How does selection cause change in allele frequencies?

1. How does selection cause change in allele frequencies? Relative fitness of genotypes affects outcome of evolution. 1.0 for AA 1.0 for Aa <1.0 for aa Deleterious recessive allele – <1.0 for AA <1.0 for Aa 1.0 for aa Deleterious dominant allele – = 1.0 for AA <1.0 for Aa <<1.0 for aa Incomplete dominance – Use AlleleA1 to try it out.

2. How does selection cause change in allele frequencies? Relative fitness of genotypes affects outcome of evolution. <1.0 for AA 1.0 for Aa <1.0 for aa Heterozygote advantage – <1.0 for AA 1.0 for Aa <<1.0 for aa Heterozygote advantage – Use AlleleA1 to try it out.

3. Case study: Sickle cell disease aa – abnormal ß hemoglobin sickle-cell disease very low fitness AA – normal ß hemoglobin Aa – both ß hemoglobins

4. Case study: Sickle cell disease aa – abnormal ß hemoglobin sickle-cell disease very low fitness AA – normal ß hemoglobin Aa – both ß hemoglobins

5. Case study: Sickle cell disease Distribution of malaria Distribution of sickle cell allele Natural selection favored heterozygotes (Aa) in some areas (parts of Africa), but not others (most of Europe).

6. 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?

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

8. 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

9. 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 = 25 (50 alleles) N = 100 (200 alleles) Does population size matter?

10. 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: What if selection continued over many generations? Use AlleleA1 to find out. N = 5 (10 alleles) N = 25 (50 alleles) N = 100 (200 alleles)

11. 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

12. 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

13. 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