hs18 popgen 02 geneticdrift - unibas.chevolution.unibas.ch/.../hs18_popgen_02_geneticdrift.pdf ·...

Evolutionary Genetics

LV 25600-01 | Lecture with exercises | 6KP

HS2018

Genetic Drift

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Population Genetics ▷ Genetic Drift

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A Primer of Ecological Genetics Chapter 3 - Genetic Drift - Page 52-55

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Populationtn

Populationtn+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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Genetic drift, also called genetic sampling error or Sewall Wright effect, a change in the gene pool of a population that takes place strictly by chance. Genetic drift can result in genetic traits being lost from a population or becoming widespread (fixed) 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.

Allendorf, Luikart, and Aitken (2013)

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PopG genetic simulation program

http://evolution.gs.washington.edu/popgen/popg.html!5


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{AB} {B}

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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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What is the rate at which genetic variation is lost from populations?

ΔH = ′H − HH ′H

t0 t1

A1A1 A2A2A1A2 A1A1 A2A2A1A2 A1A1 A2A2A1A2 A1A1 A2A2A1A2

1

0.50.25 0.125

0.0 0.00.25 0.25

0.375 0.375 0.4375 0.4375

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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 wild-type and bw75 alleles.

adopted from: Buri (1956) Gene frequency in small population of mutant Drosophila.

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probability = 12N

t t+1allele

probability = 1− 12N

t t+1

H1 = 1− 12N( )H0

Ht = 1− 12N( )t H0

Heterozygosity after one generation:

Heterozygosity after t generations:

➥ The equation indicates that the heterozygosity declines each generation at a rate inversely dependent on the population size.

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Expected heterozygosity (2pq) in populations undergoing genetic drift. The line shows the expected change in heterozygosity. Genetic drift increases homozygosity and decreases heterozygosity.

Exp

ect

ed

hete

rozy

go

sity

Generation

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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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Nindividuals=20

fixed:1

lost:0

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fixed:2

lost:8

Nindividuals=5

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fixed (p=1)

lost (p=0)

1 6 4

2 5 4

3 6 3

4 3 6

5 3 5

6 4 2

7 2 6

8 4 4

9 3 6

10 3 5

mean 3.9 4.5

Population size: 5 Generation time: 20 Number of populations: 10 Initial allele freq: q=p=0.5

fixed (p=1)

lost (p=0)

1 0 1

2 0 0

3 0 1

4 1 0

5 2 0

6 0 0

7 1 0

8 2 0

9 3 0

10 0 2

mean 0.9 0.4

Population size: 20 Generation time: 20 Number of populations: 10 Initial allele freq: q=p=0.5

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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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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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Genetic drift tend to make different populations different from each other.

–

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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.

(1− p)2NThe probability of an allele being lost after a bottleneck is:

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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")

N=2N=5N=25

(1−p)

2N

p

Prob

abili

ty o

f an

alle

le b

eing

lost

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E( ′A ) = A − (1− p)2Nj=1

A

∑

If a population is reduced to N individuals for one generation then the expected total number of alleles (A’) remaining is:

A : initial number of alleles pj :the frequency of the jth allele

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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)

E(′A)=

A−

(1−p)

2N

j=1A ∑

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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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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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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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A1A2

A3A4

A5A6

A7A8

A3A4

Nallele = 8 Nallele = 2

f(AxAy) = 100% f(AxAy) = 100%

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Random genetic drift …

[yes/no] is a random factor and therefore the outcome cannot be predicted.

[yes/no] depends on population size.

[yes/no] is more likely to remove rare alleles.

[yes/no] decreases genetic diversity over time.

[yes/no] is driving a population away from H-W equilibrium.

[yes/no] is bringing subpopulation closer to each other

[yes/no] is reducing the number of alles in a population over time.

[yes/no] is leading to fixation of alleles.

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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 alleles in the original population.

What does a founder effect and a bottleneck have in common?

___________________________________________

___________________________________________

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hs18 popgen 02 geneticdrift - unibas.chevolution.unibas.ch/.../hs18_popgen_02_geneticdrift.pdf ·...

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