Presentation_14 - 1 Lecture 14 Natural selection on simple(1-locus traits • Random mating • No mutation • No migration • No selection •

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Unformatted text preview: 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 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 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 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 5 Background • Consider single locus, two allele (A 1 , A 2 ) models....
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This note was uploaded on 09/28/2008 for the course BIOEE 2780 taught by Professor Geber during the Fall '07 term at Cornell University (Engineering School).

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Presentation_14 - 1 Lecture 14 Natural selection on simple(1-locus traits • Random mating • No mutation • No migration • No selection •

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