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Lecture 5 - Mutation and migration as evolutionary forces

Lecture 5 - Mutation and migration as evolutionary forces -...

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1 Lecture 4.1 - Balancing Selection and the Maintenance of Genetic Variation Look around - lots of genetic variation. Why? "Classical" view is that there is a single "wild-type" allele at any locus. Most mutations are deleterious, removed by selection. Occasional beneficial mutation sweeps to fixation. Idea dates back to Morgan. Alternative view, put initially put forth by Dobzhansky, is that balancing selection actively maintains genetic variation. 1. True overdominance - heterozygote is unconditionally advantageous Initial evidence came from chromosomal inversions in Drosophila, but many loci in an inversion, so not clear if any one locus is truly overdominant. - Classical example? Sickle-cell anemia. Cystic Fibrosis? (many common mutations) - Enzyme loci; different alleles may have different kinetics (e.g., optimum temperature), so het has wider "effective zone". - Some indirect evidence from Drosophila that male mating traits are maintained by overdominance Also disease-resistance loci MHC, HLA loci in vertebrates?? Probably not 2. Evolution in a variable environment Example: leaves vs. bark It is trivially obvious that the environment varies in time and space. Therefore, unless one genotype is most fit in ALL environments, selection will vary over time and space. - Can a spatially variable environment maintain genetic variation? Yes, condition for stable equilibrium is if the het is closer to the more fit homo, averaged over all env'ts. This is called "marginal overdominance" - Can a temporally variable (varying over time) environment maintain genetic variation by balancing selection? Yes, but opinions differ on how probable it is that environmental variation can maintain genetic variation. General condition is that geometric mean fitness of the het must be higher than either homo. Classical view (Haldane and Jayakar 1963) is that eventually you get enough zeros in a row to drive one allele extinct. 3. Antagonistic Pleiotropy
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2 a) Definition of "pleiotropy": One locus (gene) affects more than one phenotypic trait. -e.g., Albinism. Single aa substitution at the Tyr (tyrosinase) locus disrupts the melanin synthesis pathway and results in albinism. Affects pigmentation in skin, hair, eyes. - "Antagonistic" pleiotropy occurs when alleles that move one trait in the direction of higher fitness move another trait in the direction of lower fitness, i.e., a "fitness trade- off". - Fundamental assumption of " Life History" theory (LH ~ demography) is that there are trade-offs between "components of fitness" such as survival and reproduction. -
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Lecture 5 - Mutation and migration as evolutionary forces -...

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