Chapter 17.2: Evolution as Genetic Change in Populations

Natural Selection on Single-Gene Traits Natural selection selects for traits Genes create traits There can be a wide variation in traits and genes Some traits are a better fit for the environment than others, so the genes that create them are also a better fit

Directional Selection Polygenic traits, like height or size, have many variations One extreme phenotype in the range of variation has higher fitness than others This phenotype is favored and selected for

Stabilizing Selection Individuals with intermediate (medium, average) phenotypes have higher fitness than others This phenotype is favored and selected for

Disruptive Selection Individuals with both types of extreme phenotypes have higher fitness than others These phenotypes are selected for, and the intermediate phenotype is selected against

Genetic Drift Is a random change in allele frequency An allele can become more or less common in a population by chance Occurs more frequently in small populations Can be caused by human actions, disease, or natural disasters

Bottleneck Effect Is a change in allele frequency following a catastrophic reduction in the size of a population Can sharply reduce a population’s genetic diversity

Founder Effect Is when a small group from one population leaves and founds a new population elsewhere The founding group that left may have different allele frequencies than the original population