Last day… examined basic equation of population genetics, Hardy-Weinberg equilibrium (H-W eq.), describesexpected 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 behomozygous 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) pB) qC) p and qD) p and 2pqE) 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) 0.8464B) 0.1472C) 0.0064D) 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)

3) No mutation

4) Random mating

5) No natural selection

Conditions never completely met, but often approximately true for a locus

2) No gene flow (isolated pop.)

The 5 causes of evolution = 5 factors that disrupt Hardy-Weinberg equilibrium

1) Genetic drift (due to small population size)2) Gene flow3) Mutation4) Non-random mating5) 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:a) 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, mayimpede 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 byitself (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