Deme

A deme (plural demes) is a locally interbreeding population, or local breeding group, within a species. Demes are considered fundamental units of population structure, sharing a gene pool that can be distinct from other demes within the same species. They are often geographically or ecologically bounded and may exchange migrants with other demes at some rate, contributing to a metapopulation.

In population genetics, demes are used to model genetic variation across landscapes. Each deme experiences genetic drift somewhat independently, while migration between demes introduces gene flow that can homogenize allele frequencies. The balance of drift, migration, selection, and mutation shapes patterns of genetic differentiation among demes.

Common models include the island model, where all demes have equal size and exchange migrants with equal probability, and the stepping-stone model, where migration occurs primarily between neighboring demes. Demographic parameters such as deme size, migration rate, and spatial arrangement influence measures of differentiation like FST and related statistics used to quantify population structure.

Applications of the deme concept include understanding population structure in natural populations, informing conservation genetics and management units, and interpreting spatial genetic data in humans and other species. The term originated in population genetics in the 20th century and has been developed to explain how spatial subdivision affects evolutionary processes.

Note: the word deme also has non-biological meanings, such as a local civic subdivision in ancient Greece, which is unrelated to the biological concept described here.