Pop Gen 5 - Drift Flashcards
Genetic Drift
change in allele frequencies resulting from random sampling processes that take place within population over generations
random, however some effects are predictable
aka random genetic drift or drift
When could sampling events occur in populations
A sample of gametes is combined to make zygotes
A sample of zygotes survive to become juveniles
A sample of juveniles survive to become adults
A sample of adults contribute to the gamete pool
Principles of genetic drift
Changes in allele freq from one gen to the next are expected to be greater in smaller populations
Populations remain polymorphic (retaining genetic variance) for longer, on average, with increasing population size
**larger populations better able to retain genetic variation
How could a breeding program help maintain variation in small populations?
Avoid sexual selection - not one individual with many mates, selecting many different breeding pairs
more things!
Drift determined by…
Population size
all else being equal, acs similarly across all loci in the genome
Chance that any one allele fixes (all individuals descend from) =
it’s initial frequency in a population (assuming equal fitness of all)
H[t]
expected heterozygosity over time t = probability that two alleles drawn at random are different alleles
H[t] for a population with two alleles
H[t] = 2p[t]q[t]
General equation for heterozygosity at time t
H[t] = (1 - 1/{2N}) * H[t-1]
**Greater N value makes 1 - 1/2N MUCH closer to 1, thus retaining heterozygosity
Rate of decline in variability in a population of N diploids
1/(2N)
founder effects
effects that occur when a new population is founded by a small number of individuals from a larger population
depends on number of colonizing individuals
Even if high genetic diversity in original population, unlikely smaller population is representative
genetic bottlenecks
a severe reduction in the number of individuals in a population, which results in the loss of genetic variation in the surviving population
inbreeding
with few surviving lineages, the chance that the two alleles in an individual recently shared a common ancestor is much higher
inbreeding depression
Increasing frequency of homozygous recessive diseases due to inbreeding
probability of fixing a new mutation in larg pop when rare
Haldane (1927) showed that in a large population, the probability of fixing a new mutation is twice its selective advantage when rare (2s in haploids, 2hs in diploids)
