Evolution as Genetic Change in Populations

Learning Objectives  Explain how natural selection affects single-gene and polygenic traits.  Describe genetic drift.  Explain how different factors affect genetic equilibrium.

How Natural Selection Works An is any genetically controlled trait that increases an individual’s fitness. evolutionary adaptation

Natural Selection on Single-Gene Traits Natural selection on single-gene traits can produce changes in allele frequencies that may be reflected by simple changes in phenotype frequencies.

Natural Selection on Polygenic Traits Natural selection on polygenic traits can produce three types of selection: directional selection stabilizing selection disruptive selection

Directional Selection Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end.

Stabilizing Selection Individuals near the center of the curve have higher fitness than individuals at either end.

Disruptive Selection Phenotypes at the upper and lower ends of the curve have higher fitness than individuals near the middle.

Genetic Drift Genetic drift is a random change in allele frequency. Genetic bottlenecks The founder effect Founding populations Descendants

Evolution Versus Genetic Equilibrium If a population is not evolving, the population is in genetic equilibrium. Sexual reproduction Hardy – Weinberg principle

If p = 0.40 and q = 0.60: Probability of genotype aa: If p = 0.40 and q = 0.60: Probability of genotype aa: If p = 0.40 and q = 0.60: Probability of genotype Aa: If p = 0.40 and q = 0.60: Probability of genotype Aa: If p = 0.40 and q = 0.60: Probability of genotype AA: Hardy–Weinberg Principle and In words, this is stated: (frequency of AA) + (frequency of Aa) + (frequency of aa) = 100% and (frequency of A) + (frequency of a) = 100% 16%36% 48%

Hardy–Weinberg Principle Nonrandom mating Small population size Gene flow from immigration or emigration Mutations Natural selection

Student Worksheet Answers