Polymorphism is the occurrence of two or more clearly different morphs or forms, also referred to as alternative phenotypes, in the population of a species. In order to be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population (one with random mating).

Three mechanisms may cause polymorphism:

Genetic polymorphism - where the phenotype of each individual is genetically determined.

A conditional development strategy, where the phenotype of each individual is set by environmental cues.

A mixed development strategy, where the phenotype is randomly assigned during development.