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Variation and Gene Pools• Genetic variations (differences) are studied
in populations (group of individuals of the same species that interbreed).
• A gene pool is all of the genes that are present in a population.
• The relative frequency of an allele is the # of times that the allele occurs in a gene pool compared with the # of times other alleles occur (usually a %).
• In genetic terms, evolution is any change in the relative frequency of alleles in a population.
Sources of Genetic Variation
• There are two main sources of genetic variation:oMutations (any
change in a sequence of DNA)
o The gene shuffling that results from sexual reproduction.
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:Trait controlled by a single gene with two alleles (ex: widows peak)
Polygenic trait:Trait controlled by two or more genes, each gene has two or more alleles (ex: height)
Single-Gene Traits• Natural Selection on single-gene traits can
lead to changes in allele frequencies and thus to evolution.
Polygenic Traits
• Natural selection can affect the distributions of phenotypes in any of three ways:o stabilizing selectiono directional selectiono disruptive selection
Stabilizing Selection• When individuals near the center of the curve
have higher fitness than individuals at either end of the curve
Directional Selection
• When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end
Disruptive Selection
• When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle
Genetic Drift• A random change in allele frequency.• In small populations, individuals that carry a
particular allele may leave more descendants than other individuals do, just by chance.
• When allele frequencies change due to migration of a small group of a population it is known as the founder effect.