Deme

Definition

A local population of a species that is sufficiently isolated by geography or ecology to experience gene frequencies that drift independently from those of other such populations, yet remains capable of exchanging genes with neighboring demes through limited dispersal. In population genetics and evolutionary biology, demes are the fundamental units of spatial genetic structure, representing a level of organization between the individual and the species. Gene flow among demes maintains species cohesion, while restricted exchange permits local adaptation and can lead to incipient speciation.

Etymology

From Greek dēmos, 'district, people'; adopted into evolutionary biology via population genetics literature of the 1950s–1960s to describe spatially structured breeding populations.

Example

A metapopulation of the Glanville fritillary butterfly (Melitaea cinxia) in the Åland Islands comprises hundreds of demes occupying discrete meadow patches; each deme experiences local extinction and recolonization, with dispersal among demes determining regional persistence and genetic diversity.

Synonyms

• local population
• subpopulation
• Mendelian population

Related Terms

• metapopulation
• Gene flow
• population genetics
• effective population size
• Wahlund effect
• isolation by distance
• source-sink dynamics

Usage Notes

Deme is often used interchangeably with 'local population' or 'subpopulation,' but carries specific theoretical weight in Wright's shifting balance theory and island models of gene flow. Unlike 'ecotype,' which emphasizes phenotypic adaptation, deme emphasizes genetic structure and reproductive connectivity. The term implies potential rather than actual gene exchange—demes may be functionally isolated during a given generation yet remain part of a larger genetic continuum. In practice, deme boundaries are operationalized via F-statistics, assignment tests, or spatial autocorrelation of genotypes. Contrast with 'deme' in classical Athenian civic organization (unrelated).