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 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.
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 A1 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 bw75/bw
 Initial frequency of bw75 = 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 in
populations as allele fixation
approaches.

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.
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 A1 = 0.6, A2 = 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 19th 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.