demes

A deme is a locally interbreeding population within a larger population structure that shares a gene pool and largely reproduces within the group. In population genetics, demes are the basic units of subdivision, often defined by geographic, ecological, or behavioral factors that limit mating with individuals from other groups, though boundaries can be permeable.

Within a deme, mating is more likely among its members, and genetic drift acts on its allele frequencies. Migration between demes introduces gene flow, which can counteract drift and reduce differentiation. Theoretical models of subdivided populations include the island model, where demes exchange migrants at a rate m, and the stepping-stone model, where migration occurs primarily between neighboring demes. Coalescent approaches commonly incorporate demes as connected populations.

Genetic differentiation among demes is typically measured by statistics such as F-statistics, particularly F_ST. Low migration tends to increase differentiation, while substantial gene flow homogenizes allele frequencies across demes. Demographic structure influences evolutionary dynamics, including local adaptation and speciation, with isolation promoting divergence and gene flow potentially preventing it.

In practice, demes serve as units for sampling, analysis, and conservation planning. They can be defined by genetic data or by geographic and ecological criteria, and their boundaries may be uncertain or dynamic over time. Understanding deme structure helps illuminate patterns of diversity, migration, and evolutionary history within species.