zygote

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zygote
viability selection
survival to adult
courtship
sexual selection
fertilization
sexual selection
gamete production
fecundity selection
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Fitness individuals genetic contribution to
the next generation (zygote?zygote)
differential survival and/or reproduction
absolute fitness, Wij offspring, lifespan,
etc relative fitness, wij contribution
relative to other genotypes selection
differential, sij strength of
selection against a genotype
A1A1 A1A2 A2A2 Pr(survival) Wij
80 40 20 wij 1.0 0.5 0.25 sij
0 0.5 0.75
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Survival after selection
But what is the sum of these?
0.6875
To make them sum to one (for a new frequency) you
must divide by 0.6875
What is 0.6875?
It is the mean fitness. (p2w11 2pqw12q2w22)
New genotype frequencies 0.363 0.546
0.091
What are the new allele frequencies?
A1 0.64 (0.5) A2 0.36 (0.5)
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How does selection change genotype and allele
frequencies?
A1A1 A1A2 A2A2 geno. freq. p2 2pq q2
relative fitness, wij w11 w12 w22 geno.
freq. p2w11 2pqw12 q2w22 after selection
w w w
average relative fitness, w p2w11 2pqw12
q2w22
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Patterns of selection -- Fitness arrays
A1A1 A1A2 A2A2 w11 w12
w22 deleterious recessive 1 1
1-s recessive lethal 1 1
0 deleterious dominant 1 1
1s deleterious dominant 1 1-s
1-s deleterious intermediate 1 1-hs
1-s deleterious recessive 1 1s
1s heterozygote advantage 1-s 1
1-t heteroz. disadvantage 1s 1 1t
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Selection against a recessive allele
A1A1 A1A2 A2A2 initial g.f. P2 2pq
q2 rel. fitness 1 1 1-s w p2(1)
2pq(1) q2(1-s) 1
q2s g.f. gt selection p2(1)
2pq(1) q2(1-s) 1-q2s 1-q2s 1-q2s
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A numerical example

• A1A1 A1A2 A2A2
• gen. freq. 0.25 0.50 0.25
• wij 1.0 1.0 0.4
• gen. freq. 0.25(1) 0.5(1)
  0.25(0.4)
• gt selection 0.85 0.85
  0.85

0.294 0.588 0.118
f(A1) 0.59 (0.5) f(A2) 0.42 (0.5)
w 0.294(1) 0.588(1) 0.25(0.4) 0.982
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what is the new frequency of A2 ?
1 2
q Q H
q
q2(1-s) 1-sq2
recall that p 1 - q and q 1 - p
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change in the frequency of a lethal recessive in
Tribolium castaneum
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change in the frequency of a deleterious
recessive
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what is the new frequency of A2 ?
1 2
q Q H
q
q2(1-s) 1-sq2
recall that p 1 - q and q 1 - p
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how much has the frequency of A2 changed after
one generation of selection ?
Dq q - q -
q Dq
q(1-sq) 1-sq2
q sq2 q sq3 1-sq2
-sq2(1 q) 1-sq2
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selection against a deleterious recessive allele
Dq
q
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Selection against a dominant allele
A1A1 A1A2 A2A2 initial g.f. P2 2pq
q2 rel. fitness 1 1-s 1-s w p2(1)
2pq(1-s) q2(1-s) 1
sq(2p-q) g.f. gt selection p2(1)
2pq(1-s) q2(1-s) 1-sq(2p-q)
1-sq(2p-q) 1-sq(2p-q)
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change in the frequency of a deleterious dominant
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Selection against a dominant allele
Dq
q
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Selection favoring heterozygotes
A1A1 A1A2 A2A2 initial g.f. P2 2pq
q2 rel. fitness 1-s 1 1-t w p2(1-s)
2pq(1) q2(1-t) 1
p2s - q2t g.f. gt selection p2(1-s)
2pq(1) q2(1-t) 1 p2s - q2t 1 p2s -
q2t 1 p2s - q2t
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Dq - q and Dp - p
at equilibrium, Dq 0 and Dp 0
1 tq 1 - sp

tq sp s(1-q)
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heterozygote advantage
Dq
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heterozygote advantage at phosphoglucose isome
rase in Colias butterflies
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glycolysis
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enzyme kinetics of phosphoglucose isomerase in
Colias
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.15
.
.
.10
.
.
.05
deviation from expected heterozygosity
0
-.05
-.10
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3
11
17
July
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Selection against heterozygotes
A1A1 A1A2 A2A2 initial g.f. P2 2pq
q2 rel. fitness 1s 1 1t w p2(1s)
2pq(1) q2(1t) 1
p2s q2t g.f. gt selection p2(1s)
2pq(1) q2(1t) 1p2sq2t 1p2sq2t
1p2sq2t
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at equilibrium, Dq 0 and Dp 0
s s t
t s t
gt
gt
q and p
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heterozygote disadvantage
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heterozygote disadvantage translocation
heterozygotes in Drosophila
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simple models of selection
w11 gt w12 lt w22
w11 gt w12 gt w22
unstable polymorphism
fix A1
relative fitness of A1A1
w11 lt w12 gt w22
w11 lt w12 lt w22
stable polymorphism
fix A2
relative fitness of A2A2
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relative fitness enables different traits and
populations to be compared selection can act
at many stages in the life cycle opportunity
for opposing selection at different
stages directional selection fixes one allele
and eliminates all others from the
population heterozygote advantage can maintain a
balanced polymorphism, but cannot explain the
high levels of genetic variation found in
natural populations heterozygote disadvantage
produces an unstable polymorphism which allele
is fixed depends on chance