EEB214H1 Lecture 7: Selection

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Ecology & Evolutionary Biology
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Jennifer Carpenter

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EEB214 Lecture 7: Selection • Selection is the change in allele frequency caused by organisms with certain alleles surviving or reproducing more than those with other alleles o so certain alleles will do better • Example of cabbage: to make other vegetables and pigeons • Fitness: how good an allele is o An individual’s fitness is it’s genetic contribution to the next generation o How to measure them: mating, raw number of offspring, how many seeds or pollen a plant made o Measuring fitness is hard ▪ count “babies” ▪ example: each pigeon has different number of offspring so they have different allele ▪ but doesn’t always work because the pigeons [2 generations later] those baby pigeons might not reproduce ▪ inclusive fitness o we need to scale the number: scaling them by the maximum fitness in the population o take all fitness numbers and divide by the maximum number [use Wsymbol for fitness] o we often measure relative fitness, that is fitness compared to the best individual in the population o sometimes we’ll also scale by the average fitness • We can compare the fitness of different genotypes to make a simple model of selection • difference in fitness can show how bad the allele is • Since we’re using relative fitness the best genotype will always have a fitness = 1 • 1-s  s = difference in fitness between the two homozygotes • 1 – hs  h is the dominance coefficient, and determine which allele is recessive and by how much o the h is determined by the dominance of the alleles • 1 = measure homozygote • When h is 0, what does the heterozygote look like? o then h = 0 o The heterozygote has the same fitness as the AA homozygote, so the A allele is completely dominant o the heterozygote has the same fitness as the aa homozygote, so the allele is completely dominant • Another example: flower colour from the bees but the heterozygote is pink...they’re a mix of the homozygotes • h  how far between the two homozygotes you are • If h is 0.5 the heterozygote that means about the fitness of heterozygotes? o WAA = 1 o WAa = 1.0.8s o Waa = 1-s o if we have something between half and one it’s closer to the small allele • 3 different kinds of selection: o Directional selection is when one allele is favored, having more of that allele is better ▪ the population is moving towards a particular allele/phenotype ▪ Dawson testes how fast a recessive lethal i
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