Chapter 16 Evolution of Populations

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: o Mutations (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 selection o directional selection o 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.

16-3 The Process of Speciation

Vocab Gene Pool Relative frequency Single-gene trait Polygenic trait o stabilizing selection o directional selection o disruptive selection Genetic drift Founder effect