Monomorphic

Definition

Exhibiting a single, uniform morphological form within a population, sex, or life stage; lacking the discrete alternative phenotypes that characterize polymorphism. In arthropod biology, the term most commonly describes species or populations where males and females are externally indistinguishable (sexual monomorphism), in contrast to sexually dimorphic species where secondary sexual traits differ markedly between sexes. The term also applies to social insect colonies lacking physical castes, and to species showing no seasonal or phase-related morphological variation.

Etymology

Greek monos (one, single) + morphe (form, shape)

Example

The sweat bee Lasioglossum zephyrum is monomorphic: males and females share similar body size, coloration, and facial markings, requiring examination of genitalia or behavioral cues to distinguish sexes. This contrasts with strongly dimorphic carpenter bees (Xylocopa), where males are larger and bear pale facial markings absent in females.

Synonyms

• monomorphous

Related Terms

• dimorphic
• Polymorphism
• Sexual dimorphism
• Caste
• phenotypic plasticity
• Polyphenism
• sexual monomorphism

Usage Notes

Applied at population or species level; a species may be monomorphic for one trait (e.g., color pattern) while dimorphic for another (e.g., size). In myrmecology, 'monomorphic' specifically denotes colonies where all workers are similarly sized, as in most Formica species, versus polymorphic colonies with distinct worker size classes in Atta leafcutter ants. Not synonymous with 'uniform' or 'homogeneous' in casual usage—reserved for discrete morphological categories rather than continuous variation.