Selection coefficient

In population genetics, a selection coefficient, usually denoted by the letter s, is a measure of differences in relative 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 and respectively. Then, choosing genotype A as our point of reference, we have , and , where s measures the fitness advantage (s>0) or disadvantage (s