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Situation where new alleles enter into a population via random changes in base sequence as fast as existing alleles leave a population.
What is being balanced out with balanced mutation, or mutational balance, is on the one hand the rate of acquisition of new mutations (mutation rate), i.e., of new alleles, and, on the other hand, the rate of extinction of alleles. This is an equilibrium situation which also can be described as a mutation-selection balance if natural selection is the cause of the loss of alleles from populations.
In addition to natural selection, alleles can be lost as a consequence of genetic drift. Due to its random nature, however, genetic drift can be a somewhat inefficient means of allele loss or, instead, of allele fixation. Nevertheless, the nature of neutral mutations is to be balanced within populations, by definition, entirely as a consequence of genetic drift.
An alternative circumstance to mutation balance certainly can also exist, where mutation load, rather than remaining at equilibrium, instead is either increasing or decreasing.
Note, that mutation balance that occurs as a consequence of genetic drift can become modified as circumstances change. For instance, a formerly neutral allele can become detrimental in comparison to a newly arising mutation. Alternatively, such changes from fitness neutrality can occur as a consequence of changes in environmental conditions, or instead due to changes in epistatic interactions.
In other words, competition can arise with newly present beneficial alleles (rendering formerly neutral alleles instead detrimental), environments can change (potentially rendering formerly neutral alleles instead either beneficial or detrimental), or beneficial or detrimental interactions with newly arising alleles can occur, with some genetic backgrounds, that is, what other alleles are present within a genome, again rendering a focus allele no longer neutral.
Mutation balance that is due entirely to balance between mutation and genetic drift thus is particularly difficult to achieve or, instead, difficult to sustain if achieved. Nevertheless, at an extreme, populations that experience balanced mutation solely due to a combination of mutation and genetic drift are described as quasispecies. In real-world populations this situation can be approximated – though never truly duplicated – when mutations rates are especially high, such as can be seen with RNA viruses.
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