presentation_14_15 - Lecture 14 (cont d) Natural selection...

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1 Lecture 14 (cont±d) Natural selection at 1 locus traits Increase in frequency of best genotype & Fxation of good allele D. D. D. D. F. Selection Mode of Favors w A1A1 w A1A2 w A2A2 Outcome selection ________________________________________________________________ A 1 A 1 , A 1 A 2 111 - s ± x a t i o n o f A 1 directional A 1 A 1 1 1-hs 1-s ±xation of A 1 directional A 1 A 1 1 1-s 1-s ±xation of A 1 directional A 1 A 2 1-s 1 1-t both alleles balancing A 1 A 1 , A 2 A 2 1+s 1 1+t ±xation of A 1 or A 2 disruptive Fitness varies according to niche both alleles multiple-niche Fitness varies with genotype frequency freq.-dependent highest when rare both alleles negative highest when common best allele ±xed positive
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2 Disruptive selection Heterozygote has lowest ftness A1 or A2 is fxed, depending on which is most common initially. ftness Banding patterns oF polytene chromosomes in Drosophila allow us to see chromosomal inversions Heterozygotes For inversion have a Fertility disadvantage because oF meiotic problems
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3 Summary Modes of selection that do not maintain genetic variation at a 1-locus trait: directional, positive frequency dependent & disruptive (heterozygote disadvantage). Outcome of positive frequency- dependent and disruptive selection depends on initial allele frequencies. Both alleles & all 3 genotypes are maintained Selection Mode of Favors w A1A1 w A1A2 w A2A2 Outcome selection ________________________________________________________________ A 1 A 1 , A 1 A 2 111 - s ± x a t i o n o f A 1 directional A 1 A 1 1 1-hs 1-s ±xation of A 1 directional A 1 A 1 1 1-s 1-s ±xation of A 1 directional A 1 A 2 1-s 1 1-t both alleles balancing A 1 A 1 , A 2 A 2 1+s 1 1+t ±xation of A 1 or A 2 disruptive Fitness varies according to niche both alleles multiple-niche
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presentation_14_15 - Lecture 14 (cont d) Natural selection...

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