1Bio1B Evolution 4Last lecture:• More history - Darwin+Mendel => the neodarwinian synthesis• Mechanisms of evolution:– Evolution in populations - population genetics– Allele, genotype and phenotype frequencies– Predicting genotype freq’s: Hardy (Castle) Weinberg Equilibrium• Application: Null model for evolutionToday• Predicting genotype freq’s: Hardy (Castle) Weinberg Equilibrium– Application: Predicting heterozygote frequencies for recessive traitsEvolutionary processes• Sampling effects => “genetic drift”– Relevance in evolution - loss of variation, bottlenecks• Mutation as the ultimate source of variation; effects on fitness• Selection– fitness– Forms and consequences2Applications of HWE• A null model forevolution– Deviations fromexpected proportionsindicate somethinginteresting - but what?• Predicting frequencyof heterozygotes forrecessive alleles, e.g.cystic fibrosisCystic fibrosis: Mapped to chloridetransport gene on chromosome 7Common mutation, !F508 isrecessive and at p = 0.02 incaucasian populationF(het) = 2pq = 0.04 (carriers)F(hom) = p2 = 0.0004 (affected)3Hardy-Weinberg genotype frequencies as a function ofallele frequencies at a locus with two allelesFutuyma, 2nd Ed.4Effect of small population size - “genetic drift”• Sampling gametes => zygotes• Small population have greater sampling error =>larger fluctuations in allele frequency=> reduced variation within populationsFig. 23.85Population bottlenecks• Habitat loss or over-harvesting• Colonization of new areas (eg.islands; humans “Out of Africa”! Loss of genetic diversity! Rapid change in allelefrequencies => divergenceFig. 23.9Fig. 23.106Mutations - formsFig. 17.22HbbAHbbSFig 18.8Regulation CodingChanges in:Coding sequenceGene regulationGene copy numberChromosome number &structureExons -> mRNA -> aa7Agouti melanisticmutations:Recessive [regulatory& coding]Mc1r MelanisticmutationsDominant[structural]So what? … Mc1r & melanoma!XSome key genes in melanin production pathway8Genetic fitness• Selection acts throughthe phenotype• Fitness = Survival andreproduction relative toother phenotypes orgenotypes in thepopulation• Relative fitness can beenvironment dependentSorry Arnie….9Forms of selection (Fig. 23.13)10Relative fitnesses:Without malaria:HbbAA > HbbAS > HbbSSanaemiaWith malaria:HbbAS > HbbAA > HbbSSExample of Heterozygote advantagesickle-cell anaemiaMoreresistantto malariaNote - fitness of HbbAS depends onenvironment (+ malaria)1112Association between melanicphenotype and Mc1r allele in rockpocket mice from Pinacates lava flowHopi Hoekstra inthe field..Geographic cline13Genomic signatures of recent selectionNew mutation arisesthat increases fitnessUnder directional selectionincreases to p = 1; dragslinked sites with itResults in a region of lowvariation relative to othersVariationVariable sitehigh highhighlowStorz 200514Genomicsignatures ofselection;localizedreductions indiversitySutter et al. 2007Science 316:112What’swith mycrazy
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