Response to selection can be
fast!
Selection is
strong
Favored
allele is
partially
dominant
Both alleles
are
common
Selection is not always
“Directional”
• Heterozygote advantage
• Frequency dependence
• Selection varying in space or time
Heterozygote
advantage
Fitnes
s
AA
Aa
aa
Relative fitness of hemoglobin genotypes in Yorubans
Relative Fitness
HbA/HbA
0.88
HbA/HbS
1.0
HbS/HbS
0.14
Fitness (in symbols)
1-t
1
1-s
Selection coefficients
t=0.12
Equilibrium frequencies:
peq = s/(s+t) = 0.86/(0.12+0.86) = 0.88
qeq = t/(s+t) = 0.12/(0.12+0.86) = 0.12
Predict the genotype frequencies (at birth):
HW proportions
0.774
0.211
s=0.86
0.0144
Variable selection: genotypes have different
fitness effects in different environments
1
0.9
0.8
AA
Aa
aa
Fitness 0.7
0.6
0.5
0.4
Env. 1
Env. 2
Env. 3
Frequency-dependent selection
Selection
Whether directional or stabilizing,
causes adaptive changes in allele
frequencies
Forces causing evolution:
Random Genetic Drift
Changes in allele frequency due
to random sampling: not adaptive
10 Populations, N=15
Drift occurs even in large populations!
N=10,000
Genetic drift eliminates
genetic variation
Forces that cause evolution
Mutation
Ultimate source of all genetic variation
Mutation is generally not adaptive
How common is mutation?
Achondroplastic dwarfism
• Dominant autosomal allele
• Recurrent mutation rate: 3/200,000 =
0.000015 per generation
• q0=0.0; q1 = 0.000015, q2 = 0.000030
Mutation/Selection Balance
Even highly deleterious mutations can persist at
substantial frequency, especially if they are
recessive:
Selection against a recessive allele is s
Genotype
Fitness
AA
1
For recessive lethal, s = 1
Aa
1
aa
1-s
Mutation-selection equilibrium
Recessive deleterious alleles:
qe = √(/s)
If a recessive lethal (s=1) has a recurrent
mutation rate of 1.5*10-5, what is it’s
equilibrium frequency?
qe = 0.004
Mutation maintains substantial
genetic variation
Organism
C. Elegans
D. melanogaster
Mouse
Human
Deleterious mutations
per genome/gener’n
0.04
0.14
0.9
1.6
HIV virus is thought to have mutation rate
~10 X greater than humans!
Forces causing evolution:
Non-random mating:
Inbreeding
Mating between relatives
What happens to genotype
frequencies under inbreeding?
Most extreme form of inbreeding is selfing
P:
F1:
25% AA
Aa x Aa
50% Aa
25% aa
F2:
37.5% AA
25% Aa
37.5% aa
F3:
43.75% AA 12.5% Aa 43.75% aa
Fewer heterozygotes and more
homozygotes each generation
What happens to heterozygosity
under inbreeding?
Generations
of selfing
0
1
2
3
Heterozygosity:
Prop. of heterozygotes
100% Aa
50% Aa
25% Aa
12.5% Aa
What happens to allele
frequencies under inbreeding?
P:
F1:
F2:
F3:
25% AA
Aa x Aa
50% Aa
25% aa
37.5% AA
25% Aa
37.5% aa
43.75% AA 12.5% Aa 43.75% aa
Allele frequencies do not change under
inbreeding, but population is perturbed from
H-W proportions.
Inbreeding Depression
70
60
50
Yield
40
30
20
10
0
0
0.25
0.5
0.75
Inbreeding Coefficient
1
Pup survival relative to
Inbreeding
Inbreeding Coefficient
< 0.19
0.25-0.67
> 0.67
Survival
75%
51%
25%
Brother-sister or parent-offspring mating reduces
the heterozygosity by 25% per generation:
G0: H=1
G1: H= ?
G2: H= ?
Proportions of individuals w/ genetic
disease who are products of first
cousin marriages
Migration between
subpopulations
Tends to equalize allele
frequencies among
subpopulations, even if the allele
frequencies differ because of
differing selection pressure
Migration: island model
qm = 0.9
Migration rate=
m=0.05
q = 0.1
q' = (1-m)q + mqm = q - m(q - qm)
q' = 0.1 +0.04 = 0.14
Evolution is the result of
violating assumptions of H-W
• These ideas are straightforward.
• Mathematics can be complicated,
especially when multiple
evolutionary forces are occurring
simultaneously
Practical Considerations
• Evolution of pathogens (HIV, SARS,
West Nile Virus, etc.)
• Evolution of antibiotic resistance
• Evolution of pesticide and herbicide
resistance
• Conservation of genetic diversity in
natural, captive, and agricultural
species.