Evolution of Populations What are three ways that

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

Video Explanation The Making of the Fittest Series HHMI “Natural Selection in Humans”

Individuals in Populations with Beneficial Genetic Traits Can Leave More Offspring STEPS to EVOLUTION 1. Development of genetic variations ◦ 2. Occurs through mutations in reproductive cells Natural selection ◦ ◦ ◦ Acts on individuals Genetically based traits enhance your ability to survive & reproduce For natural selection to occur, a trait must be heritable & lead to differential reproduction

Summary of Biological Evolution Genes mutate Individuals are selected Populations evolve ◦ Are now better adapted to survive & reproduce under existing environmental conditions

Evolution by Natural Selection

3 Choices! When environmental conditions change, populations ◦ Adapt ◦ Migrate ◦ Become extinct

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

This is an example of disruptive selection because one phenotype is favored on the lava flows and the other is favored on the sandstone rocks.

Video Explanation The Making of the Fittest Series HHMI “Natural Selection and Adaptation”

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

Loss of Genetic Variation The original population is on the left with three possible founder populations on the right.

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