Polymorphism refers to the presence of two or more distinct forms which exist together within a single breeding population of a species. The forms are discontinuous, meaning that the population lacks individuals that are intermediate. Polymorphisms are found in many plants and animals and often exist at a frequency too high to be maintained solely by mutation. Polymorphisms are known to exist either as obvious physical variations easily detected by examination of the body of the organism, enzymes, or proteins of those organisms, and as chromosomal or deoxyribonucleic acid (DNA) variants.

The term discontinuous is essential to an understanding of polymorphism. Most traits of organisms exist in a continuous gradation between extremes. Consider pigment and height in human populations. Skin pigment varies from a deep brown to various light hues. Similarly, height in humans varies from short to extremely tall. The existence of innumerable intermediate forms in both...