  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 error is higher with smaller sample

  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.

Fig 6.11

 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.

6.12c

    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 (i) allele fixation and (ii) frequency of heterozygosity?

 In each population expect one of the two alleles to drift to fixation.

 Expect heterozygosity to decline populations as allele fixation approaches.

in

 Distribution of frequencies of bw became increasingly U-shaped over time.

75 allele  By end of experiment, bw 75 allele fixed in 28 populations and lost from 30.

Fig 6.16

 Frequency of heterozygotes declined steadily over course of experiment.

Fig 6.17

 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.

6.15A

6.15B

6.15C

    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.

6.15D

6.15E

6.15F

 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

 Bottlenecks and founder effects are examples of genetic drift.

A bottleneck causes genetic drift

 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.

 A bottleneck can dramatically affect population genetics.

 Next slide shows effects of a bottleneck on allele frequencies in 10 simulated replicate populations.

  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.

 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.

 More recent work that has used DNA sequencing has shown some variation in northern seals, but still much less than in southern 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.

 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.

Founder effects cause genetic drift

 Silvereyes colonized South Island of New Zealand from Tasmania in 1830.

 Later spread to other islands.

http://photogallery.canberra

birds.org.au/silvereye.htm

6.13b

 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.

Fig 6.13 c

 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.

 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.