Stable maintenance of more than one allele at a given locus within a population.
Mechanisms giving rise to balanced polymorphisms can include frequency-dependent selection, particularly stabilizing frequency-dependent selection, and the existence of neutral variation. See also balancing selection.
Other mechanisms can result in balanced polymorphisms. These include hybrid vigor and heterozygous advantage. Unlike those mechanisms considered in the previous paragraph, hybrid vigor and heterozygous advantage are available only to diploid species (e.g., not, for example, to bacteria).
Though seemingly a rather obscure concept, nevertheless balanced polymorphisms can be hugely important to the fitness of a population since they mean, in effect, that the population is not being excessively purged of genetic variation and therefore retains a greater potential to adapt given environmental change.
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