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Daily Warm-up February th 7 What is genetic drift? What are the two types? HW: -Darwin video due tomorrow -Read 21.4 Turn in: -Nothing 5 Evolutionary Agents Natural selection Sexual selection Mutation Gene flow Small population size (genetic drift) Genetic Drift Chance events that take out alleles at random Ex. Earthquake, flood More pronounced in small populations Two Types Founder effect Individuals become “founders” o f new population Bottleneck effect Population has experienced a “bottleneck” and certain alleles may be over-represented Founder effect example Cheetahs as a bottleneck example Genetic Drift Significant in small populations Causes allele frequencies to change at random Can lead to a loss of genetic diversity Can cause harmful alleles to be more prominent Gene Flow Transfer of alleles into or out of a population Question In what sense is natural selection more predictive than genetic drift? Natural Selection Only agent that consistently causes adaptive evolution Survival and reproductive advantage Can be measure by relative fitness Contribution to the gene pool by an individual relative to the other members of the population 3 Types of Evolution 1. Directional selection= conditions favor individuals at one extreme 2. Stabilizing Selection Intermediate phenotype versions are selected for by conditions 3. Disruptive Selection Intermediate phenotype is selected against, both extremes selected for by conditions. Sexual Selection Traits that may or may not improve the likelihood of obtaining a mate Sexual dimorphism- difference in secondary sex characteristics ex. Color, ornamentation, behavior Intrasexual Intersexual (mate choice) *Birds of Paradise Preservation of Genetic Variation Neutral variation- genes that do not confer any advantage or disadvantage help maintain genetic variation Also Diploidy- recessive allele is “hidden” Balancing selection- natural selection keeps two or more forms in the population Heterozygote advantage Frequency-dependent Heterozygote Advantage When heterozygotes have greater fitness than either homozygous forms Ex. Sickle-cell disease and malaria Frequency-Dependent Selection Fitness of phenotype depends on how common in the population. Ex. Fish in lake Fungi in population of insects (see video) Why doesn’t natural selection= perfect organism? 1. Selection can only act on existing variation 2. Evolution is limited by historical constraints -Limited to ancestral structures 3. Adaptations are often compromises 4. Chance, natural selection, and environment interact