The document discusses the concept of genetic drift, which is the change in allele frequencies over time due to random chance rather than natural selection. It provides examples of genetic drift occurring through bottlenecks, founder effects, and stochastic sampling in populations that have experienced reductions in size. The document then simulates genetic drift through a "Haploid Sex with Chocolate" activity, where students exchange M&M "gametes" over multiple generations to model how drift can change allele frequencies randomly over time in a finite population.

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

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Frequency
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Frequency
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Generations
Frequency
Red:
0.1667
Blue:
0.1667
Orange:
0.1667
Green:
0.1667
Brown:
0.1667
Yellow:
0.1667

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Frequency
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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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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