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{{No footnotes|date=April 2009}}
The '''mutation–selection balance''' is a classic result in [[population genetics]] 
first derived in the 1920s by [[John Burdon Sanderson Haldane]] and [[Ronald Fisher|R.A. Fisher]].  

A genetic variant that is deleterious will not necessarily disappear immediately from a 
population.  Its frequency, when it first appears in a population of N individuals, will 
be 1/N (or 1/2N in a [[diploid]] population), and this frequency might drift up and down a 
bit before returning to zero.  If the population is large enough, or if the [[mutation]] 
rate <math>\mu</math> is high enough, i.e., if <math>\mu*N</math> is high enough, then one has to consider additional 
mutations.  In a hypothetical infinite population, the frequency will never return to 
zero.  Instead, it will reach an equilibrium value that reflects the balance between 
mutation (pushing the frequency upward) and [[selection]] (pushing it downward), thus 
the name ''mutation–selection balance''. 

If 's' is the deleterious selection coefficient (the decrease in relative fitness), then the equilibrium frequency 'f' of an [[allele]] in mutation–selection balance is approximately <math>f = \mu/s</math> in [[haploid]]s, or for the case of a dominant allele in diploids.  For a recessive allele in 
a diploid population, <math>f = \sqrt{\mu \over{s}}</math>.  A useful approximation for 
alleles of intermediate dominance is that f ~ <math>\mu/(sh)</math>, where h is the coefficient of 
dominance. These formulae are all approximate because 
they ignore back-mutation, typically a trivial effect.  

The mutation–selection balance has the practical use of allowing estimates of 
mutation rates from data on deleterious alleles (see examples on pp.&nbsp;85–89 of Crow, 1986).   
For population geneticists, it provides a simple model for thinking about how 
variation persists in natural populations.

== External links ==
* http://www.blackwellpublishing.com/ridley/a-z/Mutation-selection_balance.asp
* J. F. Crow (1986). ''Basic concepts in population, quantitative, and evolutionary genetics''. New York: W.H. Freeman. p.&nbsp;273.

{{DEFAULTSORT:Mutation-selection balance}}
[[Category:Mutation]]
[[Category:Selection]]