Lecture 23 - Genetic Drift

included on exam 4

random factors in evolution

mass extinction

mutation

genetic drift

mass extinction

a random factor in evolution

loss of many species resulting from random occurrences (e.g. asteroids, volcanoes)

mutation

a random factor in evolution

a random process that gives rise to new alleles each generation at a relatively constant rate

adds allelic diversity; affects allele frequencies at a given locus

genetic drift

a random factor in evolution

change in allele frequencies due to sampling from a limited population

reduces allelic diversity; affects allele frequencies at all loci in the genome

2 main examples: founder effect, bottleneck

note: there is a very fine line between genetic drift and natural selection (does this trait directly influence the fitness of an individual, or not?)

reasons genetic drift happens

1) populations are NOT infinitely large

2) sampling error randomly changes allele frequencies if size is not infinite

by chance, some individuals/genes may leave more copies in the next generation → changes allele frequencies

predicability of genetic drift

genetic drift is random → cannot predict exactly what/when will happen

but we CAN predict probability of different results happening

genetic drift in populations

effect is faster and more dramatic in small population

each population follows a unique path (because drift is random)

with enough time, drift can cause substantial changes in allele frequencies (even in large populations)

in absence of other evolutionary forces, drift causes eventual fixation of some alleles and loss of others

as alleles drift to fixation, heterozygote frequency declines in the population

probability of fixation of an allele

X(1/2N) = X/2N

N = population size

(2N = number of alleles at locus A)

X = initial number of copies of A₁

equation: genetic drift of heterozygotes in the next generation

H_g+1 = H_g(1 - 1/2N)

H_g+1 = expected heterozygote frequency in the next generation

H_g = observed heterozygosity in the current generation

for H values, use frequencies

for N, use actual sample size

equation: genetic drift of heterozygotes in any generation

H_t = H₀(1 - 1/2N)^t

t = time (number of generations)

0 = initial generation

effective population size (N_e)

the portion of the population that actually mates

always smaller than cencus size

fewer individuals actually contribute to the next generation’s gene than all the individuals that could

assume that N_e = N for this class, but know this is not often the case in real life

why effective population size is smaller than census size

1) variance in progeny production (breeding success): extreme in polyandry and polygyny

2) skewed sex ratio

3) overlapping generations → inbreeding

4) fluctuations in N (small N has huge impact: genetic bottleneck)

bottleneck effect

if population size is severely reduced, then only a small sample of alleles is left

even if these alleles are not under selection, allele frequency is change among survivors

founder effect

if a population is founded by a small number of individuals, chance alone will cause different allele frequencies in the new population

relation between gene flow and genetic drift

they have opposite effects on allele frequencies within and among populations

drift increases fixation, decreases variation

gene flow decreases fixation, increases variation

F_ST considering gene flow AND drift

F_ST = 1 / 4N_em+1

F_ST = fixation index

N_e = effective population size

m = migration rate

note: can be rearranged to find migration rate

equation: calculating m from F_ST

N_em = [(1/F_ST) - 1] / 4

even a little bit of gene flow can prevent genetic drift