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Selection Coefficient

The selection coefficient of an individual member of a species is the relative difference in fitness between the best-adapted phenotype and that of some other less well-adapted phenotype. Fitness is measured as the likelihood of being selected again. Thus, a lower selection coefficient represents a higher fitness. The best-adapted phenotype is given a score of 0 (meaning that it is not likely to be selected against), and the less adaptive ones a portion of that down to 1.0 (meaning that it will not pass on its genotype).

The selection coefficient of any given phenotype is determined by measuring the contribution its genotype makes to the gametes of the population. The formula to determine the selection coefficient is presented as 1-W, in which W is the fitness or adaptive value of the genotype. (Fitness is the likelihood of a genotype to contribute to the next generation via reproduction.)

If a genotype results in a lethal phenotype– representing the phenotypic lack of an organ, for instance – the W value is 0, resulting in a selection coefficient of 1. A genotype that results in the survival of 65% of the young has a selection coefficient of 1-.65=.35.

This coefficient is used to calculate the rates and frequency of genetic change between generations of a population.

A different form of selection coefficient is used to study neutral genetic drift, and refers to positive and negative selection. A coefficient with a positive number is favored by natural selection, while one with a negative coefficient is selected against.

Web Resources On Selection Coefficient

Evolution and the selection coefficient
Encyclopædia Britannica Article

Book Resources On Selection Coefficient

Coefficients of natural selection by Laurence Martin Cook
The Genetical Theory of Natural Selection by Fisher & Bennett

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