Discontinuous genetic variation that results in the occurrence of several different forms or types of individuals among the members of a single species.
The most obvious example of polymorphism is the separation of most higher organisms into male and female sexes. Another example is the different blood types in humans. A polymorphism that persists over many generations is usually maintained because no one form has an overall advantage or disadvantage over the others in terms of natural selection . Some polymorphisms have no visible manifestations. The castes that occur in social insects are a special form of polymorphism that results from differences in nutrition rather than from genetic variation.