EVOLUTION AT MULTIPLE LOCIPowerPoint PresentationSlide 3Shell diameter in snailsSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24EVOLUTION AT MULTIPLE LOCII. Properties of Quantitative TraitsA. Underlying control of expression is POLYGENIC1. Simultaneous segregation of many genes2. More loci that affect the trait, greater the possible number of phenotypes3. Each of the loci affecting the trait has a small effect-phenotypic distinctions between classes are blurred -continuous distribution of phenotypesShell diameter in snails4. ADDITIVE effects of alleles at different loci-alleles at different loci add together to produce the final phenotypeA1A1 A1A2 A2A2B1B1 6 5 4B1B2 5 4 3B2B2 4 3 2With the addition of each “1” allele, 1 mm is added to the 2 mm base diameter of the shell (A2A2/B2B2)Shell diameter in snails-more than one genotype produces the same phenotypeA1A1 A1A2 A2A2B1B1 6 5 4B1B2 5 4 3B2B2 4 3 2A1A1/B1B2 and A1A2/B1B1 both produce 5 mm shells5. Strong environmental component in expression of quantitative traits-same genotype many have DIFFERENT phenotypes in different environments1.Maternal effects2.Environmental induction of different phenotypes = PHENOTYPIC PLASTICITY PhenotypeEnvironmentB. Types of Quantitative Traits1. Continuous traits-distribution of phenotypes is continuous approximating a normal distribution-height, weight, growth rate 2. Meristic traits-phenotype is expressed in discrete integral classes-flower number, offspring number, litter size3. Threshold traits-trait is governed by some threshold expression of loci involved–each locus produces a particular biochemical product and the sum of the products determines the phenotype expressedPhenotypeNo. of lociEVOLUTION AT MULTIPLE LOCIII. P = G + EA. G when all loci are Codominant1. Phenotypic trait value of heterozygote is exactly intermediate2. Alleles have ADDITIVE EFFECTS (A)Example: spot number in lady bird beetlesA1A1 = 2.0 A1A2 = 1.0 A2A2 = 0.0B. G when some loci exhibit dominance1. Types of Dominance effects (D)A1A1 A1A2 A2A2No dominance 20 15 10Complete dom. 20 20 10Partial dom. 20 17 10No dominance: G = GA onlyComplete dominance: G = GD onlyPartial dominance: G = GA = GDP = GA + GD +EEVOLUTION AT MULTIPLE LOCIIII. Estimating the components of phenotypic variation (VP)A. Mean and Variance1. Mean phenotype = µ-average of the genotypic trait values weighted by the frequencies of each genotypeExample:Genotype Trait Value Relative freq.A1A1 3.0 mm 0.04A1A2 2.0 mm 0.32A2A2 1.0 mm 0.64Mean = (3.0)(0.04) + (2.0)(0.32) + (1.0)(0.64) = 1.4 mm2. Variance (VP) is the deviation from the mean-Populations may have same mean yet different variances-Due to the genotypic effects: GA and GD-Due to environmental effects: E 1.4 mmVP = VGA + VGD + EPHENOTYPEFREQUENCYµContributions to VP3. Estimating VGA (no dominance effects)VGA = P(x11 - µ)2 + H(x12 - µ)2 + Q(x22 - µ)2Example:Genotype Trait Value Relative freq.A1A1 3.0 mm 0.04A1A2 2.0 mm 0.32A2A2 1.0 mm 0.64Population µ = 1.4 mmVGA = 0.04(3.0 - 1.4)2 + 0.32(2.0 - 1.4)2 + 0.64(1.0 - 1.4)2 = 0.1024 + 0.1152 + 0.1024 = 0.324. Only additive genetic variance (VGA) is heritableB. Narrow sense Heritability (h2)1. Extent to which variation in phenotypes is determined by the alleles transmitted by parentsh2 = VGA/VP h is not squared2. Measures the “evolvability” of a trait in a populationNo VGA or all VE50% VGA + 50% VGD and/or VE100% VGA3. Estimates of heritability for some traits in natural populations are highh2 = 0.98 for beak depth in song sparrowsProvides ample opportunityFor natural selection toOperate it has to be HeritableIV. Types of multi-locus natural selectionA. Directional selection--selection favors one or the other homozygous phenotypes-A1A1 is most fit OR A2A2 is most fitW11 > W12 > W22Or W11 < W12 < W22PhenotypeBefore selection--Gen. 1 After selection--Gen. 2FitnessB. Stabilizing selection-intermediate phenotype is most fit-heterozygote superiority but not alwaysW12 > W11 = W22Before selection--Gen. 1 After selection--Gen. 2FitnessNo. of individualsC. Disruptive selection-BOTH extremes have higher fitness than intermediate phenotype-disruptive selection can lead to speciation if two extremes become so different that they no longer exchange genesW11 = W22 > W12No. of individualsBefore selection--Gen. 1 After selection--Gen.