1Response to selection can befast!Selection isstrongFavoredallele ispartiallydominantBoth allelesarecommonSelection is not always“Directional”• Heterozygote advantage• Frequency dependence• Selection varying in space or timeHeterozygoteadvantageFitnessA a a aA A2s=0.86t=0.12Selection coefficientsFitness (in symbols)Relative Fitness1-s11-t0.141.00.88HbS/HbSHbA/HbSHbA/HbARelative fitness of hemoglobin genotypes in YorubansEquilibrium frequencies:peq = s/(s+t) = 0.86/(0.12+0.86) = 0.88qeq = t/(s+t) = 0.12/(0.12+0.86) = 0.12Predict the genotype frequencies (at birth):HW proportions 0.774 0.211 0.0144Variable selection: genotypes have differentfitness effects in different environments0.40.50.60.70.80.91Env. 1 Env. 2 Env. 3AAAaaaFitnessFrequency-dependent selectionSelectionWhether directional or stabilizing,causes adaptive changes in allelefrequencies3Forces causing evolution:Random Genetic DriftChanges in allele frequency dueto random sampling: not adaptive10 Populations, N=15Drift occurs even in large populations!N=10,000Genetic drift eliminatesgenetic variation4Forces that cause evolutionMutationUltimate source of all genetic variationMutation is generally not adaptiveHow common is mutation?• Dominant autosomal allele• Recurrent mutation rate: 3/200,000 =0.000015 per generation• q0=0.0; q1 = 0.000015, q2 = 0.000030Achondroplastic dwarfismMutation/Selection BalanceEven highly deleterious mutations can persist atsubstantial frequency, especially if they arerecessive:Selection against a recessive allele is sGenotype AA Aa aaFitness 1 1 1-sFor recessive lethal, s = 1Mutation-selection equilibriumRecessive deleterious alleles:qe = √ (µ/s)If a recessive lethal (s=1) has a recurrentmutation rate of 1.5*10-5, what is it’sequilibrium frequency?qe = 0.0045Mutation maintains substantialgenetic variation Deleterious mutationsOrganism per genome/gener’nC. Elegans 0.04D. melanogaster 0.14Mouse 0.9Human 1.6HIV virus is thought to have mutation rate~10 X greater than humans!Forces causing evolution:Non-random mating:InbreedingMating between relativesWhat happens to genotypefrequencies under inbreeding? Most extreme form of inbreeding is selfingP: Aa x AaF1: 25% AA 50% Aa 25% aaF2: 37.5% AA 25% Aa 37.5% aaF3: 43.75% AA 12.5% Aa 43.75% aaFewer heterozygotes and morehomozygotes each generationWhat happens to heterozygosityunder inbreeding?Generations Heterozygosity:of selfing Prop. of heterozygotes 0 100% Aa 1 50% Aa 2 25% Aa 3 12.5% Aa6What happens to allelefrequencies under inbreeding?P: Aa x AaF1: 25% AA 50% Aa 25% aaF2: 37.5% AA 25% Aa 37.5% aaF3: 43.75% AA 12.5% Aa 43.75% aaAllele frequencies do not change underinbreeding, but population is perturbed fromH-W proportions.0102030405060700 0.25 0.5 0.75 1Inbreeding DepressionInbreeding CoefficientYieldPup survival relative toInbreedingInbreeding Coefficient Survival< 0.19 75%0.25-0.67 51%> 0.67 25%Brother-sister or parent-offspring mating reducesthe heterozygosity by 25% per generation:G0: H=1G1: H= ?G2: H= ?Proportions of individuals w/ geneticdisease who are products of firstcousin marriages7Migration betweensubpopulationsTends to equalize allelefrequencies amongsubpopulations, even if the allelefrequencies differ because ofdiffering selection pressureMigration: island modelq' = (1-m)q + mqm = q - m(q - qm)q = 0.1Migration rate=m=0.05qm = 0.9q' = 0.1 +0.04 = 0.14 Evolution is the result ofviolating assumptions of H-W• These ideas are straightforward.• Mathematics can be complicated,especially when multipleevolutionary forces are occurringsimultaneouslyPractical Considerations• Evolution of pathogens (HIV, SARS,West Nile Virus, etc.)• Evolution of antibiotic resistance• Evolution of pesticide and herbicideresistance• Conservation of genetic diversity innatural, captive, and
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