1. 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 individualsthat contribute genes to the next generation

2. 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
(4N% )(N&)
N% + N&
Ne =
Mating systems where Sex Ratio greatly skewed alter Ne
-- dominance hierarchy
-- resource polygyny
-- leks

3. Ne and population fluctuations
if population size fluctuates frequently,
the arithmetic mean is not appropriate
hare-lynx cycles
Ne = Ni 1 n
i=1 t
humans in Egypt
Ne:
Ne = 110

4. 3 average Ne affected disproportionately by the
smallest size --> population bottleneck
results in the loss of genetic variation
With a fluctuating population use the
harmonic mean (geometric average):
Ne t Ni 3
i=1 t =
Ne: Ne ~ 70
population fluctuations characterize irruptive species
-- forest insects (e.g., gypsy moth)
-- seed-eating finches
-- voles – hare – lynx
-- coniferous trees

5. if Ne is small, what are the genetic consequences??
-- neutral alleles
-- interaction of selection
and small population size

6. genetic drift is sampling effect
“true frequency distn” vs. “sample frequency distn”
take finite samples from an infinite population ---> 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 ---> finite number of adults
(population) (sample)

7. “true” mean = 0.35
X = X =

8. Random walk of allele frequency in one dimension
- direction of change is random in each generation
- amount of change is determined by Ne (Dq % ) 1 Ne q

9. 2N = 18
2N = 100

10. 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)
With x copies of an allele in the population,
pr(fixation) = x/2N
*common alleles are more likely to be fixed

11. Ht+1 = (1 - )Ht how does drift affect heterozygosity??
because alleles are fixed or lost, heterozygosity
and within popn genetic variance declines
What is the chance that two identical alleles from an
individual unite to form a zygote?
1/2N
When they do heterozygosity is lost. What is the chance
that they do not unite?
1- 1/2N
Ht+1 = ( )Ht 1
2Ne

12. how does drift affect heterozygosity??
because alleles are fixed or lost, heterozygosity
and within popn genetic s2 decline
After t generations
Ht = ( )tH0
if 2N is large, 1/2N ~ 0, and ( ) ~ 1, Ht ~ H0
if 2N is small, 1/2N ~ ‘ ‘, and ( ) ~ 0, Ht ~ 0 1 2N 1 2N 1 2N
very
large

14. drift decreases within population genetic variance
drift changes allele frequencies within a population in a random fashion
drift decreases within population genetic variance
net change in mean allele frequency (across all popns) due to drift is 0
drift increases among population genetic variance
A2A2
A1A1
A1A2
A2A2
A1A1
A1A1
A1A2
A2A2
A1A1
A1A1
A1A2
A2A2
A1A1
A1A2
A2A2
A2A2
A2A2
A1A1
A1A2
A2A2
A1A1
A1A2
A2A2
A1A1

15. Drift in real populations
Buri 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???

17. Buri’s 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: > 0.16)

19. Genetic drift and selection
Drift fixes alleles (at random)
when Ne is small
Directional selection fixes the
favored allele
When does drift overcome selection
(or vice versa)??

20. interaction of drift and directional selection
AA Aa aa initial f(A) = 0.5
wij
N= N=10

21. interaction of drift and directional selection
AA Aa aa initial f(A) = 0.5
wij
N=10

22. interaction of drift and selection AA Aa aa initial f(A) = 0.5
wij
N=50

23. 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 >> 1/4Ne, selection determines outcome
if s << 1/4Ne, drift can overcome selection 1
4Ne

24. s = 0.01 Ne = 10 1/4Ne = 0.025

25. Drift and Selection, con’t
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 ---> small population size
- loss of heterozygosity
- fixation of deleterious alleles
---> “mutational meltdown”

26. Founder Event
Genetic drift occurs when a new population is
started by a small number of individuals
Usually, loss of rare or uncommon alleles (i.e.,
reduced polymorphism, within popn genetic s2)

28. Founder event
Genetic drift occurs when a new population is
started by a small number of individuals
Usually, loss of rare or uncommon alleles (i.e.,
reduced polymorphism, within popn genetic s2)
Loss of variation may differentially affect
social organisms

32. 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

33. 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