1 Review Define genetic drift
Relate Cause and Effect How can the founder effect
lead to changes in the allele pool
2 Infer Genetic equilibrium is uncommon in actual
populations. Why do you think this is the case
3 Evolution Do you think populations stay in genetic
equilibrium after the environment Has changed
significantly- explain.
CH 17 EVOLUTION OF POPULATIONS
17.2 Evolution as Genetic Change in Populations
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Insect populations often contain a few individuals
that are resistant to a particular pesticide
They pass on their resistance to their offspring and
soon the pesticide-resistant offspring dominate the
population.
Natural Selection on
Single-Gene Traits
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Lead to changes in allele frequencies and then to
evolution
Mutation in one gene that determines body color
in lizards can affect their lifespan
The normal color for lizards is brown, a mutation
may produce red and black forms.
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Red lizards are more visible to predators
They are less likely to survive and reproduce = red
allele is rare.
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Black lizards can absorb sunlight
Higher body temperatures may allow the lizards to
move faster, escape predators, and reproduce.
Natural Selection on
Polygenic Traits
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The fitness of individuals may vary from one end of
the curve to the other
Can affect the range of phenotypes and hence the
shape of the bell curve.
Directional Selection
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Individuals at one end of the curve have higher
fitness than individuals in the middle or at the
other end
Range of phenotypes shifts.
Stabilizing Selection
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Individuals near the center of the curve have higher
fitness than at either end
Keeps the center of the curve at its current
position, but it narrows the overall graph.
Disruptive Selection
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Individuals at the upper and lower ends of the
curve have higher fitness than individuals near the
middle
Acts against the intermediate type and can create
two distinct phenotypes.
Genetic Drift

Random change in allele frequency.
Genetic Bottlenecks
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Change in allele frequency following a dramatic
reduction in the size of a population
A disaster may kill many individuals in a
population, and the surviving population’s gene
pool may contain different gene frequencies from
the original gene pool.
The Founder Effect

Allele frequencies change as a result of the
migration of a small subgroup of a population.
Genetic Equilibrium
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Allele frequencies in the population remain the
same
Evolution is not occurring.
Hardy-Weinberg Principle
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1.
2.
3.
4.
5.
All conditions that must occur for evolution not
to occur
Very large population
No mutations
Random mating
No migration
No natural selection.
Large Population
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Genetic drift has less effect on large populations
Large population size helps maintain genetic
equilibrium.
No Mutations

If mutations occur, new alleles may be introduced
into the gene pool, and allele frequencies will
change.
Random Mating
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All members of the population must have an equal
opportunity to produce offspring
Individuals must mate with other members of the
population at random.
Random Mating

Non random mating examples
 Selective
breeding
 Self pollination
 Artificial selection
 Sexual selection
Sexual Selection

Individuals in a population showing a preference
for mating with a certain phenotype or trait.
Sexual Selection
No Migration (No Gene Flow)
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Individuals who join a population may introduce
new alleles into the gene pool
Individuals who leave may remove alleles from the
gene pool
There must be no movement of individuals into or
out of a population.
No Natural Selection

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All genotypes in the population must have equal
probabilities of surviving and reproducing
No phenotype can have a selective advantage over
another.
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Shuffling of genes during sexual reproduction
produces many different gene combinations but
does not alter the relative frequencies of alleles in
a population.