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Genetic Drift. Key Concepts . Genetic drift causes allele frequencies to change in populations Alleles are lost more rapidly in small populations. Genetic Drift.
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Key Concepts • Genetic drift causes allele frequencies to change in populations • Alleles are lost more rapidly in small populations
Genetic Drift • 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.
Genetic drift results from random sampling error Sampling error is higher with smaller sample
Genetic Drift • Assume gene pool where frequency A1 = 0.6, A2 = 0.4. • Produce 10 zygotes by drawing from pool of alleles. • Repeat multiple times to generate distribution of expected allele frequencies in next generation.
Genetic Drift • Allele frequencies much more likely to change than stay the same. • If same experiment repeated but number of zygotes increased to 250 the frequency of A1 settles close to expected 0.6.
Empirical studies on allele fixation and heterozygosity • Buri (1956) established 107 Drosophila populations. • All founders were heterozygotes for an eye-color gene called brown. Neither allele gives selective advantage. • Initial genotype bw75/bw • Initial frequency of bw75 = 0.5
Buri (1956) study • Followed populations for 19 generations. • Population size kept at 16 individuals. • What do we predict will occur in terms of allele fixation and heterozygosity?
Buri (1956) study • In each population expect one of the two alleles to drift to fixation. • Expect heterozygosity to decline in populations as allele fixation approaches.
Buri (1956) study • Distribution of frequencies of bw75 allele became increasingly U-shaped over time. • By end of experiment, bw75 allele fixed in 28 populations and lost from 30.
Buri (1956) study • 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 over time • 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 A1 = 0.6, A2 = 0.4. Simulation run for different population sizes.
Conclusions from simulations • 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.
Conclusions from simulations • 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.
Drift reduces genetic variation in a population • Alleles are lost at a faster rate in small populations • Alternative allele is fixed
Bottlenecks reduce genetic variation A bottleneck causes genetic drift
Population bottlenecks • 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.
Population bottlenecks • Simulation models show a bottleneck can dramatically affect population genetics. • Next slide shows effects of a bottleneck on allele frequencies in 10 replicate populations.
Key Concept • Even brief bottlenecks can lead to a drastic reduction in genetic diversity that can persist for generations
Empirical example of a bottleneck-northern elephant seal • The northern elephant seal (which breeds on California and Baja California) was hunted almost to extinction in the 19th century. Only about 10-20 individuals survived. • Now there are more than 100,000 individuals.
Empirical example of a bottleneck-northern elephant seal • 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.
Empirical example of a bottleneck-northern elephant seals • More recent work that has used DNA sequencing has shown some variation in northern seals, but still much less than in southern elephant seals.
Empirical example of a bottleneck-northern elephant seals • 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.
Empirical examples of sampling error: Founder Effect • 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.
Founder effect Founder effects cause genetic drift
Founder effect in Silvereye populations. • Silvereyes colonized South Island of New Zealand from Tasmania in 1830. • Later spread to other islands.
Founder effect in Silvereyes • Analysis of microsatellite DNA from populations shows Founder effect on populations. • Progressive decline in allele diversity from one population to the next in sequence of colonizations.
Founder effect in Silvereyes • Norfolk island Silvereye population has only 60% of allelic diversity of Tasmanian population.
