Last day… examined basic equation of population genetics,

1. Last day… examined basic equation of population genetics, Hardy-Weinberg equilibrium (H-W eq.), describes expected genotype frequencies in stable pop. p2 + 2pq + q2 = 1 Allows us to examine the frequencies of two alleles, and also to figure out what proportion should be homozygous and heterozygous

2. A population geneticist studies a population of American Robins and finds that the allele for the normal form of alcohol dehydrogenase has a frequency of 0.92 while a recessive allele that produces a defective form of the enzyme has a frequency of 0.08. (Robins often eat fermenting berries, and may get drunk if they have the defective form.) If this population is in Hardy-Weinberg equilibrium, what proportion of the population should be homozygous for the recessive allele? What proportion should be heterozygotes? What info have you already been given? A) p B) q C) p and q D) p and 2pq E) p2 and q2

3. p2 + 2pq + q2 = 1 0.922 + 2*0.92*0.08 + 0.082 = 1 0.8464 + 0.1472 + 0.0064 = 1 Which number represents the frequency of heterozygotes? A) 0.8464 B) 0.1472 C) 0.0064 D) 1 …and homozygous recessives = 0.0064 (or just 0.64%)

4. Hardy-Weinberg equation describes pop. that is not evolving – assumes 5 conditions: 1) Very large population (no random fluctuations) 2) No gene flow (isolated pop.) 3) No mutation 4) Random mating 5) No natural selection • Conditions never completely met, but often • approximately true for a locus

5. The 5 causes of evolution = 5 factors that disrupt Hardy-Weinberg equilibrium Genetic drift (due to small population size) 2) Gene flow 3) Mutation 4) Non-random mating 5) Natural selection

6. Genetic drift – random change of gene frequencies due • to ‘sampling errors’ in small pops. • - will cause small deviations in larger population, can • cause major changes (e.g. allele lost) in small pops.

7. Two specific ‘varieties’ of drift, other than usual: • Bottleneck Effect – Pop. temporarily reduced, very small, later increases again • - by chance, only certain alleles ‘pass through the • bottleneck’

8. e.g. Northern Elephant Seal, Cheetah

9. b) Founder Effect – New population founded by small number of colonists, do not include full genetic variation of source population e.g. retinitis pigmentosa on Tristan da Cunha • in both cases, alleles may be missing or in higher frequency • than in original population

10. 2) Gene flow – gene exchange between populations due • to movement of individuals or gametes • tends to reduce differences between populations, may • impede local adaptation • - may introduce new genetic variation Great Tit

11. Streamside Salamanders show weaker anti-predator adaptations if near fishless populations

12. 3) Mutation – change from one allele to another due to replication error, radiation damage, etc. • relatively rare (1 per locus per 105 – 106 gametes), • & often reversible, so only very small effect by • itself (but produces variation that other factors • can work on)

13. 4) Non-random mating – mating with individuals that are more similar (or more different) than expected by chance – tendency to mate with nearby individuals or even self-fertilize (plants) leads to inbreeding; decreases # of heterozygotes • no effect on allele frequency • by itself (only genotype • frequency), but may • expose alleles to selection

14. 5)Natural Selection – process by which particular genotypes consistently increase in frequency due to their superior adaptation to the environment (higher fitness) Fitness – relative contribution of a genotype to the next generation, reflecting its probability of survival & its reproductive output

15. Most successful variant: fitness = 1 - others some proportion < 1 (0.5, 0.8...) Fitness is NOT: - being ‘physically fit’ - just about survival - just about producing lots of offspring mayfly

16. Only natural selection can consistently produce • adaptation • - most important cause of evolution Jaguar