In biology, polymorphism can be defined as "the occurrence in the same habitat of two or more forms of a trait in such frequencies that the rarer cannot be maintained by recurrent mutation alone." E.B. Ford, 1940. The different forms are called morphs (or phases).

Examples

In some cases of polymorphism, the different morphs are distinct. An example from botany is heterostyly, in which flowers occur in different forms having different arrangements of the pistil and the stamens. For instance, some cowslip plants (Primula veris) have "pin flowers", in which the style (the stalk of the pistil) reaches the end of the corolla tube and the stamens extend only halfway up the tube and thus are hidden. Other cowslip plants have "thrum flowers", in which the stamens reach the end of the corolla tube and the style is hidden inside. Also, thrum flowers produce bigger pollen grains than pin flowers. This polymorphism prevents inbreeding.

In other cases of polymorphism, there is continuous variation. For instance, normal human hair color, even within the single "habitat" of northern Europe, ranges continuously from black through reddish and brownish shades to nearly white. Little is known about any adaptive value of this polymorphism.

Another example of a polymorphic species is the Scarlet tiger moth.

Still other polymorphisms are variations in an organism's DNA sequence that do not affect its phenotype. Examples include single-nucleotide polymorphisms and restriction-fragment-length polymorphisms.

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Heterostyly in the Cowslip (Primula veris L.)