What is "selection coefficient"

In population genetics, the selection coefficient is a measure of the relative fitness of a phenotype. Usually denoted by the letter s, it compares the fitness of a phenotype to another favored phenotype, and is the proportional amount that the considered phenotype is less fit as measured by fertile progeny. s = 0 then is selectively neutral compared to the favored phenotype, while s = 1 indicates complete lethality. For example, if the favored phenotype produces 100 fertile progeny, and only 90 are produced by the phenotype selected against then s = 0.1. An alternative way of expressing this is to describe the fitness of the favored phenotype as 1.0 and that of the phenotype selected against as 0.9. The terminology is used in the same way to refer to the selective differences between genotypes to which it extends in a natural fashion.

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.

Despite the common use of the symbol s to describe a selective force acting against a phenotype, in some contexts the letter s is used to describe a selective advantage instead. One can, for example, speak of "a new mutation that improves fitness by s = 0.001".