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Population Genetics and Speciation
Section 2: Genetic Change
Section 2
Population Genetics and Speciation
Section 2
Key Ideas
• What does the Hardy-Weinberg principle predict?
• How does sexual reproduction influence evolution?
• Why does population size matter?
• What are the limits of the force of natural selection?
• What patterns can result from natural selection?
Population Genetics and Speciation
Section 2
Equilibrium and Change
• genetic equilibrium
– Ratio of alleles remain the same between generations
• Same amount of alleles does not mean same amount of
genotypes
Population Genetics and Speciation
Section 2
Equilibrium and Change
• The Hardy-Weinberg principle
– frequencies of alleles and genotypes in a population will not
change unless at least one of five forces acts upon the
population.
Population Genetics and Speciation
Equilibrium and Change, continued
Gene Flow
• genes are added to or removed from a
population.
• can be caused by migration
Nonrandom Mating
• sexually reproducing populations
• limits or preferences of mate choice
Section 2
Population Genetics and Speciation
Section 2
Visual Concept: Comparing the Effects of
Random and Nonrandom Mating
Population Genetics and Speciation
Section 2
Equilibrium and Change, continued
Genetic Drift
• Chance events can cause rare alleles to be lost from one
generation to the next
Mutation
• can add a new allele to a population
Population Genetics and Speciation
Section 2
Equilibrium and Change, continued
Natural Selection
• acts to eliminate individuals with certain traits
from a population
• the alleles for those traits may become less
frequent in the population
• both allele and genotype frequencies may
change
Population Genetics and Speciation
Section 2
Sexual Reproduction and Evolution
• Sexual reproduction
– can recombine alleles and increase variation in a population
– mating patterns or behaviors can influence the gene pool.
Population Genetics and Speciation
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Sexual Reproduction and Evolution,
continued
• Sexual Selection
– females sometimes select mates based on the male’s
size, color, ability to gather food, or other
characteristics.
• Inbreeding
– individuals either self-fertilize or mate with others like
themselves
– more likely if a population is small
Population Genetics and Speciation
Population Size and Evolution
• Allele frequencies are
more likely to remain
stable in large populations
• Genetic drift occurs when
a particular allele
disappears.
Section 2
Population Genetics and Speciation
Section 2
Natural Selection and Evolution
• Natural selection is a result of the following facts:
– All populations have genetic variation.
– Individuals tend to produce more offspring than the
environment can support.
– Populations depend upon the reproduction of
individuals.
Population Genetics and Speciation
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Natural Selection and Evolution,
continued
Genetic Results of Selection
• an allele’s frequency may increase or decrease
• natural selection is not the only force that can cause
evolution, but it is a powerful force.
Population Genetics and Speciation
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Natural Selection and Evolution,
continued
Why Selection is Limited pg 408
• environment does the selecting
Natural selection is indirect
• Changes frequency of alleles
• acts on genotypes by removing unsuccessful
phenotypes
Population Genetics and Speciation
Section 2
Natural Selection and Evolution,
continued
The Role of Mutation
• Only characteristics that are expressed can be targets of
natural selection.
• selection cannot operate against rare recessive alleles
• For this reason, genetic disorders (such as cystic fibrosis
in humans) can persist in populations.
Population Genetics and Speciation
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Patterns of Natural Selection pg 409
Directional Selection
• the “peak” of a normal distribution moves in one direction
along its range.
• acts to eliminate one extreme from a range of
phenotypes
Population Genetics and Speciation
Section 2
Patterns of Natural Selection, continued
Stabilizing Selection
• the bell-curve shape becomes narrower
• eliminates individuals that have alleles for any extreme
type.
• very common in nature
Population Genetics and Speciation
Section 2
Patterns of Natural Selection, continued
Disruptive Selection
• the bell curve is pushed apart into two peaks.
• acts to eliminate individuals with average phenotype
values