Class lecture on genetic drift. Originally intended for intro undergrad.

1. Genetic Drift (and Haploid Sex
with Chocolate)
Lady Luck and the M&M’s at her mercy

2. Drift
• Genetic drift: change
in allele frequency
over time due to
stochastic effects
• Independent of
selection
– does not mean that
fitness is unimportant
• Can cause
nonadaptive or
maladaptive evolution

3. 0
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Generations
Frequency
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Generations
Frequency
0
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0 20 40 60 80 100
Generations
Frequency
Red:
0.1667
Blue:
0.1667
Orange:
0.1667
Green:
0.1667
Brown:
0.1667
Yellow:
0.1667

4. 0
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Generations
Frequency
0
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0.7
0.8
0.9
0 20 40 60 80 100
Generations
Heterozygosity
Red:
0.02
Blue:
0.013
Orange:
0.4
Green:
0.3
Brown:
0.2
Yellow:
0.067
Diversity loss due to drift

5. 3 ‘Kinds’ of Drift
• Drift is generally thought of as acting via
one of three processes, though all are
really examples of the same phenomena
– Bottleneck
– Founder Effect
– Stochastic Sampling

6. 3 ‘Kinds’ of Drift
• Drift is generally thought of as acting via
one of three processes, though all are
really examples of the same phenomena
– Bottleneck
• Reduction in population size
– Founder Effect
• Subset of population founds new population
– Stochastic Sampling
• Constant effect given finite population size

7. Bottlenecks
• Elephant Seals
– Reduced by 1890 20-50 adults
– What would happen to rare
alleles?
• Florida Panthers Felis concolor
– Isolated from other subspecies
of cougar for >100 years; range
contraction
– By 1980’s only 30-50 adults
– Congenital heart defects, low
sperm quality, and
cryptorchidism (kinked tail) all
due to low Ne

8. Bottlenecks
• California cypress
– Once very widespread
– Due to fire and
competition new
restricted habitat
– Population “islands”
• Drift caused change
in gene frequencies
– Different cone
morphologies evolved

9. Bottlenecks
• California cypress
– Once very widespread
– Due to fire and
competition new
restricted habitat
– Population “islands”
• Drift caused change
in gene frequencies
– Different cone
morphologies evolved

10. Simulating a Bottleneck
• Heterozygosity: 0.71
• Expected loss due to
one generation of
bottleneck: 1%
• Starting population of
M&Ms:
– Brown 0.013
– Orange 0.02
– Green 0.067
– Yellow 0.20
– Blue 0.4
– Red 0.3

11. Founder Effect
• Very similar to bottleneck
• A subsample of the main
population colonizes a
new population
• Cattle brought to iceland
by the Vikings
• Almost complete lack of
blood group B in
American Indians –
founder effect of crossing
the Bering Strait

12. Founder Effect
• Polydactyly -- a symptom
of the Ellis-van Creveld
syndrome.
• Common among Amish
of Pennsylvania, very
rare in the rest of the
population
• Ashkenazi Jews: idiopathic torsion dystonia (ITD) and
Tay-Sachs disease
– Small population size and unequal mating success

13. Founder Effect
• Polydactyly -- a symptom
of the Ellis-van Creveld
syndrome.
• Common among Amish
of Pennsylvania, very
rare in the rest of the
population
• Ashkenazi Jews: idiopathic torsion dystonia (ITD) and
Tay-Sachs disease
– Small population size and unequal mating success
• Variation in mating success
• Unequal sex ratio
• Population size fluctuation
• Overlapping generations

14. Simulating Founder Effect
• Starting population of
M&Ms:
– Brown 0.013
– Orange 0.02
– Green 0.067
– Yellow 0.20
– Blue 0.4
– Red 0.3
• Heterozygosity: 0.71
• Expected loss due to
founder effect: 3%

15. Stochastic Loss
• Happens in ALL finite populations
• Can cause “neutral” alleles or alleles under little
selection to change in frequency dramatically
over long periods of time
• Based on the the “bottleneck” that occurs
between generations: the number of gametes is
much greater than the number of individuals

16. Simulating Stochastic Loss
• Starting population of
M&Ms:
– Blue 0.013
– Red 0.02
– Yellow 0.067
– Brown 0.20
– Green 0.4
– Orange 0.3
• Heterozygosity: 0.71
• Expected loss due to
stochastic loss: 0.5%

17. • Each person will take 5 M&Ms of the same color. These
are your gametes (use them wisely).
• Your genotype is the same as whatever color your
gametes are.
Haploid Sex with Chocolate

18. • Each person will take 5 M&Ms of the same color. These
are your gametes (use them wisely).
• Your genotype is the same as whatever color your
gametes are.
• All students will get up from their seats and try to fertilize
as many other students as possible.
Haploid Sex with Chocolate

19. • Each person will take 5 M&Ms of the same color. These
are your gametes (use them wisely).
• Your genotype is the same as whatever color your
gametes are.
• All students will get up from their seats and try to fertilize
as many other students as possible.
• The Mating Rule: Give out as many gametes as
possible, but you can only accept ONE gamete
Haploid Sex with Chocolate

20. Haploid Sex with Chocolate
• Each person will take 5 M&Ms of the same color. These
are your gametes (use them wisely).
• Your genotype is the same as whatever color your
gametes are.
• All students will get up from their seats and try to fertilize
as many other students as possible.
• The Mating Rule: Give out as many gametes as
possible, but you can only accept ONE gamete
• At the end of the simulation the new gamete you
accepted becomes your genotype in the next generation.

21. Red:
0.020
Blue:
0.013
Orange:
0.300
Green:
0.400
Brown:
0.200
Yellow:
0.067
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Generations
Frequency
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Generations
Frequency
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Generations
Frequency

22. NO CHANGE
1 GAMETE EACH
Y:1/6 B:1/18 G:4/18 Br:4/18 R:1/9 O:4/18
Y:1/6 B:1/18 G:4/18 Br:4/18 R:1/9 O:4/18

23. MANY GAMETES
DRIFT
Y:1/6 B:1/18 G:4/18 Br:4/18 R:1/9 O:4/18
Y:1/9 B:0 G:5/18 Br:3/18 R:3/18 O:5/18