-genetic drift increases homozygosity of a population, but population do not become totally homozygous for neutral alleles
-one reason is mutation
-change the wright's F statistic over a generation (F is the probability that two gene copies are identical by descent in the absence of mutation)
-if µ is the mutation rate, then there is a µ chance that one copy at a locus will change between generations, by definition; there is a 1-µ chance that one copy does not change; and a (1-µ)^2 chance that neither copy at the locus will change; plug in that to the equation and set Foffspring=Fparental to calculate Fequilibrium and get:
Fequilibrium= 1/(4Nµ+1); when N and µ are small, heterozygosity will be lower
-SWITCH TO NON-NEUTRAL ALLELES
-even when alleles that are favored by natural selection are not guaranteed to become fixed in populations
-Haldane found looked at a simple model in which a new, slightly beneficial allele with a fitness of 1+s arises in a large population and competes with the wild-type allele that has a fitness of 1; Haldane found that the fixation probability is approximately 2s; therefore, a new beneficial mutation that confers a 1% fitness advantage has only a 2% chance of being fixed in a large population
-drift matters even in large populations, because we are looking at what happens to the initial mutant allele; so, in a large population, at very low frequencies, even a new allele with positive benefits can be lost by chance; if it does survive long enough to occur at a substantial frequency, then it is likely to go to fixation even if the benefit is small
-in a pop of 100, for example, drift causes substantial fluctuations in allele frequencies; so even if barely beneficial, it has a modest chance of becoming fixed though drift alone
-in a pop of 1000000, drift will have less effect on allele frequencies overall, but a new allele will begin at a frequency of 1 in 1000000; it will have a really long way to go to reach fixation through drift alone
-In a population of 100, for example, drift causes
substantial fluctuations in allele frequencies, but a new
allele will begin at a frequency of 1 in 100. Relatively
speaking, it doesn't have that far to go to become fixed
-In a population of 1,000,000, drift will have less effect
on allele frequencies overall, but a new allele will begin
at a frequency of 1 in 1,000,000. It will have a really
long way to go to reach fixation through drift alone. But
if it reaches even a modest frequency, selection will
begin to dominate over drift and it will reach fixation
-In BOTH scenarios, conditions occur that drive fixation, and
so the probability of fixation is more or less independent of
population size
-a low N, very high selection advantage is required to guarantee fixation over drift
-at very, very high N, even weak selective pressure can eventually overcome drift
-when s>1/2Ne, then selection dominates the selective advantage
-when s<1/2Ne, then drift dominates