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 genotypes2 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
