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I. Necessary Conditions II. Selection in Nature III. Units & Levels of Selection Natural Selection.

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Presentation on theme: "I. Necessary Conditions II. Selection in Nature III. Units & Levels of Selection Natural Selection."— Presentation transcript:

1 I. Necessary Conditions II. Selection in Nature III. Units & Levels of Selection Natural Selection

2 Necessary Conditions for Evolution by Natural Selection Variation among individuals Heritability of variation Differential reproduction of variants

3 Phenotypic SelectionGenetic Response Natural Selection [Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]

4 Sexual Selection Phenotypic Selection [Source: Endler 1986, Natural Selection in the Wild, Princeton University Press] Nonsexual Selection Fecundity Selection Mortality Selection

5 Modes of Selection on Continuous Characters [Source: Brodie et al. 1995, Trends in Ecology & Evolution 10, ] Frequency Fitness Frequency Fitness Frequency Fitness Trait Directional Selection Disruptive Selection Stabilizing Selection Variance Selection

6 Selection Differentials Directional selection Stabilizing if j < 0 Disruptive if j > 0 Variance selection variance after selection variance before selection Positive if i > 0 Negative if i < 0 [Source: Endler 1986, Natural Selection in the Wild, Princeton University Press] mean before selection mean after selection

7 Response to Selection selection differential where, response to selection heritability [Source: Falconer 1981, Introduction to Quantitative Genetics, Longman Press]

8 Selection Coefficients coefficient of directional selection where, coefficient of variance selection relative fitness [Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]

9 Advantages of Selection Coefficients Directly link traits to fitnessDirectly link traits to fitness Can be used to compare the effects of multiple traits on fitnessCan be used to compare the effects of multiple traits on fitness Allows on to visualize phenotypic selectionAllows on to visualize phenotypic selection

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11 Average lifetime contribution to the breeding population by a phenotype, relative to the contribution of other phenotypes. A Definition of Fitness [Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]

12 Fundamental (Direct) Estimated Time until Extinction (Forget about it!)Estimated Time until Extinction (Forget about it!) Derived (Indirect) GrowthGrowth SurvivalSurvival Mating SuccessMating Success FecundityFecundity Empirical Measures of Fitness [Source: Cooper 1984, Journal of Theoretical Biology 107, ]

13 Studies Demonstrating Natural Selection in the Wild [Source: Endler 1986, Natural Selection in the Wild, Princeton University Press]

14 In most cases, directional selection was not very strong. [Source: Kingsolver et al. 2001, The American Naturalist 157, ] Log (sample size) Median β

15 In most cases, directional selection was not very strong. [Source: Kingsolver et al. 2001, The American Naturalist 157, ] Absolute Value of Linear Selection Gradient ( β) Frequency

16 Strength of selection did not depend strongly on sample size. [Source: Kingsolver et al. 2001, The American Naturalist 157, ] Log (sample size) Median β

17 Selection differential accord with selection gradients. [Source: Kingsolver et al. 2001, The American Naturalist 157, ] Linear Gradient (β) Selection Differential

18 Stabilizing and disruptional selection were equally uncommon. [Source: Kingsolver et al. 2001, The American Naturalist 157, ] Quadratic Selection Gradient ( γ) Frequency

19 [Source: Hereford et al. 2004, Evolution 58, ] B μ is a measure of the strength of selection that is truly independent of the population’s phenotypic variation.


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