Polymorphism, in biology, is a discontinuous genetic variation that results in the occurrence of several different forms or types of individuals among members of a single species. A discontinuous genetic variation divides individuals in a population into two or more markedly distinct ways. The most obvious example of this is the separation of most higher organisms into the male and female sexes. Another example is the different blood types in humans.
In the continuous variation, by contrast, individuals do not fall into definite classes but fall almost imperceptibly between broad extremes. Examples include the smooth grading of height between individuals in human populations and the possible gradings between different geographic races. If the frequency of two or more discontinuous forms within a species is too high to be explained by mutation, the variation, as well as the population that exhibits it, is said to be polymorphic.
A polymorphism that persists for many generations is usually maintained because no one form possesses a general advantage or disadvantage over the others in terms of natural selection. Some polymorphisms have no visible manifestations and require biochemical techniques to identify the differences that occur between the chromosomes, proteins, or DNA of the different forms. The castes that occur in social insects are a special form of polymorphism that is attributed to differences in nutrition rather than genetic variations