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Evolution at Multiple Loci: Quantitative Genetics. I. Rediscovery of Mendel and Challenges to Natural Selection. Do traits that exhibit continuous variation have a genetic basis? - PowerPoint PPT Presentation
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Evolution at Multiple Loci:Quantitative Genetics
I. Rediscovery of Mendel and Challenges to Natural Selection
• Do traits that exhibit continuous variation have a genetic basis?
• If the only traits which have genetic variation are controlled by one or two loci then natural selection not as important as mutation
• Darwin envisioned evolution to be a continuous process of selection acting on limitless genetic variation, with small changes occurring in any one generation, but large changes occurring over long periods.
Why the normal distribution: Central Limit Theorem
Mendelian genetics can explain quantitative traits
Ex. 1: NILSSON-EHLE: Red and White Kernel Color in Wheat (red dominant, white recessive)
Ex. 2:East’s workwith tobacco
Quantitative traits are influenced by the environment as well as genotype
Yarrow plant
1. Fisher’s prediction
Mutation Effect
Pro
bab
ility
of
Fix
atio
n
2. Kimura’s modification
3. Orr’s modification
II. Neo Darwinian SynthesisTheoretical models that support vs. contend the Darwinian model
Typical results
Corolla Width (mm)
F2
BC
F1
M. micranthus M. guttatus
F1
F2
Fenster & Ritland 1994
Testing the Models:
No filter
Filtered image—“bumblevision”
SegregationOf floral typesDemonstrateGenetic basisOf trait Differences
Convergent evolution??
Yosemite Sam thinks so
in the F2 generation
MC Qc
ML QL
ML QL
MC Qc
x
MC Qc
ML QL
If the map distance is 5 cm then there is a 95% chance that the marker will be associated with the QTL in the F2:
1- r(MQ)
MC1 Qc MC2
MC1 Qc MC2
x
If the map distance between markers and QTL are 5 cm then there is a 99.5% chance that one of the markers will be associated with the QTL in the F2:
1-2 r(M1Q)(QM2)
ML1 QL ML2
ML1 QL ML2
MC1 Qc MC2
ML1 QL ML2
xx
1. Fisher’s prediction
Mutation Effect
Pro
bab
ility
of
Fix
atio
n
2. Kimura’s modification
3. Orr’s modification
Theoretical models that support or contend with the Darwinian model
Alleles with a distribution of effect sizes contribute to adaptations
III. Measuring Selection and Response to Selection on Continuous Traits
A. Heritability
Song sparrows
Galapagos finches
58 60 62 64 66 68 70 72 74
Femal eHt
0
0. 05
0. 1D
e
n
s
i
t
y
90 105 120 135 150 165 180 195 210
Femal eWt
0
0. 005
0. 01
0. 015D
e
n
s
i
t
y
Class Data
Female Wt
Female HT
100 125 150 175 200 225 250 275
Mal eWt
0
0. 005
0. 01
0. 015D
e
n
s
i
t
y
62. 5 65. 0 67. 5 70. 0 72. 5 75. 0 77. 5
Mal eHt
0
0. 05
0. 1
0. 15
D
e
n
s
i
t
y
Male Wt
Male Ht
Model Equat i on
Femal eWt = 91. 1457 + 0. 2807 Mot her Wt
100 150 200 250
Mot her Wt
100
150
200
F
e
m
a
l
e
W
t
Heritability of Female Wt
Model Equat i on
Femal eWt = 75. 0179 + 0. 3094 Fat her Wt
150 200 250 300
Fat her Wt
100
150
200
F
e
m
a
l
e
W
t
Heritability of Female Wt
Model Equat i on
Femal eWt = 57. 2357 + 0. 4499 Mi dPar ent Wt
150 200 250
Mi dPar ent Wt
100
150
200
F
e
m
a
l
e
W
t
Heritability of Female Wt
Model Equat i on
Femal eHt = 48. 4108 + 0. 2592 Mot her Ht
55 60 65 70 75
Mot her Ht
60
65
70F
e
m
a
l
e
H
t
Heritability of Female Ht
Model Equat i on
Femal eHt = 26. 1514 + 0. 5575 Fat her Ht
65 70 75
Fat her Ht
60
65
70F
e
m
a
l
e
H
t
Heritability of Female Ht
Model Equat i on
Femal eHt = 23. 3220 + 0. 6198 Mi dpar ent Ht
62 64 66 68 70 72
Mi dpar ent Ht
60
65
70F
e
m
a
l
e
H
t
Heritability of Female Ht
Model Equat i on
Mal eWt = 137. 452 + 0. 1867 Mot her Wt
100 150 200
Mot her Wt
150
200
250
M
a
l
e
W
t
Heritability of Male Wt
Model Equat i on
Mal eWt = 107. 950 + 0. 2951 Fat her Wt
150 200 250 300
Fat her Wt
150
200
250
M
a
l
e
W
t
Heritability of Male Wt
Model Equat i on
Mal eWt = 99. 5721 + 0. 3870 Mi dPar ent Wt
150 200 250
Mi dPar ent Wt
150
200
250
M
a
l
e
W
t
Heritability of Male Wt
Model Equat i on
Mal eHt = 29. 1168 + 0. 6420 Mot her Ht
60 65 70
Mot her Ht
65
70
75
M
a
l
e
H
t
Heritability of Male Ht
Model Equat i on
Mal eHt = 31. 2457 + 0. 5623 Fat her Ht
65 70 75
Fat her Ht
65
70
75
M
a
l
e
H
t
Heritability of Male Ht
Model Equat i on
Mal eHt = 14. 7069 + 0. 8275 Mi dpar ent Ht
62 64 66 68 70 72
Mi dpar ent Ht
65
70
75
M
a
l
e
H
t
Heritability of Male Ht
Conclusions from class data:
Distributions of Wts and Hts are roughly normal
Distribution indicates that Wts and Hts are likely controlledby many loci, = many loci are segregating alleles that contribute to wt and ht differences among individuals
Heritabilities for Ht >> WT 50% >> 30%
Interpretation for other human traits??
Red
Red
Black
B. Selection
Functional significance of trait variation
S=
S= t* - t
t
t*
C. Response to Selection
The “2” term is meaningless, just an historical artifact of the derivation
The slope of the best-fit line is 0.13
Stabilizing selection on a gall-making fly
Disruptive selection on bill size in the black-bellied seedcracker
Genetic by Environment Interaction
Low Altitude Site (Stanford)
High Altitude Site, Mather California
Conclusion
• Continuous traits are common• Continuous traits can be heritable• Continuous traits can respond to
selection• Darwin’s notion of natural selection
acting on continuous variation is consistent with evidence
• Genetic x Environment interactions may be important