Genetic drift in finite populations

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Genetic drift in finite populations HWE
assumes that population size is
infinite approximately true if population is
very large but, very large size unlikely for
most species -- habitat capacity --
environmental patchiness -- social
organization -- human-induced habitat
fragmentation
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Attwaters Prairie Chicken
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genetic drift is sampling effect true
frequency distn vs. sample frequency
distn take finite samples from an infinite
population ---gt most will deviate slightly
from the actual distribution (frequency of A1,
A2) if you have a large number of samples, the
average frequency over all samples will better
approximate the true distribution than any
single sample infinite number of gametes
---gt finite number of adults (population) (s
ample)
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founder event genetic drift occurs when a
new population is started by a small number
of individuals pr(homozygosity) (p2)N
(q2)N (r2)N usually, loss of rare or
uncommon alleles occasionally, increase in
the frequency of rare alleles Afrikaner
population of South Africa 40 families in 1652
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what is the probability that an allele is fixed
by drift?? if A1, A2, A3, A4, , An alleles
in a population N individuals, 2N alleles --
one is fixed, others lost if each allele is
unique, pr(Ai) is fixed 1/2N (2N possible
outcomes, pr(particular) 1/2N)
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Gen 1 2 3 4
5 6 7 8
9 10 11
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what is the rate of neutral evolution?? if A1,
A2, A3, A4, , An alleles in a population N
individuals, 2N alleles -- one is fixed, others
lost if each allele is unique, pr(Ai) is fixed
1/(2N) allow mutation neutral rate per
gene per generation total number of neutral
mutations 2N per generation rate of neutral
evolution 1/(2N) x 2N
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drift in real populations Buri (1956)
Drosophila melanogaster alleles at brown eye
color locus bw, bw75 are neutral to each
other initially f(bw) f(bw75) 0.5 107
replicate lines, Ne 16 (8, 8), each
generation randomly chose 16 new
individuals what happens to genetic variation???
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Buris results by generation 19, 30 popns fix
bw 28 popns fix bw75 which allele fixed is
random decrease in within popn variance
(58/107 fixed for one allele) increase in
among population genetic variance (s2 0
---gt 0.16)
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how does drift affect heterozygosity?? because
alleles are fixed or lost, heterozygosity and
within popn genetic s2 decline Ht (1 -
)tH0 if 2N is large, 1/2N 0, and (1 -
) 1, Ht H0 if 2N is small, 1/2N
, and (1 - ) 0, Ht 0
1 2N
1 2N
very large
1 2N
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Polymorphism in finite populations is the product
of neutral mutation without mutation f
1 - f with
mutation

( )
1 1 2N 2N
(1 u)2
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biological take-home points -- drift changes
allele frequencies within a population in a
random fashion drift decreases within
population genetic s2 -- net change in mean
allele frequency (across all popns) due to
drift is 0 drift increases among population
genetic s2 -- mutation maintains genetic
variation at neutral loci in finite populations
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Genetic drift in finite populations HWE
assumes that population size is
infinite approximately true if population is
very large but, very large size unlikely for
most species -- environmental patchiness --
social organization evolutionarily relevant
population size is not simply the number of
individuals Ne effective population size
number of individuals that contribute genes to
the next generation
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Effective population size, Ne affected by --
sex ratio (breeding system) -- fluctuations in
population size -- differences in fecundity due
to chance Ne and sex ratio for breeding
individuals only Ne Mating systems
where OSR greatly skewed alter Ne -- dominance
hierarchy -- resource polygyny -- leks
(4N )(N) N N
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Ne and population fluctuations if
population size fluctuates frequently, the
arithmetic mean is not appropriate
hare-lynx cycles
humans in Egypt
Ne 100 150 25 150 125 Ne 110
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average Ne affected disproportionately by
the smallest size --gt population
bottleneck results in the loss of genetic
variation With a fluctuating population use the
harmonic mean (geometric average)
Ne 100 150 25 150 125 Ne 70 population
fluctuations characterize irruptive species --
forest insects (e.g., gypsy moth) --
seed-eating finches -- voles hare lynx --
coniferous trees
t
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1 1 1 Ne t Ni

i1
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genetic drift and selection drift fixes alleles
(at random) when Ne is small directional
selection fixes the favored allele when does
drift overcome selection ( vice versa)?? s
strength of selection strength of
genetic drift if s gt 1/4Ne, selection
determines outcome if s lt 1/4Ne, drift can
overcome selection
1 4Ne
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s 0.01 Ne 10 1/4Ne 0.025 25
0.01 100 0.025 250
0.001
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drift and selection, cont population may not
attain equilibrium allele frequencies
predicted by selection coefficients deleterious
alleles can be fixed by drift if selection
is relatively weak slightly advantageous
alleles are less likely to be fixed in small
populations implications for conservation
biology - habitat fragmentation ---gt small
population size - loss of heterozygosity -
fixation of deleterious alleles
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Ne, the effective population size, can be
affected by mating system, temporal fluctuations
in population size, or differences in fecundity
due to chance genetic drift is sampling
error drift occurs in all finite populations,
but its relative importance is determined by
population size genetic drift reduces genetic
variation within a population by random fixation
of alleles, but increases genetic variance among
populations directional selection in small
populations is often less effective deleterious
alleles may occur at higher frequency and
may become fixed
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Genome-wide effects of drift may result in a high
genetic load of deleterious alleles and
consequent effect on individual and population
survival (mutational meltdown) Founder events
can produce new populations with radically
different patterns of genetic variation compared
to their source - loss of alleles, especially
rare alleles - changes in allele frequency -
changes in genetic variance depend on
subsequent rates of growth