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 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.

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.[3]

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".[4]

See also[edit]


  1. ^ Carroll, Robert L: "Patterns and Processes of Vertebrate Evolution", p.182. Cambridge University Press 1997
  2. ^ Ridley, Mark. "Evolution - A-Z - Selection coefficient". Accessed May 23, 2008
  3. ^ Bersaglieri, T. et al. Genetic signatures of strong recent positive selection at the lactase gene. Am. J. Hum. Genet. 74,1111-1120(2004).
  4. ^ Orr, H. A. (2010). "The population genetics of beneficial mutations". Philosophical Transactions of the Royal Society B: Biological Sciences 365 (1544): 1195–1201. doi:10.1098/rstb.2009.0282.  edit