BioConcepts

balanced polymorphismbalancierter Polymorphismus (ger.)

• Genetic polymorphism that persists at a stable level within a population, being maintained by a selective advantage favouring heterozygotic organisms; an instance of this. (OED 2011)
  category polymorphism

  1934

  It is perhaps possible that such divergence originated as a balanced polymorphism.

  Ford, E.B. (1934). Mendelism and Evolution, 2nd. ed.: 94.

  1940

  A balanced polymorphism […] may be controlled either by genetic or environmental means

  Ford, E.B. (1940). Polymorphism and taxonomy. In: Huxley, J. (ed.). The New Systematics, 493-513: 493; cf. 502; id. (1945). Polymorphism. Biol. Rev. 20, 73-88: 73.

  1945

  Ford (1940b) has subdivided polymorphism into four groups: (1) disadvantageous varieties eliminated by selection and maintained at a low level by recurrent mutation of the genes controlling them; (2) variations due to the effects of genes approximately neutral as regards survival value; (3) those dependent upon genes maintained by a balance of selective agencies; (4) advantageous varieties controlled by genes spreading through the population and displacing their alleles. Only the third and fourth groups are regarded as polymorphism proper; here two or more well-marked forms, capable of appearing among the offspring of a single female, occur with frequencies high enough to exclude the maintenance of the rarest of them by recurrent mutation. It is clearly Ford’s third group to which the material under discussion belongs; Ford calls it balanced polymorphism.

  Goldschmidt, R.B. (1945). Mimetic polymorphism, a controversial chapter of Darwinism (Concluded). The Quarterly Review of Biology 20, 205-230: 207-8.

  1982

  balanced polymorphism Polymorphism in which the genetically distinct forms are more or less permanent components of the populatton, maintained by selection in favour of diversity as in the case of the selective superiority of the heterozygote over both homozygotes;
  cf. polymorphism. 

  Lincoln, R.J., Boxshall, G.A. & Clark, P.F. (1982). A Dictionary of Ecology, Evolution and Systematics: 28.