Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Click to edit Master subtitle style 1/12/13 Chapter 6The ways of change: drift and selection1/12/13 Population genetics•Study of the distribution of alleles in populations and causes of allele frequency changes1/12/13 Key Concepts•Diploid individuals carry two alleles at every locus–Homozygous: alleles are the same–Heterozygous: alleles are different•Evolution: change in allele frequencies from one generation to the next1/12/13 Hardy-Weinberg equilibrium•Population allele frequencies do not change if:–Population is infinitely large–Genotypes do not differ in fitness–There is no mutation–Mating is random–There is no migration1/12/13 Predictions from Hardy-Weinberg•Allele frequencies predict genotype frequenciesp2 + 2pq + q2 = 11/12/13 Key Concepts•Hardy-Weinberg theorem proves that allele frequencies do not change in the absence of drift, selection, mutation, and migration•Mechanisms of evolution are forces that change allele frequencies1/12/13 Populations evolve through a variety of mechanisms1/12/13 Key Concept•Hardy-Weinberg serves as the fundamental null model in population genetics1/12/13 Genetic drift causes evolution in finite populations1/12/13 Genetic drift results from random sampling errorSampling error is higher with smaller sample1/12/13 Drift reduces genetic variation in a population•Alleles are lost at a faster rate in small populations–Alternative allele is fixed1/12/13 Key Concepts•Genetic drift causes allele frequencies to change in populations•Alleles are lost more rapidly in small populations1/12/13 Bottlenecks reduce genetic variationA bottleneck causes genetic drift1/12/13 Rare alleles are likely to be lost during a bottleneck1/12/13 Founder effectFounder effects cause genetic drift1/12/13 Key Concept•Even brief bottlenecks can lead to a drastic reduction in genetic diversity that can persist for generations1/12/13 The concept of fitness•Fitness: the reproductive success of an individual with a particular phenotype•Components of fitness:–Survival to reproductive age–Mating success–Fecundity•Relative fitness: fitness of a genotype standardized by comparison to other genotypes1/12/13 Contribution of alleles to fitness•Average excess fitness: difference between average fitness of individuals with allele vs. those withoutΔp = p x (aA1/ )ϖ1/12/13 Natural selection more powerful in large populations•Drift weaker in large populations•Small advantages in fitness can lead to large changes over the long term1/12/13 Pleiotropy may constrain evolution•Pleiotropy: mutation in a single gene affects many phenotypic traits–Can be antagonistic–Net effect on fitness determines outcome of selection1/12/13 Pesticide resistance and pleiotropy1/12/131/12/13 Pesticide resistance and pleiotropy1/12/13 Experimental evolution provides important insights about selection1/12/13 Natural selection in actionAlleles that lower fitness experience negative selectionAlleles that increase fitness experience positive selection1/12/13 Relationships among alleles at a locus•Additive: allele yields twice the phenotypic effect when two copies present•Dominance: dominant allele masks presence of recessive in heterozygote1/12/13 Effects of selection on different types of alleles1/12/13 Mutation generates variation •Mutation rates for any given gene are low•But, considering genome size and population size many new mutations arise each generation–Estimate in humans: 9.8 billion new mutations•Source of variation for selection and drift to act1/12/13 Mutation-selection balance•Equilibrium frequency reached through tug-of-war between negative selection and new mutation•Explains persistence of rare deleterious mutations in populations1/12/13 Balancing selection•Some forms of selection maintain diversity in populations:–Negative frequency-dependent selection–Heterozygote advantage1/12/13 Negative frequency-dependent selection1/12/13 Heterozygote advantage and sickle-cell anemia1/12/13 Key Concepts•Selection occurs when genotypes differ in fitness•Outcome of selection depends on frequency of allele and effects on fitness•Population size influences power of drift and selection–Drift more powerful in small population–Selection more powerful in large population1/12/13 Key Concepts•Alleles may have pleiotropic effects–When fitness effects oppose each other environment determines direction of selection•Laboratory evolution studies reveal how alleles rise and spread through populations•Rare alleles almost always carried in a heterozygous state–Recessive alleles invisible to selection–Selection cannot drive dominant to fixation1/12/13 Key Concepts•Mutations are the source of new genetic variation in populations–Can be many in a large population•Balancing selection maintains multiple alleles in populations–Negative frequency-dependent–Heterozygote advantage1/12/13 Inbreeding and the Hapsburg dynasty1/12/13 Inbreeding coefficient•Probability that two alleles are identical by descent1/12/13 Inbreeding depression results in reduced fitness•Rare deleterious alleles more likely to combine in homozygotes1/12/13 Key Concepts•Alleles are identical by descent if they both descended from a single mutational event•Inbreeding increases percentage of loci that are homozygous for alleles identical by descent•Genetic bottlenecks often go hand in hand with inbreeding and selection–Recessive alleles exposed to