Chapter 16: Evolution of Populations
16.1 Genes and Variation
Variation and Gene Pools
 Gene Pool – all genes in a population of organisms
 Allele Frequency – the number of times the allele occurs in a gene pool
 In genetic terms, evolution is any change in the frequency of alleles in a population.
Sources of Genetic Variation
 Mutations
 Any change in a sequence of DNA
 Gene Shuffling
 Crossing-over
 Sexual reproduction
 Random arrangement of chromosomes in Metaphase I of meiosis
Single-Gene and Polygenic Traits
 Single Gene Traits – traits controlled by one gene that has two alleles
 Example: Widow’s Peak – AA or Aa - have widow’s peak
aa - have no widow’s peak
 Polygenic Traits – traits controlled by two or more genes
 Example: Height in humans
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16.2 Evolution as Genetic Change
 If an individual dies without reproducing, it does not contribute its alleles to
population’s gene pool.
 If an individual produces many offspring, its alleles stay in the gene pool and may
increase in frequency.
 Populations, not individual organisms, can evolve over time.
 Natural selection can affect the distributions of phenotypes in three ways:
1. Directional Selection
 Phenotypes shift toward homozygous dominant or homozygous recessive
 Example: Darwin’s Finches
 The finches had beaks of different sizes to eat different food. What if the
supply of small seeds disappeared only leaving the large, hard seeds?
Those finches with big beaks would survive causing a shift to that
phenotype
2. Stabilizing Selection
 Heterozygote of a trait are favored and alleles that specify extreme forms
(homozygote) are eliminated from a population
 Example: Darwin’s Finches
 What if the supply of seeds was mostly medium size seeds?
3. Disruptive Selection
 Forms at both extremes of the range of variation
 Extremes are favored and the intermediate form is selected against
 Example: Darwin’s Finches
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 What would happen if the supply of medium seeds disappeared?
Genetic Drift
 Genetic Drift – a random change in allele frequencies over the generations
 Genetic drift has a greater effect on small populations.
 Two Kinds of Genetic Drift
 Founder Effect –
 Occurs when allele frequencies in a group of migrating individuals are by
chance not the same as that of their original population
 Example: beetles
 Two small groups of different beetles leave the population.
 These two small groups start their own population.
 The two new populations are genetically different from the original
population.
 Bottleneck
 Occurs when the population undergoes a dramatic decrease in size.
 Causes:
 Natural catastrophes
 Predation
 Disease
Evolution Versus Genetic Equilibrium
 1908 Hardy and Weinberg independently suggested a scheme whereby evolution
could be viewed as changes in the frequency of alleles in a population of organisms
 Hardy-Weinberg – allele frequencies in a population will remain constant unless one
or more factors cause those frequencies to change
 Genetic Equilibrium – When allele frequencies remain constant
 5 conditions are required to maintain genetic equilibrium from generation to
generation:
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1.
2.
3.
4.
5.
there must be random mating
population must be very large
there can be no movement of genes into or out of the population
no mutations
no natural selection – all genotypes must have an equal rate of survival and
reproduction
16.3 The Process of Speciation
 Natural selection and chance events can change the relative frequencies of alleles in
a population and lead to speciation.
 Speciation – formation of a new species
 Species – a group of organisms that breed with one another and produce fertile
offspring.
Isolating Mechanisms
 As new species evolve, populations become reproductively isolated from each other.
 When the members of two populations cannot interbreed and produce fertile
offspring, reproductive isolation has occurred.
 3 Types of Reproductive Isolation
1. Behavioral Isolation
 Occurs when a species does not recognize another species as a mating
partner because it does not perform the correct courtship rituals, display the
proper visual signals, sing the correct mating songs or release the proper
chemicals
2. Geographic Isolation
 Occurs when two populations are separated by geographic barriers such as
rivers or mountains.
3. Temporal Isolation
 Occurs when two species mate or flower during different seasons or at
different times of the day
17.4 Patterns of Evolution
 Macroevolution – large-scale evolutionary patterns and processes that occur over
long periods of time
 Extinction
 More than 99% of all species that have ever lived are now extinct
 What effects have mass extinctions had on the history of life? Mass
extinctions have:
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 provided ecological opportunities for organisms that survived
 resulted in bursts of evolution that produced many new species
 Divergent Evolution
 Two or more species that originate from a common ancestor
 Adaptive radiation – a type of divergent evolution – the process by which a
species evolves into several different species.
 The disappearance of dinosaurs then resulted in the adaptive radiation of
mammals.
 Convergent Evolution
 The process by which unrelated organisms come to resemble one another.
 Convergent evolution has resulted in sharks, dolphins, seals, and penguins.
 Coevolution
 The process by which two species evolve in response to changes in each
other over time.
 Example: predator prey
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Chapter 16: Evolution of Populations
16.1 Genes and Variation
Variation and Gene Pools
 Gene Pool –
 Allele Frequency –
 In genetic terms, evolution is any change in the frequency of alleles in a population.
Sources of Genetic Variation
 Mutations
 Any change in a sequence of ___________
 Gene Shuffling

 _______________ reproduction
 Random arrangement of chromosomes in Metaphase I of meiosis
Single-Gene and Polygenic Traits
 Single Gene Traits –
 Example: Widow’s Peak – AA or Aa - have widow’s peak
aa - have no widow’s peak
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 Polygenic Traits –
 Example: Height in humans
16.2 Evolution as Genetic Change
 If an individual dies without reproducing, it does ______ contribute its alleles to
population’s _________ __________.
 If an individual produces __________ offspring, its alleles stay in the gene pool and
may ______________ in frequency.
 ______________, not individual organisms, can evolve over time.
 Natural selection can affect the distributions of phenotypes in three ways:
1. ________________ Selection
 Phenotypes shift toward _________________ ____________ or
____________ ______________
 Example:
 The finches had beaks of different sizes to eat different food. What if the
supply of small seeds disappeared only leaving the large, hard seeds?
Those finches with big beaks would survive causing a shift to that
phenotype.
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2. ____________________ Selection
 Heterozygote of a trait are ___________ and alleles that specify extreme
forms (homozygote) are eliminated from a population
 Example: Darwin’s Finches
 What if the supply of seeds was mostly medium size seeds?
3. ____________ Selection
 Forms at both extremes of the range of variation
 _____________ are favored and the heterozygous form is selected against
 Example: Darwin’s Finches
 What would happen if the supply of medium seeds disappeared?
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Genetic Drift
 Genetic Drift –
 Genetic drift has a greater effect on ______________ populations.
 Two Kinds of Genetic Drift
1. _____________ ______________
 Occurs when allele frequencies in a group of ____________ individuals are
by chance not the same as that of their ______________ population.
 Example: beetles
 Two small groups of different beetles leave the population.
 These two small groups start their own population.
 The two new populations are genetically different from the original
population.
2. _____________
 Occurs when the population undergoes a dramatic ___________ in size.
 Causes:



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Evolution Versus Genetic Equilibrium
 1908 Hardy and Weinberg independently suggested a scheme whereby evolution
could be viewed as changes in the frequency of alleles in a population of organisms
 Hardy-Weinberg – allele frequencies in a population will remain ___________
unless one or more factors cause those frequencies to change.
 Genetic Equilibrium –
 5 conditions are required to maintain genetic equilibrium from generation to
generation:
1. There must be ___________ _____________
2. Population must be very ______________
3. There can be no movement of genes _________ or _______ of the
population
4. _____ _____________
5. _____ __________ _____________ – all genotypes must have an equal
rate of survival and reproduction
16.3 The Process of Speciation
 Natural selection and chance events can change the relative frequencies of alleles in
a population and lead to speciation.
 Speciation –
 Species –
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Isolating Mechanisms
 As new species evolve, populations become reproductively isolated from each other.
 When the members of two populations cannot ____________ and produce
___________ offspring, reproductive isolation has occurred.
 3 Types of Reproductive Isolation
1. ______________ Isolation
 Occurs when a species does not recognize another species as a mating
partner because it does not perform the correct ___________ rituals, display
the proper _____________ signals, sing the correct mating __________ or
release the proper _____________.
2. _____________ Isolation
 Occurs when two populations are separated by geographic barriers such as
____________ or ________________.
3. ______________ Isolation
 Occurs when two species mate or flower during different ____________ or at
different ______________ of the day
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17.4 Patterns of Evolution
 Macroevolution –
 Extinction
 More than ________ of all species that have ever lived are now extinct
 What effects have mass extinctions had on the history of life? Mass
extinctions have:
 Provided ecological opportunities for organisms that survived
 Resulted in bursts of evolution that produced many new species
 Divergent Evolution

 Adaptive radiation – a type of divergent evolution –
 The disappearance of ____________________ then resulted in the adaptive
radiation of mammals.
 Convergent Evolution
 Convergent evolution –
 Convergent evolution has resulted in sharks, dolphins, seals, and penguins.
 Coevolution
 Coevolution –
 Example:
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