10/17/20111Genetic change may occur through:• Natural Selection• Genetic DriftA genetic definition of evolution:Evolution is any genetic changein a population.• MigrationMutation1.There must be phenotypic variation in the population, and that phenotypic variation has Selection… happens when individuals vary in fitness, due tosome having higher rates of survival and/orreproduction.to affect fitness.2.The phenotypic variation that affects fitness must be inherited from parent to offspring. (Non-genetic phenotypic variation does not contribute to evolution.)Definition of FITNESS (of a genotype)Selection… happens when individuals vary in fitness, due tosome having higher rates of survival and/or reproduction.The average genetic contribution of individuals of that genotype make to the next generation.This means fitness is strictly a function ofreproductive success!However, you have to survive to reproduce,so survival (of the fittest) also matters.Definition of FITNESS(of a genotype)The average genetic contribution of individuals of that genotype make to the next generation.In real life, we can never experimentally measure “fitness”. The best we can do is estimate it based on phenotypes we think are good predictors of fitness.For example:• survival•mating success•number of progeny producedTwo common modes of natural selection:1.Purifying selection•testing for purifying selection•mutation-selection balance2DitilltiWays in which selection can act on phenotypes, regardless 2.Directional selection•genetically simple directional selection•testing for directional selectionof the underlying genetic basis.opulationectionPurifying (stabilizing) selectionOrange arrows indicate phenotypes disfavoredby selection.phenotypic valuefrequency in pbefore sele
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