Polymorphism (biology) - Terminology

Terminology

Although in general use polymorphism is quite a broad term, in biology it has been given a specific meaning.

• 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. 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).