1. in biology, the condition of having multiple behavioral or physical types within a species or population. In some fish species, there are two distinct sizes of males: Larger males defend territory and attract females to mate with them; much smaller males, often with the physical appearance of females, stay close to the large male and inseminate some of the eggs. Peppered moths in England exist as black morphs (forms) in polluted areas and white morphs in nonpolluted areas.

2. in genetics, the presence in a population of two or more variants of a gene (i.e., alleles) at a given genetic locus. For example, the variety of human blood groups is due to polymorphism of particular genes governing the characteristics of red blood cells. See also single-nucleotide polymorphism. —polymorphic adj.