in the relative frequency of alleles in a
population
Sources of Genetic Variation
• The 2 main sources of genetic variation are ________________________________
and the ________________________________________ that results from
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Mutations - _____________________________________________________________
• Mutations can occur because of
o
o
• Some mutations don’t affect the phenotype but some do
Gene shuffling during sexual reproduction
• Mutations are not the only source of variation
• Most heritable differences are due to gene shuffling that occurs during the production
of _________________________________
• The 23 pairs of chromosomes can produce ______________________________
different combinations of genes
• _________________________________ further increases the number of different
genotypes that can also appear in offspring
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4. Single – Gene and Polygenic Traits
• The number of phenotypes produced for a given trait depends on how many genes
control the trait
Single – gene trait - _______________________________________________________
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• Variation in these genes leads to only 2 distinct phenotypes
Polygenic traits - _________________________________________________________
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• Each gene of a polygenic trait has 2 or more alleles
• As a result one polygenic trait can have many possible genotypes and phenotypes
Ex.) height
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5. Name _______________________________ Date _____________________ Per ______
16 – 1 Section Review
1. What two processes can lead to inherited variation in populations?
2. How does the range of phenotypes differ between single-gene traits and polygenic
traits?
3. What is a gene pool? How are allele frequencies related to gene pools?
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6. 4. How could you distinguish between a species in which there is a lot of variation and
two separate species?
5. How does the process known as independent assortment relate to the genetic variation
that results from sexual reproduction?
16 – 2 Evolution as Genetic Change
• A genetic view of evolution offers a new way to look at key evolutionary concepts
• If each time an organism reproduces, it passes copies of its genes to its offspring…
• We can therefore view evolutionary fitness as an organism’s success in passing genes
to the next generation
• We can also view an evolutionary adaptation as any genetically controlled
physiological, anatomical, or behavioral trait that increases an individuals ability to
pass along its genes
• Remember that evolution is any change over time in the relative frequency of alleles
in a population. This reminds us that it is populations, not individual organisms that
can evolve overtime
Natural Selection on Single – Gene Traits
• Natural selection on single gene traits can lead to changes in allele frequencies and
thus to evolution
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7. Natural Selection on Polygenic Traits
• Natural selection can affect the distributions of phenotypes in any of three ways
1. Directional Selection - ___________________________________________________
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8. 2. Stabilizing Selection - ___________________________________________________
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9. 3. Disruptive Selection - ___________________________________________________
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• Can create 2 distinct phenotypes
Genetic Drift
• Natural Selection is not the only source of evolutionary change
• In small populations, an allele can become more or less common by chance
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10. Genetic Drift - ___________________________________________________________
Q: How does genetic drift take place?
A:
• These individuals may carry alleles in different relative frequencies than did the
larger population from which they came
• If so, the population that they found will be genetically different from the parent
population
• This cause is not natural selection, but _____________________________
Founder effect - __________________________________________________________
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Evolution vs. Genetic Equilibrium
• To clarify how evolutionary change operates, scientists often find it helpful to
determine what happens when no change takes place
Hardy – Weinberg principle - _______________________________________________
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Genetic equilibrium - ______________________________________________________
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11. _______________________________________________________________________
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• Five conditions are required to maintain genetic equilibrium from generation to
generation
1.
2.
3.
4.
5.
Name _______________________________ Date ______________________ Per _____
16 – 2 Section Review
1. Describe how natural selection can affect traits controlled by single genes.
2. Describe three patterns of natural selection on polygenic traits. Which one leads to
two distinct phenotypes?
3. 3. How does genetic drift lead to a change in a population’s gene pool?
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12. 4. What is the Hardy-Weinberg principle?
5. How are directional selection and disruptive selection similar? How are they
different?
16 – 3 The Process of Speciation
• Factors such as natural selection and chance events can change the relative
frequencies of alleles in a population
• But how do these changes lead to speciation?
Speciation - _____________________________________________________________
Isolating Mechanisms
• Since members of the same species share a common gene pool, in order for a species
to evolve into 2 new species, the gene pools must be separated into 2
• As new species evolve, populations become reproductively isolated from each other
Reproductive isolation - ____________________________________________________
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Behavioral Isolation - ______________________________________________________
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13. _______________________________________________________________________
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Geographical Isolation - ____________________________________________________
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Temporal Isolations - ______________________________________________________
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Testing Natural Selection in Nature
Q: Can evolution be
observed in nature?
A:
• Darwin hypothesized
that finches had
descended from a
common ancestor and
overtime, natural
selection shaped the
beaks of different bird populations as they adapted to eat different foods
• The Grants, realized that Darwin’s hypothesis relied on two testable assumptions
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14. 2.
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Variation
• The Grants identified and measured every variable characteristic of the birds on the
island
• Their data indicated that there is a great variation of heritable traits among the
Galapagos finches
Natural Selection
• During the…
• Rainy season – ________________________________________________________
• Dry season – __________________________________________________________
• At that time, differences in beak sizes can mean the difference between life and death
• Birds become feeding specialists
• The Grants discovered that individual birds with different size beaks had different
chances of survival during a drought
Speciation in Darwin’s Finches
• Speciation in the Galapagos finches occurred by founding of a new population,
geographical isolation, changes in the new population’s gene pool, reproductive
isolation and ecological competition
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15. Founders Arrive
• Many years ago, a few finches from South
American mainland
• Species A, flew or were blown to one of
the Galapagos Islands
Geographic Isolation
• Later on, some birds from species A
crossed to another island in the Galapagos
group
• The finches then became unable to fly
from island to island and become isolated
from each other and no longer share a
common gene pool
Changes in the Gene Pool
• Overtime, populations on each island
became adapted to their local
environments
Reproductive Isolation
• Now imagine that a few birds from the
second island cross back to the first island
Q: Will the population A birds, breed with the
population B birds?
A:
Ecological Competition
• As these two new species live together in
the same environment, they compete with
each other for available seeds
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16. • The more different birds are, the higher fitness they have, due to
__________________________________
Continued Evolution
• This process of isolation on different
islands, genetic change, and reproductive
isolation probably repeated itself time and
time again across the entire Galapagos
island chain
• Over many generations, it produced the 13
different finch species found there today
Studying Evolution Since Darwin
• It is useful to review and critique the strength and weakness of evolutionary theory
• Darwin made bold assumptions about heritable variation, the age of the Earth, and the
relationships among organisms
• New data from genetics, physics, and biochemistry could have proved him wrong on
many counts, and ____________________________
• Scientific evidence supports the theory that living species descended with
modification from common ancestors that lived in the past
Limitations of Research
• The Grants data shows how competition and climate change affects natural selection
• However, they did not observe the formation of a new species
Unanswered Questions
• Many new discoveries have led to new hypotheses that refine and expand Darwin’s
original ideas
• No scientist suggests that all evolutionary processes are fully understood. Many
unanswered questions remain
Why Understanding Evolution is Important?
• Evolution continues today
Ex.)
o
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17. o
• Evolutionary theory helps us understand and respond to these changes in ways that
improve human life
Name _________________________________ Date _____________________ Per ____
16 – 3 Section Review
1. How is reproductive isolation related to the formation of new species?
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18. 2. What type of isolating mechanism was important in the formation of different
Galápagos finch species?
3. Explain how behavior can play a role in the evolution of species.
4. Leopard frogs and tree frogs share the same habitat. Leopard frogs mate in April; tree
frogs mate in June. How are these species isolated from each other?
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