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 Genetic drift causes allele frequencies to change in populations  Alleles are lost more rapidly in small populations.

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Presentation on theme: " Genetic drift causes allele frequencies to change in populations  Alleles are lost more rapidly in small populations."— Presentation transcript:



3  Genetic drift causes allele frequencies to change in populations  Alleles are lost more rapidly in small populations

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

5 Sampling error is higher with smaller sample

6  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.

7 Fig 6.11

8  Allele frequencies more likely to change than stay the same.  If same experiment repeated but number of zygotes increased to 250 the frequency of A 1 settles close to expected 0.6.

9 6.12c

10  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

11  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  In each population expect one of the two alleles to drift to fixation.  Expect heterozygosity to decline in populations as allele fixation approaches.

13  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.

14 Fig 6.16


16  Frequency of heterozygotes declined steadily over course of experiment.

17 Fig 6.17

18  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.

19 6.15A

20 6.15B

21 6.15C

22  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  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  Alleles are lost at a faster rate in small populations › Alternative allele is fixed

28  Bottlenecks and founder effects are examples of genetic drift.

29 A bottleneck causes genetic drift

30  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  A bottleneck can dramatically affect population genetics.  Next slide shows effects of a bottleneck on allele frequencies in 10 simulated replicate populations.



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


37  Two studies in the 1970’s and 1990’s that examined 62 different proteins for evidence of heterozygosity found zero variation.  In contrast, southern elephant seals show plenty of variation.


39  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  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  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 effects cause genetic drift

44  Silvereyes colonized South Island of New Zealand from Tasmania in 1830.  Later spread to other islands.

45 http://photogallery.canberra

46 6.13b

47  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  Norfolk island Silvereye population has only 60% of allelic diversity of Tasmanian population.

50  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.

51  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.

52  Homozygotes have achromotopsia.  Achromotopsia rare in most populations (<1 in 20,000 people). Among the 3,000 Pingelapese frequency is 1 in 20.

53  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.

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