Polymorphism

Definition

The occurrence of two or more discontinuous phenotypes within a single interbreeding population, maintained by selection or genetic drift rather than mutation pressure alone. In arthropods, polymorphism may be genetic (determined by allelic variation), developmental (environmentally cued, as in castes or seasonal morphs), or continuous (cryptic genetic variation underlying quantitative traits).

Etymology

Greek poly- (many) + morphē (form)

Example

The butterfly Papilio dardanus exhibits female-limited polymorphism with multiple mimetic morphs resembling different unpalatable model species, each controlled by alleles at a single supergene locus; males are monomorphic.

Synonyms

• polymorphy
• genetic polymorphism

Related Terms

• polytypism
• Polyphenism
• Sexual dimorphism
• mimicry
• Supergene
• balanced polymorphism
• frequency-dependent selection

Usage Notes

Distinguish from polyphenism (environmentally induced, continuous reaction norms) and polytypism (geographic subspecies variation). True polymorphism requires stable coexistence of discrete forms within one population. In entomology, wing polymorphism in aphids and grasshoppers, color polymorphism in lady beetles, and caste polymorphism in social insects are classic study systems. The term is sometimes loosely applied to any intraspecific variation; specialists reserve it for cases with demonstrated genetic basis and stable equilibrium frequencies.