Response to selection can be fast Selection is strong Favored allele is partially dominant Both alleles are common Selection is not always Directional Heterozygote advantage Fitness AA Aa aa Heterozygote advantage Frequency dependence Selection varying in space or time 1 Relative fitness of hemoglobin genotypes in Yorubans Relative Fitness HbA HbA HbA HbS 0 88 1 0 Fitness in symbols 1 t Selection coefficients t 0 12 1 HbS HbS 0 14 1 s Variable selection genotypes have different fitness effects in different environments 1 0 9 0 8 Equilibrium frequencies peq s s t 0 86 0 12 0 86 0 88 qeq t s t 0 12 0 12 0 86 0 12 Predict the genotype frequencies at birth HW proportions 0 774 0 211 s 0 86 AA Aa aa Fitness 0 7 0 6 0 5 0 4 0 0144 Frequency dependent selection Env 1 Env 2 Env 3 Selection Whether directional or stabilizing causes adaptive changes in allele frequencies 2 10 Populations N 15 Forces causing evolution Random Genetic Drift Changes in allele frequency due to random sampling not adaptive Drift occurs even in large populations N 10 000 Genetic drift eliminates genetic variation 3 How common is mutation Achondroplastic dwarfism Forces that cause evolution Mutation Ultimate source of all genetic variation Mutation is generally not adaptive Mutation Selection Balance Even highly deleterious mutations can persist at substantial frequency especially if they are recessive AA 1 For recessive lethal s 1 Aa 1 Mutation selection equilibrium Recessive deleterious alleles qe s Selection against a recessive allele is s Genotype Fitness Dominant autosomal allele Recurrent mutation rate 3 200 000 0 000015 per generation q0 0 0 q1 0 000015 q2 0 000030 aa 1 s If a recessive lethal s 1 has a recurrent mutation rate of 1 5 10 5 what is it s equilibrium frequency qe 0 004 4 Mutation maintains substantial genetic variation Organism C Elegans D melanogaster Mouse Human Deleterious mutations per genome gener n 0 04 0 14 0 9 1 6 Forces causing evolution Non random mating Inbreeding Mating between relatives HIV virus is thought to have mutation rate 10 X greater than humans What happens to genotype frequencies under inbreeding Most extreme form of inbreeding is selfing P F1 25 AA Aa x Aa 50 Aa 25 aa F2 37 5 AA 25 Aa 37 5 aa F3 43 75 AA 12 5 Aa 43 75 aa Fewer heterozygotes and more homozygotes each generation What happens to heterozygosity under inbreeding Generations of selfing 0 1 2 3 Heterozygosity Prop of heterozygotes 100 Aa 50 Aa 25 Aa 12 5 Aa 5 What happens to allele frequencies under inbreeding Inbreeding Depression 70 P F1 F2 F3 25 AA Aa x Aa 50 Aa 25 aa 37 5 AA 25 Aa 37 5 aa 60 43 75 AA 12 5 Aa 43 75 aa 50 Yield 40 30 20 10 0 Allele frequencies do not change under inbreeding but population is perturbed from H W proportions Pup survival relative to Inbreeding Inbreeding Coefficient 0 19 0 25 0 67 0 67 0 0 25 0 5 0 75 1 Inbreeding Coefficient Proportions of individuals w genetic disease who are products of first cousin marriages Survival 75 51 25 Brother sister or parent offspring mating reduces the heterozygosity by 25 per generation G0 H 1 G1 H G2 H 6 Migration between subpopulations Tends to equalize allele frequencies among subpopulations even if the allele frequencies differ because of differing selection pressure Migration island model qm 0 9 Migration rate m 0 05 q 0 1 q 1 m q mqm q m q qm q 0 1 0 04 0 14 Evolution is the result of violating assumptions of H W Practical Considerations These ideas are straightforward Mathematics can be complicated especially when multiple evolutionary forces are occurring simultaneously Evolution of pathogens HIV SARS West Nile Virus etc Evolution of antibiotic resistance Evolution of pesticide and herbicide resistance Conservation of genetic diversity in natural captive and agricultural species 7
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