Extra credit question from last Tuesday s lecture Do these genotype frequencies match HW expectations MM 298 MN 489 NN 213 Give 2 value df 1 P value and state whether you reject or do not reject the hypothesis that the observed frequencies match the expected frequencies Forces that cause deviation from H W evolution 1 2 3 4 5 Selection Mutation Genetic Drift Nonrandom Mating Gene Flow Migration Selection Consistent differences in survival or reproduction between genotypes genotypic specific differences in fitness When fitness values are expressed on a scale such that highest fitness 1 then the values are called relative fitness To conveniently calculate change in allele frequency due to selection need concept of average fitness Change in allele frequency Genotype AA Genotype Frequency p2 Relative Fitness Waa WAA Aa aa 2pq q2 WAa W average fitness p2WAA 2pqWAa q2WAa Freq of A after one gen of selection p p2 WAA W pqWAa W Freq of a after one gen of selection 1 p or q q2 Waa W pqWAa W CCR5 Example p 0 9 q 32 0 1 Genotype frequency p2 0 81 32 2pq 0 18 32 32 q2 0 01 Relative Fitness W 0 99 W 32 0 99 W 32 32 1 0 Average fitness W 0 81 0 99 0 18 0 99 0 01 1 0 9901 q q2W 32 32 W pqW 32 W 0 01009 0 089991 0 100091 p 1 q 0 89999 Next generation genotype freq p2 0 80998 2pq 0 18016 q2 0 01002 q q2 Selection will increase the frequency of 32 allele Selection is relatively weak The favored allele is recessive and the favored genotype is very rare The change in allele frequency response to selection will be relatively slow 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 Frequency dependence Selection varying in space or time Heterozygote advantage Fitnes s AA Aa aa Relative fitness of hemoglobin genotypes in Yorubans Relative Fitness HbA HbA 0 88 HbA HbS 1 0 HbS HbS 0 14 Fitness in symbols 1 t 1 1 s Selection coefficients t 0 12 s 0 86 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 0 0144 Variable selection genotypes have different fitness effects in different environments 1 0 9 0 8 AA Aa aa Fitness 0 7 0 6 0 5 0 4 Env 1 Env 2 Env 3 Frequency dependent selection Forces causing evolution Random Genetic Drift Changes in allele frequency due to random sampling Genetic drift eliminates genetic variation 10 Populations N 15 Drift occurs even in large populations N 10 000 Forces that cause evolution Mutation How common is mutation Achondroplastic dwarfism Dominant autosomal allele Recurrent mutation rate 3 200 000 0 000015 per generation q0 0 0 q1 0 000015 q2 0 000030 Mutation Selection Balance Even highly deleterious mutations can persist at substantial frequency especially if they are recessive Selection against a recessive allele is s Genotype Fitness AA 1 For recessive lethal s 1 Aa 1 aa 1 s Mutation selection equilibrium Recessive deleterious alleles qe 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 Mutation maintains substantial genetic variation Deleterious mutations Organism per genome gener n C Elegans 0 04 D melanogaster 0 14 Mouse 0 9 Human 1 6 HIV virus is thought to have mutation rate 10 X greater than humans Forces causing evolution Non random mating Inbreeding Mating between relatives 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 Prop of heterozygotes 0 100 Aa 1 50 Aa 2 25 Aa 3 12 5 Aa What happens to allele frequencies under inbreeding P F1 F2 F3 25 AA Aa x Aa 50 Aa 25 aa 37 5 AA 25 Aa 37 5 aa 43 75 AA 12 5 Aa 43 75 aa Allele frequencies do not change under inbreeding F is a measure of inbreeding F 1 Observed Het Expected Het F 1 H 2pq H 2pq 1 F Pop allele frequencies p q 0 5 Pop In H W 0 25 AA 0 50 Aa 0 25 aa 1 Gen Selfing F 0 5 0 375 AA 0 25 Aa 0 375 aa Corn yield in relation to Inbreeding 70 60 50 40 30 20 10 0 0 0 25 0 5 0 75 1 Inbreeding Coefficient Pup survival relative to Inbreeding Inbreeding Coefficient 0 19 0 25 0 67 0 67 Survival 75 51 25 Proportions of individuals w genetic disease who are products of first cousin marriages Evolution is the result of violating assumptions of H W Selection Mutation Non random mating Genetic Drift Migration Evolution is the result of violating assumptions of H W These ideas are straightforward Mathematics can be complicated especially when multiple evolutionary forces are occurring simultaneously Practical Considerations 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 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 p 0 5 q 0 5 Genotypes Number AA 25 Aa aa 50 25 Survival to reproduction 25 100 1 50 100 1 20 80 0 8 Gamete contribution 25 95 A 25 95 A 25 95 a 20 95 a New allele frequencies p 25 25 95 0 53 q 25 20 95 0 47 New genotype frequencies assume random mating AA Aa aa 0 28 0 50 0 22
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