1Bio1B Evolution 3Last lecture:• Natural selection - principles, lines of evidence in the “Origin”• Descent with modification• Estimation & interpretation of phylogeny• Some major insights about the “Tree of Life”– 3 kingdoms: Archaea, Bacteria, Eukarya– Metazoan origins & relationships (not covered)Today• 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 evolution• Application: Predicting heterozygote frequencies for recessive traits2Mendel’s principles ofinheritance (1865) [see Ch 14]• Alternative versions of genes (alleles)account for variation in inheritedcharacters• For each character, an organism inherits2 alleles, one from each parent• If the 2 alleles at a locus differ, then thedominant allele determines phenotype• The 2 alleles for a heritable charactersegregate during gamete formation (Lawof Segregation)• Each pair of alleles segregatesindependently of others during gameteformation [for unlinked genes]Mendel’s garden; BrnoDominance of purple (P) overwhite (p) flower color: Fig. 14-5Appearance:P GenerationGenetic makeup:GametesF1 GenerationAppearance:Genetic makeup:Gametes:F2 GenerationPurple flowersPpPp1212PpF1 spermF1 eggsPP PpPp ppPp3 : 1PurpleflowersPPWhiteflowersppPpRedCRCRGametesP GenerationCRCWWhiteCWCWPinkCRCWCRGametes CWF1 GenerationF2 GenerationEggsCRCWCRCRCRCRCWCRCWCWCWCWSperm121212121212Co-dominance - heterozygote isintermediate (pink) in snapdragons:Fig. 14.104Genotype and allele frequencies for a locus with two allelesFutuyma, 2nd Ed.Gametes for each generation aredrawn at random from the gene poolof the previous generation:80% CR (p = 0.8)20% CW (q = 0.2)SpermCR(80%)CW(20%)pqp216%CRCW64%CRCREggsCW(20%)CR(80%)16%CRCWqp4%CWCWq2q2A2A2qpA2A1pqA1A2p2A1A1male gametesf(A1) = p f(A2) = qfemale gametesf(A2) = q f(A1) = pHardy-Weinberg Equilibriumgeneral caseA1A1 = p2A1A2 = 2pqA2A2 = q2ExpectedgenotypefrequenciesFig 23.76Hardy-Weinberg Equilibrium• Predicts genotype (& phenotype) frequencies fromallele frequencies• Genotype frequencies in expected proportions ina single generation• Allele (& genotype) frequencies constant acrossgenerations => inheritance alone does not causeevolution• Assumptions– Random mating (for this gene/trait)– No mutation, selection, migration– Large population (no drift)7Applications 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)8Hardy-Weinberg genotype frequencies as a function ofallele frequencies at a locus with two allelesFutuyma, 2nd
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