Mutation can change population genetic structure

1. Mutationcan change population genetic structure Mutation rates are low typically <1/10,000 per generation (μ ≤ 0.0001) Loss-of-function mutations are much more common than back mutations that restore function WHY? Howdoes selection cause change in allele frequencies? Allele freq. before mutation: 0.5A 0.5a Mutation rate: 0.0001 A a Allele freq. after mutation: Did allele frequencies change? What if mutation continued over many generations?

2. How does mutation cause change in allele frequencies? Most mutations are deleterious should be eliminated by selection (μ ≤ 0.0001) Mutation-selection balance removal by selection offsets recurrent mutations more mutations per generation  stronger selection against mutant allele 

3. Genetic driftcan change population genetic structure Genetic drift – random change in allele frequencies due tosampling error Genetic drift – does not lead to adaptation, because it is purely random – most pronounced in small populations

4. Howdoes genetic drift cause change in allele frequencies? Simulation of sampling error at the “football” locus Parental allele freq: 0.6 Fb10.4 Fb2 Draw 10 alleles from the “gene pool” (N=5 individuals) Count your alleles Allele frequencies when: N = 5 (10 alleles) Did allele frequencies change? Was the change predictable? N = 90 (180 alleles) Does population size matter?

5. Key points about genetic drift • Each population has a unique trajectory. • Drift has greater influence in small populations. • Drift can lead to large changes in allele frequencies over time. • Over time, alleles can be lost. different alleles fixed in different populations

6. Forms of genetic drift Founder effect allele frequencies may differ due to sampling error ancestral population descendent population Population bottleneck allele frequencies may differ due to sampling error ancestral population descendent population

7. Case study: Porphyria in Dutch South Africans • Porphyria • rare autosomal dominant disorder • sensitivity to light, skin lesions • mental disturbances, excess hair • (is this basis for werewolf myths?) • in the 1600’s, 80 Dutch immigrants settled in South Africa •  over 1 million descendents • one immigrant couple had the gene for porphyria

8. Migrationcan change population genetic structure Migration – movement of alleles between populations

9. Howdoes migration cause change in allele frequencies? Population 1 (“island”) 950 AA 0Aa 0 aa Population 2 (“mainland”) 0% AA 0%Aa 100% aa “fixed” “fixed” 50 aa Allele freq. before migration: Did allele frequencies change? Allele freq. after migration: Is the population in Hardy-Weinberg equilibrium? p2 = 2pq = q2 = What if mutation continued over many generations?

10. Migrationmakes population more similar AlleleA1 simulation – one-way migration (gene flow) Population 1 (“island”) Population 2 (“mainland”) A a a A A a a A A a a A a A a A a A a a a a A A a A a A a A A a a a a A A a a a A A a A A a a a A a A a A a a a Real life – gene flow can be one-way or two-way Population 1 Population 2 a a A A a a A a A a A a A A a A a A a a a A a a A A a A a A a a A A a a a a a A A a A A A A a a a a A A a a a A a