Mutation-selection balance
Encyclopedia
The mutation-selection balance is a classic result in population genetics
first derived in the 1920s by John Burdon Sanderson Haldane and 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
is high enough, i.e., if 
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
in haploids, or for the case of a dominant allele in diploids. For a recessive allele in
a diploid population,
. A useful approximation for
alleles of intermediate dominance is that f ~
, 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. 85–89 of Crow, 1986).
For population geneticists, it provides a simple model for thinking about how
variation persists in natural populations.
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
first derived in the 1920s by John Burdon Sanderson Haldane and R.A. Fisher
Ronald Fisher
Sir Ronald Aylmer Fisher FRS was an English statistician, evolutionary biologist, eugenicist and geneticist. Among other things, Fisher is well known for his contributions to statistics by creating Fisher's exact test and Fisher's equation...
.
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
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
rate
is high enough, i.e., if is high enough, then one has to consider additionalmutations. 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
Selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
(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
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
in mutation-selection balance is approximately
in haploids, or for the case of a dominant allele in diploids. For a recessive allele ina diploid population,
. A useful approximation foralleles of intermediate dominance is that f ~
, where h is the coefficient ofdominance. 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. 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. 273.