Migration

1. Announcements Migration Gene flow between populations Effects Receiving population: Change in allele and genotype frequencies (away from H-W) Between populations: homogenization Prevents evolutionary divergence due to selection Migration-selection balance analogous to mutation-selection

2. unbanded snakes are cryptic and selected on island, yet banded snakes persist? why are there no unbanded snakes on mainland? • unbanded banded banded unbanded

3. Migration migration-selection balance analogous to mutation-selection balance migration selection pressure selection pressure population 2 population 1

4. Genetic Drift • Sampling error: chance differences due to finite population size • Effects • Change in allele and genotype frequencies (away from H-W) • 1) Causes loss of alleles (genetic diversity) • 2) Increases homozygosity Measures of genetic diversity H: mean fraction of individuals in a population heterozygous at a locus Fraction polymorphic: the fraction of loci in a population that have 2+ alleles (in practice, allele freq > 0.01) Allelic richness: Average number of alleles/locus

5. Drift – loss of genetic diversity Allele frequency fluctuates inversely with population size (N). Heterozygosity is maximized when f(A1) = 0.5 What happens to Hbar?

6. Drift causes a loss of H -directly proportional to time -inversely proportional to N Hg+1 = Hg [1 – 1/2n]

7. Measures of H in four plant species Fraction polymorphic: the fraction of loci in a population that have 2+ alleles (in practice, allele freq > 0.01)

8. Drift causes a loss of genetic diversity • Conservation implications of reduced genetic diversity: • Genetic diversity is the raw material for adaptive evolution. • As H decreases, homozygosity increases; therefore deleterious recessive alleles drag down average fitness (inbreeding depression).

9. Drift and rate of molecular evolution What happens when drift is the only evolutionary force at work? Rate of evolution: rate of substitution (fixation) of one allele for another. what is the fate of a new mutation: extinction, mutation-selection balance, or, fixation generation 1

10. Drift and rate of molecular evolution • Molecular biology of DNA and proteins radically changed evolutionary biology • Zuckerland and Pauling ’65: rate of a.a. sequence divergence constant in vertebrates (molecular clock) not episodic (due to occasional beneficial mutations). • Rate of substitution is surprisingly high (1/2 per year) during course of vertebrate evolution (Kimura ’68). That is, every two years an amino acid substitution is occurring within a population. • Kimura proposed Neutral theory: the vast majority of molecular evolution (substitution) is due to drift of mutations of nofitness consequence. • 1) beneficial mutations are extremely rare • 2) deleterious mutations are removed by selection

11. Drift and rate of molecular evolution • Kimura proposed Neutral theory: the vast majority of molecular evolution (substitution) is due to drift of mutations of no fitness consequence. • 1) beneficial mutations are extremely rare • 2) deleterious mutations are removed by selection • Kimura model: • • If all 2N copies are selectively equivalent (neutral), each has a chance of becoming the • fixed allele = 1/2N • • In every generation, mutation will introduce new alleles at rate v. The number of new mutants each generation = 2Nv. • • Therefore, the rate of fixing new mutants in the population = ( 2Nv)(1/2N) = v. • This means that the rate of sequence evolution, if all alleles are neutral, equals the mutation rate. • Conclusions of Kimura's Neutral Theory • Most of molecular evolution is neutral. • •New beneficial mutations are extremely rare • •Most non-neutral mutations are deleterious, and eliminated by selection. • 2) Population size doesn't matter. • •Rate of neutral evolution is independent of population size. • •v represents the maximum rate of evolutionary change.

12. Drift and rate of molecular evolution Evidence for neutral molecular evolution: (replacement) Fitness effect: neutral often deleterious + rarely beneficial Prediction of neutral theory: synonymous (neutral) mutations should be far more common than nonsynonymous, b/c the latter tend to be deleterious.

13. Drift and rate of molecular evolution Evidence for neutral molecular evolution from influenza virus: Neutral substitutions occur 4x more frequently

14. Inbreeding (a form of non-random mating) Effect on H-W expectation: increases homozygosity Example: selfing: Homozygotes produce identical homozygotes Heterozygotes produce ½ heterozygotes, ½ homozygotes Can no longer predict genotypes from allele frequencies. Allele frequencies haven’t changed (this is why non-random mating is not necessarily a cause of evolution)

15. Inbreeding (a form of non-random mating) Inbreeding: The probability of sharing an allele by descent. F: Inbreeding coefficient Example: half sib mating F=1/16 (red allele) + 1/16 (blue allele) = 1/8

16. ws wo Inbreeding depression δ = 1 – delta: inbreeding depression coefficientws = selfed fitnesswo = out-crossed fitness

17. small N Inbreeding depression Drift Conservation genetics Prairie chicken Westemeier et al., ‘98 Habitat destruction 1) N 2) Fragments N Mutational meltdown (Lynch & Gabriel, ‘90) Extinction vortex (Soule’ and Mills, ’98) Extinction vortex Conservation effort

18. small N Inbreeding depression Drift Conservation genetics • Prairie chicken • Extinction vortex test (Bouzat et al., ’98) • Prediction Result • Genetic diversity should be lower • compared to past yes • compared to other pop’s yes • polymorphism 3.6 V. 5.5 • Treatment: Gene flow • 1992: Migration from Minnesota, Kansas • and Nebraska