Selection coefficient

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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 favoured 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 favoured phenotype, while s=1 indicates complete lethality. For example, if the favoured 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 favoured phenotype as 1.0 and that of the phenotype selected against as 0.9.^[1] The terminology is used in the same way to refer to the selective differences between genotypes^[2] to which it extends in a natural fashion. It should be realised, however, that selection can only ever act directly on phenotypic differences.

A slightly different convention is used in the study of genetic drift, where it is convenient to express selective differences both for and against a phenotype by using positive values to refer to a relative selective advantage, and negative values to refer to a relative selective disadvantage.^[2]