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Polymorphism〔(Greek: ''πολύ'' = many, and ''μορφή'' = form, figure, silhouette)〕 in biology is said to occur when two or more clearly different phenotypes exist in the same population of a species—in other words, the occurrence of more than one ''form'' or ''morph''. In order to be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population (one with random mating).〔Ford E.B. 1965. ''Genetic polymorphism''. Faber & Faber, London.〕 Polymorphism as described here involves morphs of the phenotype. The term is also used somewhat differently by molecular biologists to describe certain point mutations in the genotype, such as SNPs (see also RFLPs). This usage is not discussed in this article. Polymorphism is common in nature; it is related to biodiversity, genetic variation and adaptation; it usually functions to retain variety of form in a population living in a varied environment.〔Dobzhansky, Theodosius. 1970. ''Genetics of the Evolutionary Process''. New York: Columbia U. Pr.〕 The most common example is sexual dimorphism, which occurs in many organisms. Other examples are mimetic forms of butterflies (see mimicry), and human hemoglobin and blood types. According to the theory of evolution, polymorphism results from evolutionary processes, as does any aspect of a species. It is heritable and is modified by natural selection. In polyphenism, an individual's genetic make-up allows for different morphs, and the switch mechanism that determines which morph is shown is environmental. In ''genetic polymorphism'', the genetic make-up determines the morph. Ants exhibit both types in a single population.〔 Polymorphism also refers to the occurrence of structurally and functionally more than two different types of individuals, called zooids within the same organism. It is a characteristic feature of Cnidarians.〔name="Ford unk.">Ford E.B. 1965. ''Genetic polymorphism''. Faber & Faber, London.〕 For example, in ''Obelia'' there are feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the medusae. == Terminology == Although in general use polymorphism is quite a broad term, in biology it has been given a specific meaning, being distinguishable from monomorphism (having only one form). A more specific term, when there are only two forms, is dimorphism. * The term omits characters showing ''continuous variation'' (such as weight), even though this has a heritable component. Polymorphism deals with forms in which the variation is discrete (discontinuous) or strongly bimodal or polymodal.〔 * Morphs must occupy the same habitat at the same time: this excludes geographical races and seasonal forms.〔Sheppard, Philip M. 1975. ''Natural Selection and Heredity'' (4th ed.) London: Hutchinson.〕 The use of the words ''morph'' or ''polymorphism'' for what is a visibly different ''geographical race or variant'' is common, but incorrect. The significance of geographical variation is in that it may lead to allopatric speciation, whereas true polymorphism takes place in panmictic populations. * The term was first used to describe ''visible forms'', but nowadays it has been extended to include ''cryptic morphs'', for instance blood types, which can be revealed by a test. * Rare variations are not classified as polymorphisms; and mutations by themselves do not constitute polymorphisms. To qualify as a polymorphism there has to be some kind of balance between morphs underpinned by inheritance. The criterion is that the frequency of the ''least'' common morph is too high simply to be the result of new mutations〔 or, as a rough guide, that it is greater than 1 percent (though that is far higher than any normal mutation rate for a single allele).〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Polymorphism (biology)」の詳細全文を読む スポンサード リンク
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