Genetic drift 10/13/2014. Random factors in evolution. Sampling error. Genetic drift. Random walk. Genetic drift

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1 Random factors in evolution Mutation is random is random is random fluctuations in frequencies of alleles or haplotypes Due to violation of HW assumption of large population size Can result in nonadaptive evolution accounts for much of the DNA variation between species Considered the null hypothesis to explain an evolutionary observation Sampling error Sample drawn from a population is likely to vary from the population by chance Genes that make it into the next generation can just be a random sample of genes in the population (assume alleles are neutral with respect to fitness) Imagine: 50 individuals, every allele unique 100 alleles, q i = 0.01 Random mating: Draw gametes from population at random, with replacement By chance, 10 individuals don t get sampled 20 alleles are lost! Some other alleles were drawn more than once - their frequency is now > 0.01 Random walk Frequency of one gene will eventually reach 0 or 1 by chance if there is no stabilizing force to return the frequency towards 0.5 Allele frequencies fluctuate at random within a population, until eventually one or another allele becomes fixed Frequency of heterozygotes H = 2p(1-p) declines Rate of decline in heterozygosity is used as a measure of rate of genetic drift Frequency of an allele is equal to its probability of fixation 1

2 The probability that a given allele will become fixed is equal to the initial frequency of that allele An allele is more likely to be fixed in a small population than a large population A new mutation, p = 1/2N Therefore as N gets larger, p gets smaller Happens faster in small populations Average time is 4N generations Coalescence As time goes on, more lineages become extinct All alleles are ultimately descended from a common ancestor Populations coalesce back to a single common ancestor One allele becomes fixed 2

3 Effective population size (N e ) Effective population size is smaller than censused population size Nonbreeding individuals are not included in effective population size Factors that reduce N e Variation in number of progeny Uneven sex ratio Natural selection can change progeny number Generations overlap Fluctuations in population size Population bottlenecks Restrictions in size through which populations may pass Founder effect occurs when a new population is established by a small number of colonists Bottleneck Mauna Kea Silversword Down to 16 Individuals Monocarpic perennials Self-incompatible Most s-alleles lost by drift Some are left with no possible mate! Bottleneck Northern Elephant Seal N 1890 s =20 N 1990 s = 120,000 Extremely low genetic diversity no variation in 24 loci 3

4 Founder Effect Amish Colonies Founded , N<200 Married within religion, few converts after ,000 in Lancaster Co. PA, 100 founders Lancaster Amish: Ellis-van Creveld: p=0.13 Pyruvate kinase deficiency Ohio Amish: High frequency of hemophilia Neutral theory Asserts that the great majority of mutations that are fixed are neutral with respect to fitness Fixed by genetic drift Creates molecular clock DNA sequencing supports neutral theory Synonymous mutations happen more often than replacements Rates of substitutions are higher in introns and pseudogenes Rate of evolution is higher in genes that are least likely to affect function Gene flow Rate of fixation is inversely proportional to population size Drift is counteracted by gene flow (m) from other populations 4

5 Measuring gene flow Migration counteracts divergence by drift N m = ((1/F ST )-1))/4 N m is number of immigrants per generation Assume alleles are neutral Assume that allele frequencies have reached an equilibrium between genetic drift and gene flow Nm F ST = 1 / (4Nm + 1) Fst max differentiation Pocket gophers 5