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Selection

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Quantifying Darwin’s postulates
1. Individuals within species vary.
2. Some of these variations are heritable.
3. More offspring are produced than can survive.
4. Survival and reproduction are nonrandom.
The individuals that survive & reproduce the most are those
with variations that best suit their environment.
Quantitative genetics allows us to measure the degree to which
variation in a trait is heritable (and therefore can respond to selection).
We can also measure the strength of selection.
Combining heritability and strength of selection allows us to predict
evolutionary change in response to selection.
Requirements for evolution: Selection
number of
individuals
Selection differential
body size (or other trait)
5 6
S=
mean trait value
of selected individuals
-
S=6–5=1
mean trait value
before selection
Requirements for evolution: Selection
Selection gradient
1) absolute fitness of each individual
-
survival to reproduction? (0 = no; 1 = yes)
fecundity (number of offspring)
2) calculate population mean fitness
3) convert absolute fitness to relative fitness
-
divide absolute fitness by population mean fitness
spot
# of
absolute
brightness offspring
fitness
5
6
7
7
8
8
8
9
9
9
9
10
10
10
10
11
11
11
12
12
0
0
5
10
5
0
15
10
20
5
20
10
25
15
25
20
30
30
35
20
300/20
= 15
Requirements for evolution: Selection
Selection gradient
1) absolute fitness of each individual
-
survival to reproduction? (0 = no; 1 = yes)
fecundity (number of offspring)
2) calculate population mean fitness
3) convert absolute fitness to relative fitness
-
divide absolute fitness by population mean fitness
4) Plot relative fitness as a function of trait value
equation for best-fit line:
y = 0.3055x + 1.7802
slope = selection gradient
relative fitness
2.5
2
1.5
1
0.5
0
4
6
8
10
spot brightness
12
spot
# of
relative
brightness offspring
fitness
5
6
7
7
8
8
8
9
9
9
9
10
10
10
10
11
11
11
12
12
0
0
0.33
0.67
0.33
0
1.0
0.67
1.33
0.33
1.33
0.67
1.67
1.0
1.67
1.33
2.0
2.0
2.33
1.33
Selection x heritability = evolutionary response
R = h2 x S
response
to selection
heritability
selection
differential
The amount of evolutionary change in a population depends on the
strength of selection (selection differential) and the heritability of the trait.
Selection x heritability = evolutionary response
h2 ≈ 0.4
R
R = h2 x S
h2 = R / S
Midparent
S
Quantifying Darwin’s postulates
1. Individuals within species vary.
2. Some of these variations are heritable.
3. More offspring are produced than can survive.
4. Survival and reproduction are nonrandom.
The individuals that survive & reproduce the most are those
with variations that best suit their environment.
Combining heritability and selection differential allows us to predict
evolutionary change in response to selection.
R = h2 x S
Forms of selection
Directional selection
Stabilizing selection
Disruptive selection
Directional
selection
Directional
selection
changes the
population mean
Stabilizing
selection
Stabilizing
selection
reduces variation
Disruptive
selection
Disruptive
selection
increases variation
Forms of selection
Directional selection
change in population mean
Stabilizing selection
reduced variance (no change in mean)
Disruptive selection
increased variance (no change in mean)
Evolutionary biology after Darwin
• Evolution was widely accepted.
• Natural selection was debated as its primary mechanism.
– Mechanisms of inheritance were not understood:
• Can acquired features be inherited?
• Will favorable new variants be swamped out by blending?
• Discovery of Mendel’s work (published in 1866, �discovered’ in 1900)
and the principles of genetics
– Disproves blending inheritance.
– Natural selection requires discrete inheritance, or favorable
new mutations will simply be swamped out.
• Discovery of mutations (early 1900’s)
– Showed how variability could be generated.
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