Genetic Drift

1. Genetic Drift

3. Key Concepts • Genetic drift causes allele frequencies to change in populations • Alleles are lost more rapidly in small populations

4. Genetic Drift • Genetic drift results from the influence of chance. • When population size is small, chance events more likely to have a strong effect.

5. Genetic drift results from random sampling error Sampling error is higher with smaller sample

6. 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.

7. Fig 6.11

8. Genetic Drift • Allele frequencies 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.

9. 6.12c

10. 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

11. Buri (1956) study • Followed populations for 19 generations. • Population size kept at 16 individuals. • What do we predict will occur in terms of (i) allele fixation and (ii) frequency of heterozygosity?

12. Buri (1956) study • In each population expect one of the two alleles to drift to fixation. • Expect heterozygosity to declinein populations as allele fixation approaches.

13. 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.

14. Fig 6.16

16. Buri (1956) study • Frequency of heterozygotes declined steadily over course of experiment.

17. Fig 6.17

18. 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.

19. 6.15A

20. 6.15B

21. 6.15C

22. Important conclusions about genetic drift • Populations follow unique paths • Genetic drift most strongly affects small populations. • Given enough time, even large populations can be affected by drift. • Genetic drift leads to fixation or loss of alleles, which increases homozygosity and reduces heterozygosity.

23. 6.15D

24. 6.15E

25. 6.15F

26. 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.

27. Drift reduces genetic variation in a population • Alleles are lost at a faster rate in small populations • Alternative allele is fixed

28. Bottlenecks and Founder Effects • Bottlenecks and founder effects are examples of genetic drift.

29. Bottlenecks reduce genetic variation A bottleneck causes genetic drift

30. Population bottlenecks • A bottleneck occurs when a population is reduced to a few individuals and subsequently expands. • Many alleles are lost because they do not pass through the bottleneck. • As a result, the population has little genetic diversity.

32. Population bottlenecks • A bottleneck can dramatically affect population genetics. • Next slide shows effects of a bottleneck on allele frequencies in 10 simulated replicate populations.

34. Rare alleles most likely to be lost during a bottleneck

35. Empirical example of a bottleneck-northern elephant seal • The northern elephant seal was almost wiped out in the 19th century. Only about 10-20 individuals survived. • Now there are more than 100,000 individuals.

37. Empirical example of a bottleneck-northern elephant seal • Two studies in the 1970’s and 1990’s that examined 62 different proteins for evidence of heterozygosity found zerovariation. • In contrast, southern elephant seals show plenty of variation.

39. 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.

41. Empirical example of a bottleneck-northern elephant seals • Museum specimens collected before the bottleneck exhibit much more variation than does current population. • Clearly, the population was much more genetically diverse before the bottleneck.

42. Empirical examples of sampling error: Founder Effect • Founder Effect: when a population is founded by only a few individuals only a subset of alleles will be included and rare alleles may be over-represented.

43. Founder effect Founder effects cause genetic drift

44. Founder effect in Silvereye populations. • Silvereyes colonized South Island of New Zealand from Tasmania in 1830. • Later spread to other islands.

45. http://photogallery.canberrabirds.org.au/silvereye.htm

46. 6.13b

47. 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.

48. Fig 6.13 c

49. Founder effect in Silvereyes • Norfolk island Silvereye population has only 60% of allelic diversity of Tasmanian population.

50. Founder effect in human populations • 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.