Selection Coefficient

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.^{[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.

• Evolutionary pressure