In biology, polymorphism is the genetic variation that divides individuals of a population into two or more distinct forms. This variation must be discontinuous as opposed to continuous, meaning that the polymorphic expression will have sharply distinct forms rather than occurring along a gradient1. Examples of polymorphism include blood type (A, B, AB or O) and sexual dimorphism. In contrast, height would not be considered polymorphic since it is a trait with continuous variation1,2. The different observed forms expressed in a polymorphic population are called "morphs"2.
A polymorphism that persists over several generations is usually maintained because no specific form is evolutionarily advantageous in terms of natural selection1. In such a case, balanced or stable polymorphism occurs as different forms are able to coexist in equilibrium from generation to generation2. In contrast, in transient polymorphism, one form is gradually replaced by another as a result of directional natural selection2.
1. "Polymorphism: Biology." Encyclopædia Britannica. Encyclopædia Britannica, 2015, Web. 16 March 2015.
2. "Polymorphism." New World Encyclopedia. New World Encyclopdia, 2008, Web. 16 March 2015.