Cutter_Lecture_09 - 1 Population Genetics of Natural...

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Unformatted text preview: 1 Population Genetics of Natural Selection • Natural selection ≠ Evolution – Evolution requires 2 steps: • Origin of genetic variation (Mutation, Recomb, Migration/Hyb) • Changes in allele frequency (Drift, Selection) – Evolution by natural selection requires heritable differences • Differences in reproductive success (fitness) • Not just any heritable difference…must affect fitness to be subject to selection – Fitness varies due to phenotypic differences • Useful to talk about the fitness of genotypes that underlie the phenotypic differences • Phenotype is proximate cause, Genotype is ultimate cause, of variation in fitness Genetical Theory of Natural Selection 2 • Consequences of NS depend on… Tracing evolution 3 2. Relationship between phenotype and genotype 3. Relationship between fitness and genotype 4. Evolutionary change 0. Phenotypic variation 1. Relationship between phenotype and fitness Natural selection occurs Heritable phenotypic variation Heritable variation in fitness • 3 primary modes of selection on a quantitative trait (continuous variation) • Given heritable variation and selection… Relationship between phenotype and fitness 4 Breeding individuals NON ‐ breeding individuals • 1 ‐ locus 2 ‐ alleles: – A 2 A 2 individuals largest, A 1 A 1 individuals smallest – Directional selection for large size • Directional selection increases relative frequency of genotypes that encode the more extreme trait values Effects of directional selection on genotypes 5 size Breeding individuals NON ‐ breeding individuals • 1 ‐ locus 2 ‐ alleles: – A 2 A 2 individuals largest, A 1 A 1 individuals smallest – Directional selection for medium size – E.g. Heterozygote advantage • Stabilizing selection decreases phenotypic variance Effects of stabilizing selection on genotypes 6 size • 1 ‐ locus 2 ‐ alleles: – A 2 A 2 individuals largest, A 1 A 1 individuals smallest – Directional selection for large OR small size – E.g. Heterozygote disadvantage • Diversifying = disruptive selection increases phenotypic and genotypic...
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This note was uploaded on 04/22/2009 for the course EEB 318 taught by Professor Cutter during the Spring '09 term at University of Toronto.

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Cutter_Lecture_09 - 1 Population Genetics of Natural...

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