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

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

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%)

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

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

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.

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’

e.g. Northern Elephant Seal, Cheetah

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

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

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

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)

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

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

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

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