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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 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.[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.

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]

See alsoEdit


  1. Carroll, Robert L: "Patterns and Processes of Vertebrate Evolution", p.182. Cambridge University Press 1997
  2. 2.0 2.1 Ridley, Mark. "Evolution - A-Z - Selection coefficient". Accessed May 23, 2008

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