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Evolutionary Genetics
LV 25600-01 | Lecture with exercises | 6KP
Genetic Drift
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Population Genetics ▷ Hardy-Weinberg Principle
A Primer of Ecological Genetics
Chapter 3 - Genetic Drift - Page 52-55
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Population Genetics ▷ Genetic Drift
Population
tn
Population
tn+1
In each generation, some individuals may, just by chance, leave behind a few more
descendants (and genes) than other individuals. The genes of the next generation will
be the genes of the “lucky” individuals, not necessarily the healthier or “better”
individuals. That, in a nutshell, is genetic drift. It happens to ALL populations—there’s
no avoiding the vagaries of chance.
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Population Genetics ▷ Genetic Drift
Genetic drift, also called genetic sampling error or Sewall Wright effect, a change in the gene
pool of a small population that takes place strictly by chance. Genetic drift can result in genetic
traits being lost from a population or becoming widespread in a population without respect to
the survival or reproductive value of the alleles involved.
Results of computer simulations of changes in allele
frequency by genetic drift for each of three population
sizes (N) with an initial allele frequency of 0.5.
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Allendorf, Luikart, and Aitken (2013)
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Population Genetics ▷ Genetic Drift
PopG genetic simulation program
http://evolution.gs.washington.edu/popgen/popg.html
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Population Genetics ▷ Genetic Drift
{AB}
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{B}
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Population Genetics ▷ Genetic Drift
Genetic drift has several important effects on evolution:
1.Drift reduces genetic variation in populations, potentially
reducing a population’s ability to evolve in response to new
selective pressures.
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Population Genetics ▷ Genetic Drift
Allele frequencies for 107 D. melanogaster populations where 16 individuals (eight of each sex) were
randomly chosen to start each new generation. Initially, all 107 populations had equal numbers of the wildtype and bw75 alleles.
adopted from: Buri (1956) Gene frequency in small population of mutant Drosophila.
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Population Genetics ▷ Genetic Drift
What is the rate at which genetic variation is lost from populations?
t0
t1
H
H′
ΔH = H ′ − H
1
0.5
0.25
0.0
0.25
0.375
0.0
A1 A1 A1 A2 A2 A2
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0.25
0.375
A1 A1 A1 A2 A2 A2
A1 A1 A1 A2 A2 A2
0.4375
0.4375
0.125
A1 A1 A1 A2 A2 A2
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Population Genetics ▷ Genetic Drift
t+1
t
t+1
t
allele
probability =
1
2N
probability = 1 −
Heterozygosity after one generation:
H 1 = (1 −
Heterozygosity after t generations:
H t = (1 −
1
2N
1
2N
)H
0
)H
1 t
2N
0
➥ The equation indicates that the heterozygosity declines each
generation at a rate inversely dependent on the population size.
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Expected heterozygosity
Population Genetics ▷ Genetic Drift
Generation
Expected heterozygosity (2pq) in populations undergoing genetic drift. The line
shows the expected change in heterozygosity. Genetic drift increases
homozygosity and decreases heterozygosity.
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Population Genetics ▷ Genetic Drift
Genetic drift has several important effects on evolution:
1.Drift reduces genetic variation in populations, potentially
reducing a population’s ability to evolve in response to new
selective pressures.
2.Genetic drift acts faster and has more drastic results
in smaller populations. This effect is particularly
important in rare and endangered species.
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Population Genetics ▷ Genetic Drift
Nindividuals=20
fixed:1
lost:0
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Population Genetics ▷ Genetic Drift
Nindividuals=5
fixed:2
lost:8
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Population Genetics ▷ Genetic Drift
Population size: 5
Generation time: 20
Number of populations: 10
Initial allele freq: q=p=0.5
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fixed
(p=1)
lost
(p=0)
1
6
4
2
5
3
Population size: 20
Generation time: 20
Number of populations: 10
Initial allele freq: q=p=0.5
fixed
(p=1)
lost
(p=0)
1
0
1
4
2
0
0
6
3
3
0
1
4
3
6
4
1
0
5
3
5
5
2
0
6
4
2
6
0
0
7
2
6
7
1
0
8
4
4
8
2
0
9
3
6
9
3
0
10
3
5
10
0
2
m
3.9
4.5
m
0.9
0.4
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Population Genetics ▷ Genetic Drift
Large changes in allele frequency from one generation to the next are likely
in small populations due to chance. This effect may cause an increase in
frequency of alleles that have harmful effects (e.g. inbreeding
depression). Such deleterious alleles are continually introduced by mutation
but are kept at low frequencies by natural selection. Moreover, most of these
harmful alleles are recessive, so their harmful effects on the phenotype are
only expressed in homozygotes. It is estimated that every individual in a
population harbours several of these harmful recessive alleles in a
heterozygous condition without any phenotypic effects.
(Cruz et al. 2008, vonHoldt et al. 2010)
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Population Genetics ▷ Genetic Drift
Genetic drift has several important effects on evolution:
1.Drift reduces genetic variation in populations, potentially
reducing a population’s ability to evolve in response to new
selective pressures.
2.Genetic drift acts faster and has more drastic results
in smaller populations. This effect is particularly
important in rare and endangered species.
3.Genetic drift tend to make different populations different
from each other. It can contribute to speciation.
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Population Genetics ▷ Genetic Drift
Genetic drift tend to make different populations different from each other.
–
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Population Genetics ▷ Genetic Drift
A population bottleneck is a significant reduction in the size of a population
that causes the extinction of many genetic lineages within that population,
thus decreasing genetic diversity. Population bottlenecks have occurred in the
evolutionary history of many species, including humans.
The probability of an allele being lost after a bottleneck is:
(1 − p)
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2N
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(1 − p)2 N
Probability of an allele being lost
Population Genetics ▷ Genetic Drift
N=2
N=5
N=25
p
plot(p,(1-p)^(2*N1),xlab="p",type="l",col="blue",ylab="")
points(p,(1-p)^(2*N2),col="green",type="l")
points(p,(1-p)^(2*N3),col="red",type="l")
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Population Genetics ▷ Genetic Drift
If a population is reduced to N individuals
for one generation then the expected total
number of alleles (A’) remaining is:
A
E( A′ ) = A − ∑ (1 − p)
2N
j =1
A : initial number of alleles
pj :the frequency of the jth allele
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j =1
A
E( A′ ) = A − ∑ (1 − p)2 N
Population Genetics ▷ Genetic Drift
Probability of retaining a rare allele (p = 0.01, 0.05, or 0.10)
after a bottleneck of size N for a single generation.
Allendorf, Luikart, and Aitken (2013)
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Population Genetics ▷ Genetic Drift
The founder effect is a particular example of the influence of random sampling. It was
defined by Ernst Mayr: "The establishment of a new population by a few original founders (in
an extreme case, by a single fertilised female) which carry only a small fraction of the total
genetic variation of the parental population."
The founding of a new population by a small number of individuals could
cause abrupt changes in allele frequency and loss of genetic variation.
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Population Genetics ▷ Genetic Drift
Species with lower population growth rates may persist at small population
sizes for many generations, during which heterozygosity is further eroded.
Therefore, bottlenecks and founder events have a more long-lasting effect on
the loss of genetic variation in species with smaller growth rate.
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Population Genetics ▷ Genetic Drift
Simulated loss of heterozygosity and
allelic diversity at eight microsatellite
loci during a bottleneck of two
individuals for five generations. The
initial allele frequencies are from a
population of brown bears from the
Western Brooks Range of Alaska.
Redrawn from Luikart and Cornuet
(1998).
Bottlenecks and founder events have a greater effect on the allele diversity
(number of alleles) in a population than on heterozygosity.
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Population Genetics ▷ Genetic Drift
A1A2
A3A4
A3A4
A5A6
A7A8
Nallele = 8
f(AxAy) = 100%
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Nallele = 2
f(AxAy) = 100%
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Population Genetics ▷ Genetic Drift
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Population
Genetics
▹
Exercises
Population Genetics ▷ Genetic Drift
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Population Genetics ▷ Genetic Drift
A founder effect occurs when a new colony is started
by a few members of the original population. This
small population size means that the colony may
have:
• _________ genetic variation from the original
population.
• a _______________ of the genes in the original
population.
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Population Genetics ▷ Genetic Drift
[1] Exercise Huntington Disease
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Population Genetics ▷ Genetic Drift
Huntington’s chorea is a devastating but rare (30-70 cases per million people in most Western
countries) human genetic disease. People who inherit this genetic disease have an abnormal
dominant allele that disrupts the function of their nerve cells, slowly eroding their control over
their bodies and minds and ultimately leading to death.
In 1993, a collaborative research group discovered the culprit responsible for Huntington’s: a
stretch of DNA that repeats itself over and over again:
HTT at chr4:3046206-3215485
1 matleklmka feslksfqqq qqqqqqqqqq qqqqqqqqqq pppppppppp pqlpqpppqa
61 qpllpqpqpp ppppppppgp avaeeplhrp kkelsatkkd rvnhcltice nivaqsvrns
121 pefqkllgia melfllcsdd aesdvrmvad eclnkvikal mdsnlprlql elykeik...
CAG is the genetic code for the amino acid glutamine (Gln or Q), so a series of them results in
the production of a chain of glutamine known as a polyglutamine tract (or polyQ tract), and the
repeated part of the gene, the PolyQ region.
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Population Genetics ▷ Genetic Drift
In the fishing villages located near Lake Maracaibo in
Venezuela (see map at right), there are more people with
Huntington’s disease than anywhere else in the world. In some
villages, more than half the people may develop the disease.
(Q1) How is it possible that such a devastating genetic disease
is so common in some populations?
(Q2) Shouldn’t natural selection remove genetic defects from
human populations?
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Population Genetics ▷ Genetic Drift
MORE
THINGS
CONSIDERED
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Population Genetics ▷ Genetic Drift
Population genetics theory predicts that severe population bottlenecks result in a loss of genetic
variation (Nei et al. 1975, Lacy 1997, Frankham 1995). This loss increases the likelihood of
inbreeding, reducing individual fitness and overall population viability (Lande 1988). Inbreeding can
reduce fitness through the production of homozygotes. Homozygotes result in reduced fitness when
(i) heterozygotes for rare lethal or nearly lethal alleles interbreed (Lande 1988), or (ii) when
homozygotes are produced at loci where over-dominance (heterozygote advantage) is acting. In
addition to these well known inbreeding effects, theory also predicts that smaller populations are
more likely to respond to genetic drift than to selection even when selection is acting (Barton and
Charlesworth 1984, Ohta 1995). Overall, a loss of genetic diversity reduces individual fitness and
mean population fitness, and results in less evolution through natural selection and more evolution via
genetic drift. The empirical effects of a genetic bottleneck include a loss of heterozygosity (Nei 1987,
Frankham et al. 1999), a decrease in allele frequency, a loss of alleles (Bouzat et al. 1998, Glenn et
al. 1999), and an increase in frequency or fixation of alleles that may be deleterious (Lacy 1997, Ralls
et al. 2000). However since these measures are only meaningful if they can be used to demonstrate a
loss or change, they are best interpreted through comparisons with a pre-bottleneck sample from the
same population (Bouzat et al. 1998, Matocq and Villablanca 2000). Otherwise, we may
erroneously attribute low genetic diversity to a demographic bottleneck (change) when in
fact it reflects historically low levels of diversity (no change).
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Population Genetics ▷ Genetic Drift
Effect of bottleneck size (smallest number of individuals recorded in the population) and percentage
hatching failure in 51 bird species. Hatching failure is plotted on a linear scale and bottleneck size is plotted
on a logarithmic scale, although both were log transformed in analyses (Heber and Briskie, 2010).
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Population Genetics ▷ Genetic Drift
Northern elephant seals (Mirounga angustirostris) have
reduced genetic variation probably because of a population
bottleneck humans inflicted on them in the 1890s. Hunting
reduced their population size to as few as 20 individuals at the
end of the 19th century. Their population has since rebounded
to over 30,000—but their genes still carry the marks of this
bottleneck: they have much less genetic variation than a
population of southern elephant seals that was not so
intensely hunted.
The California Condor (Gymnogyps californianus) has
recently survived a severe population bottleneck. The entire
population was reduced to 27 individuals in 1982. The
number of genetic founders was even smaller. Of the 169
fertile California Condor eggs laid in captivity through 1998,
five resulted in severely deformed embryos. These birth
defects were diagnosed as chondrodystrophy, a lethal form
of dwarfism.
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Population Genetics ▷ Genetic Drift
The main cause of the catastrophic decline of the population of Mauritius
kestrels (Falco punctatus) was the destruction of a huge amount of the
native forest habitat. Introduced predators such as black rats (Rattus
rattus), feral cats (Felis catus), and mongooses (Herpestes auropunctatus),
also took their toll. In the mid 20th Century, pesticides such as DDT were
widely used on the island and this further decimated the population;
predators such as the kestrel that are at the top of the food chain are
particularly susceptible to the build up of these chemicals. By 1974, only
four birds remained in the wild, and this minute population was incredibly
vulnerable. Today there are more than 800 mature birds, with numbers
rising; it is estimated that the remaining habitat allows for a carrying
capacity of maybe 50-150 more.
The Golden Toad (Bufo periglenes) was
once abundant in a small region of highaltitude cloud-covered tropical forests,
about 30 square kilometers in area, above
the city of Monteverde, Costa Rica. It was
first described in 1966 and since 1989, not
a single B. periglenes is reported to have
been seen. It is classified by the IUCN as an
extinct species
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The Passenger Pigeon (Ectopistes
migratorius) was a species of pigeon that
lived in enormous migratory flocks sometimes containing more than two
billion birds.
The Bali Tiger (Panthera tigris balica) was a
subspecies of tiger which was found solely
on the small Indonesian island of Bali. This
was one of three sub-species of tiger found
in Indonesia along with the Javan tiger (also
possibly extinct) and Sumatran tiger (critically
endangered).
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Population Genetics ▷ Genetic Drift
Genetic drift—one of the basic mechanisms of evolution—is simply the evolutionary
equivalent of a sampling error.
Imagine a game in which you have a bag holding 100 marbles, 50 of which are brown and 50
green. You are allowed to draw 10 marbles out of the bag. Now imagine that the bag is
restocked with 100 marbles, with the same proportion of brown and green marbles as you
have just drawn out. The game might play out like this:
The ratio of brown to green marbles “drifts” around (5:5, 6:4, 7:3, 4:6 . . .)
This drifting happens in populations of organisms. Due to many random factors, the genes in
one generation do not wind up in identical ratios in the next generation, and this is evolution. It
is possible for the frequency of a certain gene to increase in a population without the help of
natural selection. While this is evolution, it is evolution due to chance, not selection.
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Population Genetics ▷ Genetic Drift
Imagine that our random draws from the marble bag produced the following pattern: 5:5, 6:4,
7:3, 4:6, 8:2, 10:0, 10:0, 10:0, 10:0, 10:0... Why did we keep drawing 10:0? Because if the
green marbles fail to be represented in just one draw, we can’t get them back—we are “stuck”
with only brown marbles.
The same thing can happen to populations. If the gene for green coloration drifts out of the
population, the gene is gone for good—unless, of course, a mutation or gene flow reintroduces
the green gene again.
The 10:0 situation illustrates one of the most important effects of genetic drift: it reduces the
amount of genetic variation in a population. And with less genetic variation, there is less for
natural selection to work with. If the green gene drifts out of the population, and the population
ends up in a situation where it would be advantageous to be green, the population is out of luck.
Selection cannot increase the frequency of the green gene, because it’s not there for selection to
act on. Selection can only act on what variation is already in a population; it cannot
create variation.
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Population Genetics ▷ Genetic Drift
t+1
t
t+1
t
allele
probability =
1
2N
probability = 1 −
Heterozygosity after one generation:
H 1 = (1 −
Heterozygosity after t generations:
H t = (1 −
1
2N
1
2N
)H
0
)H
1 t
2N
0
➥ The equation indicates that the heterozygosity declines each generation at a
rate inversely dependent on the population size.
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