Evolution of Populations What are the three ways that natural selection can change the distribution of a trait? How does genetic drift lead to the evolution of populations?

Natural Selection in Populations Traits are normally distributed in a bell- shaped curve Microevolution is the change in the frequencies of alleles over time The frequencies can change in one of three ways

Natural Selection in Populations Directional Selection occurs when one extreme phenotype is favored

Natural Selection in Populations Stabilizing Selection occurs when intermediate phenotypes are favored

Natural Selection in Populations Disruptive Selection occurs when both extreme phenotypes are favored

Other Mechanisms of Evolution Gene flow – movement of alleles between populations Increase in movement, increases genetic variation Decrease in movement, decreases genetic variation but increases evolution of new species

Other Mechanisms of Evolution Genetic drift Changes in allele frequency due to chance Affects small populations more than large populations Two causes: Bottleneck effect – event reduces size of population Founder effect – small number of individuals colonize a new area

Founder Effect and Ellis-van Creveld Syndrome

Other Mechanisms of Evolution Sexual Selection occurs when traits increase mating success Intrasexual selection is between males of same species Intersexual selection occurs when males display traits that attract the female