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1977 Natural selection favors reduced variance in fitness
CHAPTER FORTY TWO
1977 Natural selection favors reduced variance in fitness The concept Gillespie (1977) showed that when offspring numbers for genotypes vary over time (or between individuals), natural selection favors those genotypes with the smallest variance. Thus, adaptation to variable environments will be accomplished both by increasing the number of offspring and reducing the variance in the number of offspring.
The explanation Gillespie (1977) attacked the problem of environmental variation very differently than Levins (1968). Gillespie (1973, 1974) focused on two sources of variation in offspring production: (i) individuals of a single genotype may vary in offspring production within the same generation due to environmental variation they encounter during their lifetimes; or (ii) offspring production may vary from one generation to the next due to changes in the environment. When variation is within a generation, fitness is m N1 s2 , where m is the mean number of offspring, N is the population size, and s2 is the variance in offspring number (Gillespie, 1974). When offspring numbers vary over time, the geometric mean is the best measure of fitness and that is equal to m 12s2 (Gillespie, 1973). Gillespie goes on to show that the genotypes with the smallest variance will be favored by natural selection. These formulations of fitness could lead to the mean number of offspring declining if it was accompanied by a sufficiently large decrease in variance. When there is variance within a generation, the impact of selection on the variance in offspring number is greater in smaller populations. These ideas are also related to the advantage of iteroparity in a variable environment that may produce bad environments capable of wiping out or severely reducing offspring production (see Chapter 29). Iteroparous organisms will have multiple opportunities to reproduce and hence can reduce their variance in offspring production. Conceptual Breakthroughs in Evolutionary Ecology ISBN: 978-0-12-816013-8 https://doi.org/10.1016/B978-0-12-816013-8.00042-9
© 2020 Elsevier Inc. All rights reserved.
99
j
100
Conceptual Breakthroughs in Evolutionary Ecology
Impact: 10 Gillespie’s (1977) simple development of fitness in variable environments led to great evolutionary insights, and led to another of Gillespie’s other major accomplishments, which was a model of selection on protein polymorphism (Chapter 43, Gillespie, 1978).
References
Gillespie, J.H., 1973. Natural selection with varying selection coefficients e a haploid model. Genet. Res. 21, 115e120. Gillespie, J.H., 1974. Natural selection for within-generation variance in offspring number. Genetics 76, 601e606. Gillespie, J.H., 1977. Natural selection for variances in offspring numbers: a new evolutionary principle. Am. Nat. 111, 1010e1014. Gillespie, J.H., 1978. A general model to account for enzyme variation in natural populations. V. The SAS-CFF model. Theor. Popul. Biol. 14, 1e45. Levins, R., 1968. Evolution in changing environments. In: Monographs in Population Biology, vol. 2. Princeton University Press, Princeton, N.J.
1977 Natural selection favors reduced variance in fitness The concept Gillespie (1977) showed that when offspring numbers for genotypes vary over time (or between individuals), natural selection favors those genotypes with the smallest variance. Thus, adaptation to variable environments will be accomplished both by increasing the number of offspring and reducing the variance in the number of offspring.
The explanation Gillespie (1977) attacked the problem of environmental variation very differently than Levins (1968). Gillespie (1973, 1974) focused on two sources of variation in offspring production: (i) individuals of a single genotype may vary in offspring production within the same generation due to environmental variation they encounter during their lifetimes; or (ii) offspring production may vary from one generation to the next due to changes in the environment. When variation is within a generation, fitness is m N1 s2 , where m is the mean number of offspring, N is the population size, and s2 is the variance in offspring number (Gillespie, 1974). When offspring numbers vary over time, the geometric mean is the best measure of fitness and that is equal to m 12s2 (Gillespie, 1973). Gillespie goes on to show that the genotypes with the smallest variance will be favored by natural selection. These formulations of fitness could lead to the mean number of offspring declining if it was accompanied by a sufficiently large decrease in variance. When there is variance within a generation, the impact of selection on the variance in offspring number is greater in smaller populations. These ideas are also related to the advantage of iteroparity in a variable environment that may produce bad environments capable of wiping out or severely reducing offspring production (see Chapter 29). Iteroparous organisms will have multiple opportunities to reproduce and hence can reduce their variance in offspring production. Conceptual Breakthroughs in Evolutionary Ecology ISBN: 978-0-12-816013-8 https://doi.org/10.1016/B978-0-12-816013-8.00042-9
© 2020 Elsevier Inc. All rights reserved.
99
j
100
Conceptual Breakthroughs in Evolutionary Ecology
Impact: 10 Gillespie’s (1977) simple development of fitness in variable environments led to great evolutionary insights, and led to another of Gillespie’s other major accomplishments, which was a model of selection on protein polymorphism (Chapter 43, Gillespie, 1978).
References
Gillespie, J.H., 1973. Natural selection with varying selection coefficients e a haploid model. Genet. Res. 21, 115e120. Gillespie, J.H., 1974. Natural selection for within-generation variance in offspring number. Genetics 76, 601e606. Gillespie, J.H., 1977. Natural selection for variances in offspring numbers: a new evolutionary principle. Am. Nat. 111, 1010e1014. Gillespie, J.H., 1978. A general model to account for enzyme variation in natural populations. V. The SAS-CFF model. Theor. Popul. Biol. 14, 1e45. Levins, R., 1968. Evolution in changing environments. In: Monographs in Population Biology, vol. 2. Princeton University Press, Princeton, N.J.