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.^[1] Suppose that there are two genotypes A and B in a population with relative fitnesses ${\displaystyle w_{A}}$ and ${\displaystyle w_{B}}$ respectively.^{[clarification needed]} 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 (s>0) or disadvantage (s<0) of B.

For example, the lactose-tolerant allele spread from very low frequencies to high frequencies in less than 9000 years since farming with an estimated selection coefficient of 0.09-0.19 for a Scandinavian population. Though this selection coefficient might seem like a very small number, over evolutionary time, the favored alleles accumulate in the population and become more and more common, potentially reaching fixation.^[2]