Fixation (population genetics)
Encyclopedia
In population genetics
, fixation is the change in a gene pool
from a situation where there exist at least two variants of a particular gene (allele
) to a situation where only one of the alleles remains. The term can refer to a gene in general or particular nucleotide
position in the DNA chain (locus
).
In the process of substitution, a previously non-existent allele arises by mutation
and undergoes fixation by spreading through the population by random genetic drift
and/or positive selection
. Once the frequency
of the allele is at 100%, i.e. being the only gene variant present in any member, it is said to be "fixed" in the population.
Similarly, genetic differences between taxa are said to have been fixed in each species
.
in the population at that time. For example, if a population includes allele A with frequency equal to 20%, and allele a with frequency equal to 80%, there is an 80% chance that after an infinite number of generations a will be fixed at the locus (assuming genetic drift is the only operating evolutionary force).
For a diploid population of size N and neutral mutation rate
, the initial frequency of a novel mutation is simply 1/(2N), and the number of new mutations per generation is 
. Since the fixation rate is the rate of novel neutral mutation multiplied by their probability of fixation, the overall fixation rate is 
. Thus, the rate of fixation for a mutation not subject to selection is simply the rate of introduction of such mutations.
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...
, fixation is the change in a gene pool
Gene pool
In population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...
from a situation where there exist at least two variants of a particular gene (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...
) to a situation where only one of the alleles remains. The term can refer to a gene in general or particular nucleotide
Nucleotide
Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
position in the DNA chain (locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
).
In the process of substitution, a previously non-existent allele arises by 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...
and undergoes fixation by spreading through the population by random genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
and/or positive selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
. Once the frequency
Allele frequency
Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for...
of the allele is at 100%, i.e. being the only gene variant present in any member, it is said to be "fixed" in the population.
Similarly, genetic differences between taxa are said to have been fixed in each species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
.
Probability of fixation
Under conditions of genetic drift alone, every finite set of genes or alleles has a "coalescent point" at which all descendants converge to a single ancestor (i.e. they 'coalesce'). This fact can be used to derive the rate of gene fixation of a neutral allele (that is, one not under any form of selection) for a population of varying size (provided that it is finite and nonzero). Because the effect of natural selection is stipulated to be negligible, the probability at any given time that an allele will ultimately become fixed at its locus is simply its frequency in the population at that time. For example, if a population includes allele A with frequency equal to 20%, and allele a with frequency equal to 80%, there is an 80% chance that after an infinite number of generations a will be fixed at the locus (assuming genetic drift is the only operating evolutionary force).For a diploid population of size N and neutral mutation rate
Mutation rate
In genetics, the mutation rate is the chance of a mutation occurring in an organism or gene in each generation...
, the initial frequency of a novel mutation is simply 1/(2N), and the number of new mutations per generation is . Since the fixation rate is the rate of novel neutral mutation multiplied by their probability of fixation, the overall fixation rate is . Thus, the rate of fixation for a mutation not subject to selection is simply the rate of introduction of such mutations.