In population genetics, the selection coefficient is a standardized difference between the fitness of the genotype of interest and the most fit genotype. Generally, the most fit genotype is said to have a fitness of 1, and the fitness of another genotype is 1 - s, where s is the selection coefficient.

Specifically, the selection coefficient is the difference between fitnesses of two competing types scaled to the fitness of other type. Thus, a new mutant allele A1 conveying fitness 2.0 that appears in a haploid bacterial population originally fixed for A2 giving fitness 1.5 will have a selection coefficient (2.0-1.5)/(1.5) = 1/3.

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