Evolutionary Mechanisms

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
1) 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:
a) Bottleneck Effect – Pop. temporarily reduced,
very small, later increases again
- by chance, only certain alleles ‘pass through the
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
Only natural selection can consistently produce
- most important cause of evolution