Download Experimental design II: artificial selection

Comparison of Experimental Designs
Artificial selection
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Artificial Selection
Artificial Selection
R1
μ2
μ1
μ0
Trait value
offspring
Rcum
R2
Trait value parents
R = h2S
μ0
μ0,S= μ1 μ1,S= μ2
S1
S2
Scum
2
Artificial Selection
Wing-length in milkweed bugs
(Oncopeltus fasciatus)
Artificial Selection
R1 = (μ1 − μ 0 ) = h 2 (μ S , 0 − μ 0 ) = h 2 S1
R2 = (μ 2 − μ1 ) = h 2 (μ1, S − μ1 ) = h 2 (μ1, S − μ 0, S ) = h 2 S 2
R2,cum = R1 + R2 = h 2 S1 + h 2 S 2 = h 2 (S1 + S 2 )
n
n
t =1
t =1
Rt ,cum = ∑ Rt =h 2 ∑ S t = h 2 S t ,cum
→ h2 =
Rt ,cum
S t ,cum
Eq. 1
Eq. 2
t: index for generation
One way to obtain an estimate of the „realized heritability“ from selection
experiments is to calculate the proportion of the total evolutionary response over
the total amount of applied selection (Eq. 2). More commonly used is, however,
the method of fitting a regression line of the cumulative response to selection on
the cumulative selection differential over the course of the selection experiment
(Eq. 1; see next slide).
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Artificial Selection
Wing-length in milkweed bugs
(Oncopeltus fasciatus)
Artificial Selection
The intensity of selection i
A generalized way of expressing the selection differential in terms of the
phenotypic standard deviation (standardized scale). (see standard normal
distribution)
R = h 2 S ; S = iσ p
i and proportion selected
R = ih 2σ p
Net selection on a trait when the applied
selection is not equal between the sexes
corresponds to the mean of selection on the
two sexes
S=
1
(Sm + S f )
2
If selection is only on females:
1
1
S = (0 + S f ) = S f
2
2
1 2
→R= h S
2
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Artificial Selection
The intensity of selection i
This artificial selection experiment on 6-week body weight mice (carried out
over 30 generations in the upward and 24 generations in the downward
direction) demonstrates how the intensity of selection (proportion selected)
predicts the response to selection. Natural selection opposing artificial
selection in the downward line due to fertility problems are visible).
From Falconer & Mackay 1996
Artificial Selection
Population size and the response to selection
The „Robertson model“ (1960)
Rmax = 2 N eih 2σ p
= 2Neh2S
Ne: effective population size
Maximal response to selection after an infinite
number of generations
Faster loss of additive genetic variance in
small populations due to inbreeding. This
equation only models the loss of variance
from inbreeding, not due to selection. And it
does not incorporate the effect of mutation on
maintained variance. Under these
assumptions, the optimal selection strategy
that maximizes the total response is to select
50% from the population.
From Falconer & Mackay 1996; Roff, 1997
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Artificial Selection
Limits of selection
• faster in small populations due to
inbreeding
• faster under higher intensity of
selection
Trait value
1. Loss of additive genetic variation
2. Natural selection opposing artificial selection
Generations
3. Mutational variance to small (by definition rare at occurrence)
4. Extrinsic limit for trait
(e.g., there are specific limits for how small an organism can become)
Artificial Selection
(No) Limits of selection
Reversal of selection
2. Mutations
R = 2 N ei
VA + VM
σp
VM: additive variance generated by
mutations
The longer the duration of artificial
selection experiments, the larger
the influence of „new mutations“ on
the process. Realized heritabilities
from long-term artificial selection
experiments do not represent the
heritability in the founder
population.
Extrinsic limit
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Artificial Selection
Trait value
Asymmetric responses to selection
Asymmetric
response to
selection
→ There can only be one
possible heritability for any trait
Generations
Artificial Selection
Asymmetric responses to selection
From Roff, 1997
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Artificial Selection
Asymmetric responses to selection
Possible underlying causes:
• Random drift (solution: replicate lines)
• Natural selection asymmetrically interferring with artificial
selection (solution: measure natural selection)
• Biased mutation (mutations tending to affect phenotype in
one direction)
• Inbreeding depression (solution: control lines)
• Maternal effects
• Genetic asymmetry
• Scalar asymmetry
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