Natural Selection
I. Necessary Conditions
II. Selection in Nature
III. Units & Levels of Selection
Necessary Conditions for Evolution by
Natural Selection
• Variation among individuals
• Heritability of variation
• Differential reproduction of
variants
Natural Selection
Phenotypic Selection
Genetic Response
[Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]
Phenotypic Selection
Nonsexual Selection
Mortality
Selection
Sexual Selection
Fecundity
Selection
[Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]
Fitness
Frequency
Fitness
Frequency
Frequency
Fitness
Modes of Selection on Continuous Characters
Trait
Trait
Trait
Directional
Selection
Stabilizing
Selection
Disruptive
Selection
Variance
Selection
[Source: Brodie et al. 1995, Trends in Ecology & Evolution 10, 313-318]
Selection Differentials
Directional selection
Variance selection
i  Xa  Xb
j  va  vb
Xb 
mean before selection
Xa 
mean after selection
Positive if i > 0
Negative if i < 0
v 
b
va 
variance before selection
variance after selection
Stabilizing if j < 0
Disruptive if j > 0
[Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]
Response to Selection
R ih
2
where,
R  response to selection
i  selection differential
h2  heritability
[Source: Falconer 1981, Introduction to Quantitative Genetics, Longman Press]
Selection Coefficients
Y  a  1 X 1  2 X 2
where,
Y
relative fitness
X1  X  X
X2  ( X  X )
1  coefficient of directional selection
2
2 
coefficient of variance selection
[Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]
Advantages of Selection Coefficients
• Directly link traits to fitness
• Can be used to compare the effects of
multiple traits on fitness
• Allows on to visualize phenotypic
selection
A Definition of Fitness
Average lifetime contribution to the breeding
population by a phenotype, relative to the
contribution of other phenotypes.
[Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]
Empirical Measures of Fitness
Fundamental (Direct)
• Estimated Time until
Extinction (Forget about it!)
Derived (Indirect)
•
•
•
•
Growth
Survival
Mating Success
Fecundity
[Source: Cooper 1984, Journal of Theoretical Biology 107, 603-629]
Studies Demonstrating Natural Selection in the Wild
Numbers of Species
Animals
Cont.
Morphological
Disc.
Plants
Both
Total
Cont.
Disc.
All Species
Both
Total
Cont.
Disc.
Both
Total
33
4
34
71
4
0
10
14
37
4
44
85
7
0
2
9
8
7
3
18
15
7
5
27
Biochemical
—
—
10
10
—
—
2
2
—
—
12
12
Two or More
0
8
1
9
4
4
0
8
4
12
1
17
40
12
47
99
16
11
15
42
56
23
62
141
Physiological
Total
[Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]
Median β
In most cases, directional selection was not very strong.
Log (sample size)
[Source: Kingsolver et al. 2001, The American Naturalist 157, 245-261]
Frequency
In most cases, directional selection was not very strong.
Absolute Value of
Linear Selection Gradient (β)
[Source: Kingsolver et al. 2001, The American Naturalist 157, 245-261]
Median β
Strength of selection did not depend strongly on sample size.
Log (sample size)
[Source: Kingsolver et al. 2001, The American Naturalist 157, 245-261]
Selection
Differential
Selection differential accord with selection gradients.
Linear Gradient (β)
[Source: Kingsolver et al. 2001, The American Naturalist 157, 245-261]
Frequency
Stabilizing and disruptional selection were equally uncommon.
Quadratic Selection Gradient (γ)
[Source: Kingsolver et al. 2001, The American Naturalist 157, 245-261]
Bμ is a measure of the
strength of selection that
is truly independent of the
population’s phenotypic
variation.
[Source: Hereford et al. 2004, Evolution 58, 2133-2143]