Balancing selection refers to forms of
natural selection which work to maintain genetic
polymorphisms (or multiple
alleles) within a
population. Balancing selection is in contrast to
directional selection which favor a single allele. A balanced polymorphism is a situation in which balancing selection within a population is able to maintain stable frequencies of two or more
phenotypic forms. Evidence for balancing selection can be found by increased levels of
genetic variation between alleles or haplotypes in a species. Note that balancing selection will not always result in an observable phenotypic difference because the
genotype may not be
one-to-one with the phenotype.
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Selection involving opposing forces in which selective advantages and disadvantages cancel each other out. Heterozygote advantage (or overdominant selection) is an example in which an allele selected against in the homozygous state is retained because of the superiority of heterozygotes. Other balanced states may occur including when: an allele is favored at one developmental stage and is selected against at another (antagonistic pleiotropy); an allele is favored in one sex and selected against in another (sexual antagonism); an allele is favored when it is rare and selected against when it is common (negative frequency dependent selection).
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