Selection coefficient, usually denoted by the letter ''s'', is a measure used in
population genetics
Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, s ...
to quantify the relative
fitness of a
genotype
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
compared to other genotypes. Selection coefficients are central to the quantitative description of evolution, since fitness differences determine the change in genotype frequencies attributable to
selection
Selection may refer to:
Science
* Selection (biology), also called natural selection, selection in evolution
** Sex selection, in genetics
** Mate selection, in mating
** Sexual selection in humans, in human sexuality
** Human mating strat ...
.
The selection coefficient is typically calculated using fitness values. The fitness (
) of a genotype is a measure of its reproductive success, often expressed as a fraction of the maximum reproductive success in the population. The formula to calculate the selection coefficient
for a genotype is:
, where
is the relative fitness of the genotype, ranging between 0 and 1.
Suppose we have two genotypes,
and
, with relative fitness values of 1 (most fit, standard reference) and 0.8, the selection coefficient (
) for
is
(no selection against this genotype); the selection coefficient (
) for
is
(this indicates that the
genotype has 20% reduction in fitness compared to the
genotype).
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.
[Bersaglieri, T. et al. Genetic signatures of strong recent positive selection at the lactase gene. Am. J. Hum. Genet. 74,1111-1120(2004).]
See also
*
Evolutionary pressure
References
{{Population genetics
Population genetics
Evolutionary biology