Chapter 17:Evolution of PopulationsSection 17-2: Evolution as Genetic Change in Populations

How Natural Selection Works

•Evolutionary fitness = success in passing on genes

•Evolutionary adaptation = any genetically controlled trait that increases an organism’s ability to pass along its alleles

Natural Selection on Single-Gene Traits•Changes allele frequencies•Ex: Body color in lizards

Natural Selection on Polygenic Traits•Range of phenotypes•Fitness varies throughout the curve•Natural selection can affect phenotype

range, changing shape of bell curve

Directional Selection

•Occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

Stabilizing Selection

•Occurs when individuals near the center of the curve have higher fitness than individuals at either end

•Keeps center of curve, narrows overall graph

Disruptive Selection

•Occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle

•Acts against the intermediate phenotype, can create two distinct phenotypes

Genetic Drift

•Occurs in small populations•Allele becomes more or less common by

chance•Random

Genetic Bottlenecks

•The bottleneck effect is a change in allele frequency following a dramatic reduction in population size

•Ex: Natural disaster

The Founder Effect

•Occurs when allele frequencies change as a result of the migration of a small subgroup of a population

•Can create new populations very different from original group

The Founder Effect

Evolution vs Genetic Equilibrium•If allele frequencies in a population

remain the same it is in genetic equilibrium

•No evolution•The Hardy-Weinberg principle

describes the conditions under which evolution will not occur

•States that allele frequencies will remain constant unless something causes them the change

Hardy-Weinberg Principle

•5 conditions can disturb genetic equilibrium and cause evolution:▫Nonrandom mating – sexual selection▫Small population size▫Immigration or emigration▫Mutations▫Natural selection

Sexual Reproduction and Allele Frequency

•Sexual reproduction alone does not change relative allele frequency