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 errors are very likely when small
samples are taken from populations.
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 much 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
allele fixation and 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.

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).
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 has strongest effects on small
populations.
 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.

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
A bottleneck causes genetic drift
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 expands. Even though the
population is large it may not be
genetically diverse as few alleles passed
through the bottleneck.


Simulation models show a bottleneck
can dramatically affect population
genetics.

Next slide shows effects of a bottleneck
on allele frequencies in 10 replicate
populations.

Even brief bottlenecks can lead to a
drastic reduction in genetic diversity that
can persist for generations
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.

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.

Founder Effect: when population
founded by only a few individuals allele
frequencies likely to differ from that of
source population.

Only a subset of alleles likely to be
represented 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
(complete color blindness, extreme light
sensitivity, and poor visual acuity).
 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.