Editing Balancing selection (section)

{{Short description|Type of biological selective process}}
'''Balancing selection''' refers to a number of [[natural selection|selective processes]] by which multiple [[allele]]s (different versions of a [[gene]]) are actively maintained in the [[gene pool]] of a [[population]] at frequencies larger than expected from [[genetic drift]] alone. Balancing selection is rare compared to purifying selection.<ref>{{Cite journal |last1=Charlesworth |first1=Deborah |last2=Willis |first2=John H. |date=November 2009 |title=The genetics of inbreeding depression |url=https://www.nature.com/articles/nrg2664 |journal=Nature Reviews Genetics |language=en |volume=10 |issue=11 |pages=783–796 |doi=10.1038/nrg2664 |pmid=19834483 |s2cid=771357 |issn=1471-0064|url-access=subscription }}</ref> It can occur by various mechanisms, in particular, when the [[heterozygotes]] for the alleles under consideration have a higher [[fitness (biology)|fitness]] than the [[homozygote]].<ref>{{cite book|author1=King, R.C.|author2=Stansfield, W.D.|author3=Mulligan, P.K.|year=2006|title=A dictionary of genetics|url=https://archive.org/details/dictionarygeneti02king|url-access=limited|edition=7th|publisher=Oxford University Press|location=Oxford|page=[https://archive.org/details/dictionarygeneti02king/page/n56 44]}}</ref> In this way [[Polymorphism (biology)|genetic polymorphism]] is conserved.<ref name=Ford1940>{{cite book | last = Ford | first = E.B. | year = 1940 | authorlink = E. B. Ford | title = The New Systematics | chapter = Polymorphism and taxonomy | editor = J. Huxley|editor-link=Julian Huxley| publisher = Clarendon Press | location = Oxford | pages = 493–513}}</ref>
 
Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is ubiquitous in [[Panmixis|panmictic]] populations.

There are several mechanisms (which are not exclusive within any given population) by which balancing selection works to maintain polymorphism. The two major and most studied are heterozygote advantage and frequency-dependent selection.