Genetic Drift
Key concept
Genetic drift is the term used to describe random changes in the frequencies of alleles in a population.
When drift is occurring allele frequencies do not increase or decrease on a clear trajectory as they might
if they were, for example, experiencing strong selection. Hence, an allele’s frequency may drift around
unpredictably becoming more common or rare and eventually going extinct or becoming fixed
(becoming the only allele in the gene pool).
Genetic drift results from the influence of chance or luck. When a population is small, chance events
are more likely to have a strong effect on allele frequencies because sampling errors are very likely
when small samples are taken from populations. Larger samples are more likely to resemble the original
population from which they are drawn. It is common for alleles to be lost from small populations and as
a result the population will become less genetically diverse and there will be fewer heterozygotes and
an increase in homozygosity.
Genetic drift results from random sampling error. Sampling errors are bigger with smaller sample. In the
image below the larger sample of beads results in sample that more closely resembles the original
population.
Below is another demonstration of the effects of sampling error. Assume the gene pool only two alleles
where their frequencies are A1 = 0.6 and A2 = 0.4. From this gene pool we produce 10 zygotes by
drawing at random from the pool of alleles. We then repeat this process multiple times to generate a
distribution of expected allele frequencies in next generation.
It is clear from looking at the distribution of outcomes that allele frequencies much more likely to
change than stay the same when samples of 10 are used. However, if we repeat the same experiment
but this time increase the number of zygotes to 250 the frequency of A1 settles close to expected 0.6.
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 a selective advantage.
Initial genotype bw75/bw
Initial frequency of bw75 = 0.5
Buri tracked the populations for 19 generations and in each generation kept population size at 16
individuals.
What do we predict will occur in terms of allele fixation and heterozygosity?
Because of repeated small sampling we would expect in each population one of the two alleles to drift
to fixation. We would also expect the number of heterozygotes to decline in populations as allele
fixation approaches.
That was what was found. Distribution of frequencies of bw75 allele became increasingly U-shaped over
time, by which I mean one or other of the two alleles became fixed in an increasingly large percentage
of populations as time went by. By the end of the experiment, the bw75 allele was fixed in 28
populations and lost from 30.
In addition, the frequency of heterozygotes declined steadily over course of experiment. Heterozygosity
actually declined faster than expected because effective population size was smaller than initial size of
16 (effective refers to number of actual breeders; some flies died, some did not get to mate).
Population data are graphed on the next page.
Effects of genetic drift over time
The 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. The simulation was run for different population sizes (4, 40 and 400).
Conclusions from simulations
Populations follow unique paths and genetic drift has the strongest effect on small populations.
However, 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.
Genetic drift produces steady decline in heterozygosity. Frequency of heterozygotes is highest at
intermediate allele frequencies. However, as one allele drifts to fixation the number of heterozygotes
inevitably declines.
Drift reduces genetic variation in a population. Alleles are lost at a faster rate in small populations
Alternative allele is fixed.
Bottlenecks and Founder Effects
Another way in which populations may be exposed to the effects of drift is if the population experiences
a bottleneck.
A bottleneck occurs when a population is reduced to a few individuals and subsequently increases. Even
though the population may become large after the bottleneck, it may not be genetically diverse because
few alleles passed through the bottleneck and some alleles disappeared from the gene pool.
In the original population above, there are three different alleles, represented here by blue, black, and
yellow balls. A bottleneck cuts population size dramatically, leading to shifts in allele frequency simply
by chance. Compare the frequency of black and blue balls before and after the bottleneck. Bottlenecks
can even result in the loss of certain alleles. The yellow allele is lost in this example.
Simulation models show a bottleneck can dramatically affect population genetics. The images below
show the effects of a bottleneck on allele frequencies in 10 replicate populations.
In the previous image we see the results from simulations of 10 replicate populations, each with 1000
diploid individuals, going through a brief population bottleneck. Each population starts with the A1 and
A2 alleles each at frequency 50%. One sample trajectory is highlighted for emphasis. Allele frequencies
drift gradually until the population bottleneck, at which point the drift accelerates dramatically, causing
large changes in allele frequency. As the populations are restored to their original sizes, the rate of allele
frequency fluctuation slows.
As a result of a bottleneck, rare alleles are likely to be lost.. In the graph below the rarest allele (which
has a frequency of 0.01) is far more likely to go extinct than the other alleles
Key concept
Even brief bottlenecks can lead to a drastic reduction in genetic diversity that can persist for generations.
Empirical example of a bottleneck-northern elephant seal.
The northern elephant seal (which breeds on California and Baja California) was hunted almost to
extinction in the 19th century. Only about 10-20 individuals survived. Now there are more than 100,000
individuals.
The northern elephant seal population should show evidence of the bottleneck. Two studies in the
1970’s and 1990’s that examined 62 different proteins for evidence of heterozygosity found zero
variation. In contrast, similar studies of 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.
Low variation in northern elephant seals: DNA sequence data. DNA sequence studies at three different
highly variable loci reveal much greater variation in southern elephant seal populations than in northern
elephant seal populations. Adapted from Hoelzel et al. (1999b).
Examination of museum specimens collected before the bottleneck have shown much more variation in
these specimens than in current populations, which shows that the population was much more
genetically diverse before the bottleneck.
Empirical examples of sampling error: Founder Effect
Founder Effect: when a population is founded by only a few individuals its allele frequencies are likely to
differ from that of the source population. Only a subset of alleles likely to be represented and rare
alleles may be over-represented.
Founder effect in Silvereye populations.
 Silvereyes colonized South Island of New Zealand from Tasmania in 1830.
 Later spread to other islands.
Analysis of microsatellite DNA from populations shows Founder effect on populations. There is a
progressive decline in allele diversity from one population to the next in sequence of colonizations.
Norfolk island Silvereye population has only 60% of allelic diversity of Tasmanian population.
Founder effect in human populations
Founder effects are 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. The normal
allele codes for a protein found in cone cells of retina, which are responsible for color vision. Four
generations after the typhoon, homozygotes for the allele began to be born. The homozygotes have
achromotopsia (complete color blindness, extreme light sensitivity, and poor visual acuity).
Achromotopsia is rare in most populations (<1 in 20,000 people). However, among the 3,000
Pingelapese the frequency of the allele is 1 in 20. The high frequency of the 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 of the allele.