Chapter 7: Migration, genetic drift and non-random mating

1. Chapter 7: Migration, genetic drift and non-random mating • Migration: movement of alleles between populations. • Migration can cause allele and genotype frequencies to deviate from Hardy-Weinberg equilibrium.

2. Migration • Consider Continent-Island migration model. • Migration from island to continent will have no effect of continental allele frequencies. Continental population much larger than island. • However continent to island migration can greatly alter allele frequencies.

3. Empirical example of migration’s effects • Lake Erie water snakes. Snakes range in appearance from unbanded to strongly banded. • Banding caused by single locus: banded allele dominant over unbanded.

4. http://animaldiversity.ummz.umich.edu/site/accounts/pictures/Nerodia_sipedon.htmlhttp://animaldiversity.ummz.umich.edu/site/accounts/pictures/Nerodia_sipedon.html

5. Lake Erie water snakes • Mainland: almost all snakes banded. • Islands many snakes unbanded. • Unbanded snakes have selective advantage: better camouflage on limestone rocks. Camouflage very valuable when snake is young.

6. Fig 6.6

7. Lake Erie water snakes • If selection favors unbanded snakes on islands why aren’t all snakes unbanded? • Migration introduces alleles for banding.

8. Fig 6.7 A unbanded, B+C some banding, D strongly banded

9. Lake Erie water snakes • Migration of snakes from mainland makes island populations more like mainland. • This is general effect of migration: Homogenizes populations (making them resemble each other).

10. 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.

11. 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.

12. Fig 6.11

13. 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.

14. 6.12c

15. 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.

16. Founder effect in Silvereye populations. • Silvereyes colonized South Island of New Zealand from Tasmania in 1830. • Later spread to other islands. http://www.derwenttraders.com.au/contents /media/silvereye-460.jpg

17. 6.13b

18. 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.

19. Fig 6.13 c

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

21. 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.

22. Pingelap Atoll http://people.brandeis.edu/~msitzman/docs/pingelap_large.html

23. Founder effect in human populations • One survivor was heterozygous carrier of a recessive loss of function allele of CNGB3 gene. • That gene codes for protein in cone cells of retina. • 4 generations after typhoon homozygotes for allele began to be born.

24. Founder effect in human populations • People homozygous for the allele have achromotopsia (complete color blindness, extreme light sensitivity, and poor visual acuity). • Achromotopsia is rare in most populations (<1 in 20,000 people). Among the 3,000 Pingelapese islanders the frequency is 1 in 20.

25. Founder effect in human populations • High frequency of allele for achromotopsia is not due to a selective advantage, just a result of chance. • Founder effect followed by further genetic drift resulted in current high frequency.

26. 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.

27. 6.15A

28. 6.15B

29. 6.15C

30. 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.

31. 6.15D

32. 6.15E

33. 6.15F

34. Conclusions from simulations • Genetic drift produces steady decline in heterozygosity. • Frequency of heterozygotes is highest at intermediate allele frequencies. As one allele drifts to fixation the number of heterozygotes inevitably declines.

35. Empirical studies on fixation • 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

36. 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?

37. 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.

38. 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.

39. Fig 6.16

41. Buri (1956) study • Frequency of heterozygotes declined steadily over course of experiment. • Declined faster than expected because effective population size was smaller than initial population size of 16 (effective refers to number of actual breeders; some flies died, some did not get to mate).

42. Fig 6.17

43. Allele fixation in natural populations • Templeton et al. (1990) Studied Collared Lizards in Ozarks of Missouri • Desert species occurs on remnant pieces of desert-like habitat called glades.

44. Templeton et al. (1990) • Human fire suppression has resulted in loss of glade habitat and loss of crossable savannah habitat between glades. Areas between glades overgrown with trees.

45. Templeton et al. (1990) • Based on small population sizes and isolation of collared lizard populations Templeton et al. (1990) predicted strong effect of genetic drift on population genetics. • Expected low genetic diversity within populations, but high diversity between populations.

46. Templeton et al. (1990) • Found expected pattern. Genotype fixation common within populations and different genotypes were fixed in different populations. • Lack of genetic diversity leaves populations vulnerable to extinction. • Found >66% of glades contained no lizards.

47. Templeton et al. (1990) • What conservation measures could be taken to assist Collared Lizard populations?

48. Templeton et al. (1990) • Repopulate glades by introducing lizards. • Burn oak-hickory forest between glades to allow migration between glades.

49. Non-Random mating • The last of the five Hardy-Weinberg assumptions is that random mating takes place. • The most common form of non-random mating is inbreeding which occurs when close relatives mate with each other.

50. Inbreeding • Most extreme form of inbreeding is self fertilization. • In a population of self fertilizing organisms all homozygotes will produce only homozygous offspring. Heterozygotes will produce offspring 50% of which will be homozygous and 50% heterozygous. • How will this affect the frequency of heterozygotes each generation?