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Evolution Genetic variation There can be no evolution without variation Mutations are the ultimate source of new variation in DNA Germ-line mutations are important for evolution Genetic variation in natural populations Natural populations exhibit great variation. Heterozygosity is approximately 10% for all loci. Maintaining genetic variation Neutral genetic diversity Balance between origin of variation by mutation and loss by drift Adaptive genetic diversity Balanced between origin of variation by mutation and the loss by selection Overdominance Selection favoring overdominance o favors heterozygotes o results in stable gene frequencies fitness SS SA AA Overdominance: Sickle Cell Anemia Overdominance: Sickle Cell Anemia SS = sickle-cell anemia SA = malaria resistance AA = Malaria susceptability Sickle-cell anemia Balancing Selection Case in which natural selection maintains genetic variation at frequencies above levels of mutation. …in this case, balancing selection makes the population as a whole more resistant to malaria Prion disease in Fore Tribespeople Kuru (prion disease related to Mad Cow disease) resulted from cannibalism Only heterozygotes for the Prion Protein Gene (PRNP) are resistant Underdominance Underdominance o heterozygotes have a lower fitness o results in an unstable equilibrium o Uncommon and few natural examples fitness SS SA AA Other forms of selection…. Frequency-dependent selection male andromorph gynomorph andromorph Rare male effect (negative frequency dependent mating success) Rare male effect Stabilizing Selection Selection that eliminates the extremes of a distribution and favors the centre Stabilizing Selection Stabilizing selection is common in quantitative traits. Disruptive Selection Selection that favors the extremes and eliminates the middle of a frequency distribution of trait values Dull yearlings allowed to nest on territory Bright yearlings can hold their own Disruptive Selection Selection that favors the extremes and eliminates the middle of a frequency distribution of trait values Expression of variation Expression of variation Feather modification Feather modification Environmentally-induced phenotypic responses Induced response: One genotype produces multiple phenotypes Environmentally induced response Daphnia can produce helmets and spines if predators are present Other responses Barnacles will change shape of growth if a predator snail present Snails change offspring number, shell size, shell organic content in utero in presence of crayfish And Matt’s favorite…. Phenotypes and reaction norms GENOTYPE PHENOTYPE Phenotypes and reaction norms GENOTYPE X ENVIRONMENT PHENOTYPE Phenotypes and reaction norms GENOTYPE X ENVIRONMENT PHENOTYPE No genetic variation underlying number of digits, so no sensitivity to population density Fecundity is much more sensitive to fecundity, and some genotypes more than others So, is phenotypic plasticity adaptive? The reaction norms themselves can be subject to selection Selection to increase sensitivity to population density would lead to the increase in frequency of G1 Selection to decrease sensitivity to density would increase the frequency of G3