Selection coefficient

In population genetics, a selection coefficient, usually denoted by the letter s, is a measure of differences in fitness. Selection coefficients are central to the quantitative description of evolution, since fitness differences determine the change in genotype frequencies attributable to selection.

The following definition of s is commonly used. Suppose that there are two genotypes A and B in a population with relative fitnesses ${\displaystyle w_{A}}$ and ${\displaystyle w_{B}}$ respectively. Then, choosing genotype A as our point of reference, we have ${\displaystyle w_{A}=1}$, and ${\displaystyle w_{B}=1+s}$, where s measures the fitness advantage (positive s) or disadvantage (negative s) of B.

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