Chapter 16 Evolution of Populations 1
May 11, 2015
Chapter
16
Evolution of Populations
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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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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
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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
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
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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 phenotypesEx.) height
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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?
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?
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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
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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2. Stabilizing Selection - ___________________________________________________
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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
Genetic Drift - ___________________________________________________________
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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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Five conditions are required to maintain genetic equilibrium from generation to generation
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1.
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4.
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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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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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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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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…
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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
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
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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
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
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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 todayEx.)
o
o
Evolutionary theory helps us understand and respond to these changes in ways that improve human life
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Name _________________________________ Date _____________________ Per ____
16 – 3 Section Review
1. How is reproductive isolation related to the formation of new species?
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.
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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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