Genetic drift causes allele frequencies to change in populations Alleles are lost more rapidly in small populations
Genetic drift results from the influence of chance. When population size is small, chance events more likely to have a strong effect. Sampling errors are very likely when small samples are taken from populations.
Assume gene pool where frequency A 1 = 0.6, A 2 = 0.4. Produce 10 zygotes by drawing from pool of alleles. Repeat multiple times to generate distribution of expected allele frequencies in next generation.
Buri (1956) established 107 Drosophila populations. All founders were heterozygotes for an eye-color gene called brown. Neither allele gives selective advantage. Initial genotype bw 75 /bw Initial frequency of bw 75 = 0.5
Followed populations for 19 generations. Population size kept at 16 individuals. What do we predict will occur in terms of allele fixation and heterozygosity?
In each population expect one of the two alleles to drift to fixation. Expect heterozygosity to decline in populations as allele fixation approaches.
Distribution of frequencies of bw 75 allele became increasingly U-shaped over time. By end of experiment, bw 75 allele fixed in 28 populations and lost from 30.
Frequency of heterozygotes declined steadily over course of experiment. Declined faster than expected because effective population size was smaller than initial size of 16 (effective refers to number of actual breeders; some flies died, some did not get to mate).
Effects of genetic drift can be very strong when compounded over many generations. Simulations of drift. Change in allele frequencies over 100 generations. Initial frequencies A 1 = 0.6, A 2 = 0.4. Simulation run for different population sizes.
Populations follow unique paths Genetic drift has strongest effects on small populations. Given enough time even in large populations genetic drift can have an effect. Genetic drift leads to fixation or loss of alleles, which increases homozygosity and reduces heterozygosity.
Genetic drift produces steady decline in heterozygosity. Frequency of heterozygotes highest at intermediate allele frequencies. As one allele drifts to fixation number of heterozygotes inevitably declines.
Alleles are lost at a faster rate in small populations › Alternative allele is fixed
Another way in which populations may be exposed to the effects of drift is if the population experiences a bottleneck. A bottleneck occurs when a population is reduced to a few individuals and subsequently expands. Even though the population is large it may not be genetically diverse as few alleles passed through the bottleneck.
Even brief bottlenecks can lead to a drastic reduction in genetic diversity that can persist for generations
The northern elephant seal (which breeds on California and Baja California) was hunted almost to extinction in the 19 th century. Only about 10-20 individuals survived. Now there are more than 100,000 individuals.
The northern elephant seal population should show evidence of the bottleneck. Two studies in the 1970’s and 1990’s that examined 62 different proteins for evidence of heterozygosity found zero variation. In contrast, similar studies of southern elephant seals show plenty of variation.
Examination of museum specimens collected before the bottleneck have shown much more variation in these specimens than in current populations, which shows that the population was much more genetically diverse before the bottleneck.
Founder Effect: when population founded by only a few individuals allele frequencies likely to differ from that of source population. Only a subset of alleles likely to be represented and rare alleles may be over-represented.
Norfolk island Silvereye population has only 60% of allelic diversity of Tasmanian population.
Founder effect common in isolated human populations. E.g. Pingelapese people of Eastern Caroline Islands are descendants of 20 survivors of a typhoon and famine that occurred around 1775.
One survivor was heterozygous carrier of a recessive loss of function allele of CNGB3 gene. Codes for protein in cone cells of retina. 4 generations after typhoon homozygotes for allele began to be born.
Homozygotes have achromotopsia (complete color blindness, extreme light sensitivity, and poor visual acuity). Achromotopsia rare in most populations (<1 in 20,000 people). Among the 3,000 Pingelapese frequency is 1 in 20.
High frequency of allele for achromotopsia not due to a selective advantage, just a result of chance. Founder effect followed by further genetic drift resulted in current high frequency.