Genetic Drift

Tyson AdamsTyler StewartSteve Peper

Genetic Drift• What is Genetic Drift?– Change in allele frequency due to the chance of

random sampling error– A cause of Evolution

• What factors play a role in Genetic Drift?– Population size– Chance

• How does it effect a population?– Can lead to a fixation of an allele– The changes caused by drift can be neutral, beneficial,

or harmful

Founder Effect

• Small population that have moved or been moved to new location (founded)

• By chance allele frequencies in new population are likely to be different than source population

• Due to sampling error– Example. (35 alleles on single locus for lizards and

15 individuals are founded, the chance is very low that they will contain all of the 35 alleles)

Silvereyes

• Australia and Tasmania– Tasmania to South Island in 1830– South Island to Chatham and Palmerston in 1856– Palmerston to Auckland in 1865– Auckland to Norfolk Island in 1904

• Blood samples from 7 groups• Genotypes from 6 loci to compare groups

Which would you expect to have the most alleles in common?

A. Tasmania and AucklandB. South Island and Norfolk IslandC. Chatham Island and Palmerston NorthD. Tasmania and Norfolk IslandE. Palmerston North and Norfolk Island

Human Populations

• Pingelapese People– 20 survivors from typhoon– Heterozygous carrier of a recessive loss-of-

function allele– Achromatopsia (complete color blindness,

sensitivity to light, poor visual acuity)– Usually affects 1/20,000– 1/20 on island

Fixation of Alleles and Heterozygosity

• Activity for Fixation

• What is heterozigosity?

• As alleles drift to fixation or loss, the frequency of the heterozygotes in the population declines

Bottlenecking

• Occurs when population is reduced to a small size for a short period of time

• Only rare alleles are lost• What will this do to Genetic Variation?

Bottleneck Example

What’s the difference between founder effect and bottlenecking?

• Founder Effect– Small group leaves to new environment– More extinctions, changes and stress– Need more time to adapt

• Bottlenecking– Stay in same place– Due to sudden decrease in population size– Well adapted and bounce back faster

PopGen Lab

In general what happens to allele frequencies and genotype frequencies in a small population?

A) FixationB) They stay the sameC) They promote heterozygosityD) There is a loss of heterozygosityE) Both A & D

Heterozygosity in a finite population is lost.

A) TrueB) False

What effect does heterozygosity have on a population?

A) Increases a population’s ability to adaptB) Loss of genetic variationC) Decreases a population’s ability to adaptD) Heterozygosity has no effect on a population

What is the effect of population size on genetic variation?

A. Larger population causes more variationB. Smaller population causes more variationC. Variation doesn’t depend on population size

What is meant by fixation of alleles?

A. All allele frequencies are fixed and will not change from generation to generation

B. Allele frequencies drift between 0 and 1.0C. Once allele frequency reaches 0 it is lost

forever and if it reaches 1.0 it is fixedD. B and C

Which apply to founder effect?

1. More adaption time2. Stay in same place (not relocated)3. High stress and changes4. Decrease in population size5. Recover faster than in bottlenecking

A. 1,3,4B. 1 & 2C. 1,2,3,4,5D. 3,4,5