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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