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presentation_14 - Lecture 14 Natural selection on...

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1 Lecture 14 Natural selection on simple (1-locus) traits Random mating No mutation No migration No selection Population is infinitely large - no chance events No evolution: The frequency of variant alleles and genotypes does not change over time. Assumptions of H-W
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2 H-W model is a “null” model - no process that could result in evolution takes place. Deviations from H-W predictions about allele and genotype frequencies mean that one or more assumptions of this model are violated Next two lectures: remove assumption of no natural selection. H-W as null hypothesis Individuals in a population vary in offspring production. Individuals vary for some character(s). Variation in offspring production is consistently related to variation in the character states (phenotypes). Variation in phenotypes are at least partially heritable. Evolution by natural selection: The frequency of these successful variants increases in the next generation. Natural Selection
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3 What happens to allele frequencies over time? Is genetic variation maintained? Questions In general, the average success of individuals in a population increases across generations. However, it is not always the “best” genotype that increases in frequency (see heterozygote disadvantage. With a single locus trait, genotypes can be substituted for phenotypes. Fitness = average reproductive success of a genotype. Fitness is equivalent to the represent- ation of genotypes in subsequent generations. Fitness
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4 Two genotypes (with different per capita growth rates) in an asexually reproducing population Fitness is environment specific Growth of spectinomycin susceptible (wild type) & resistant strains of Chlamydia bacteria Spectinomycin in growth medium Growth rate of wild type > resistant strains Absent Present
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5 Background Consider single locus, two allele (A 1 , A 2 ) models.
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