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Chapter 16:
Evolution of Populations
16.1 Genes and Variation
Variation and Gene Pools
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Gene pool – all genes in a population of
organisms
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Allele frequency – the number of times the
allele occurs in a gene pool
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In genetic terms, evolution is any change in
the frequency of alleles in a population.
Gene Pool for Fur Color in Mice
Sources of Genetic Variation
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Mutations
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Any change in a sequence of DNA
Gene Shuffling
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Crossing-over
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Sexual reproduction
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Random arrangement of chromosomes in
Metaphase I of meiosis
Single-Gene and Polygenic Traits
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Single Gene Traits – traits controlled by one
gene that has two alleles
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Example: Widow’s Peak –
AA or Aa - have widow’s peak
aa - have no widow’s peak
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Polygenic Traits – traits controlled by two or
more genes
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Example: Height in humans
Bell shaped
curve is normal
distribution
16.2 Evolution as Genetic Change
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If an individual dies without reproducing, it
does not contribute its alleles to
population’s gene pool.
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If an individual produces many offspring, its
alleles stay in the gene pool and may
increase in frequency.
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Populations, not individual organisms, can
evolve over time.
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Natural selection can affect the distributions of
phenotypes in three ways:
1. Directional Selection
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Phenotypes shift toward homozygous
dominant or homozygous recessive
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Example: Darwin’s Finches
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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
Small
Medium
Beak Size
Large
2. Stabilizing Selection
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When intermediate forms (heterozygote) of
a trait are favored and alleles that specify
extreme forms (homozygote) are eliminated
from a population
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Example: Darwin’s Finches
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What if the supply of seeds was mostly
medium size seeds?
Number of Birds in Population
Beak Size
Small
Medium
Beak Size
Large
3. Disruptive Selection
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Forms at both extremes of the range of
variation
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Extremes are favored and the
intermediate form is selected against
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Example: Darwin’s Finches
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What would happen if the supply of
medium seeds disappeared?

Example: Darwin’s finches
• What would happen if the supply of medium seeds
disappeared?
Small
Medium
Beak Size
Large
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Genetic Drift
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Genetic Drift – a random change in allele
frequencies over the generations
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Genetic drift has a greater effect on small
populations.
1. Founder Effect
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Occurs when allele frequencies in a group
of migrating individuals are by chance not
the same as that of their original
population
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Example: beetles
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Two small groups of different beetles
leave the population.
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These two small groups start their own
population.
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The two new populations are genetically
different from the original population.
Population A
Population B
2. Bottleneck
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Occurs when the population undergoes a
dramatic decrease in size.
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Causes:
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Natural catastrophes
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Predation
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Disease
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Evolution Versus Genetic Equilibrium
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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
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Hardy-Weinberg – allele frequencies in a
population will remain constant unless one
or more factors cause those frequencies to
change.
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Genetic Equilibrium – When allele
frequencies remain constant
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5 conditions are required to maintain genetic
equilibrium from generation to generation:
1. There must be random mating
2. Population must be very large
3. There can be no movement of genes into or
out of the population
4. No mutations
5. No natural selection – all genotypes must
have an equal rate of survival and
reproduction
16.3 The Process of Speciation
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Natural selection and chance events can
change the relative frequencies of alleles in a
population and lead to speciation.
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Speciation – formation of a new species
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Species – a group of organisms that breed
with one another and produce fertile
offspring.
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Isolating Mechanisms
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As new species evolve, populations
become reproductively isolated from each
other.
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When the members of two populations
cannot interbreed and produce fertile
offspring, reproductive isolation has
occurred.
3 Types of Reproductive Isolation
1. Behavioral Isolation
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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
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Occurs when two populations are separated
by geographic barriers such as rivers or
mountains.
3. Temporal Isolation
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Occurs when two species mate or flower
during different seasons or at different times
of the day
17.4 Patterns of Evolution
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Macroevolution – large-scale evolutionary
patterns and processes that occur over long
periods of time.
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Extinction
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More than 99% of all species that have
ever lived are now extinct
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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
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Resulted in bursts of evolution that
produced many new species
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Divergent Evolution
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Two or more species that originate from a
common ancestor.
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Adaptive radiation – a type of divergent
evolution – the process by which a species
evolves into several different species
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The disappearance of dinosaurs then
resulted in the adaptive radiation of
mammals.
time A
time B
time C
parent species
time
time D
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Convergent Evolution
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Convergent evolution – the process by
which unrelated organisms come to
resemble one another.
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Convergent evolution has resulted in
sharks, dolphins, seals, and penguins.
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Coevolution
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Coevolution – the process by which two
species evolve in response to changes in
each other over time.
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Example: predator prey