sympatry

Sympatry is a term in biogeography and evolutionary biology describing a situation in which two or more populations, species, or taxa occupy and interact within the same geographic region, with potential for direct contact and gene flow. It is defined relative to allopatry, where populations are geographically separated, and parapatry, where ranges are adjacent and meet at a boundary.

In ecology, sympatric distributions occur when species coexist in the same area, often by exploiting different ecological niches or resources. Even with geographic overlap, interactions such as competition, predation, and mating can influence how closely related populations diverge. Gene flow is possible in sympatry, which can slow divergence unless strong divergent selection, ecological specialization, or assortative mating reduces interbreeding.

Sympatric speciation refers to the evolution of reproductive isolation within a single geographic area. Mechanisms proposed to facilitate this process include disruptive selection on resource use, polyploidy (especially in plants), and the evolution of mate preferences that favor individuals from the same ecological niche. While compelling in theory, many putative cases are debated because distinguishing sympatric from allopatric histories and estimating gene flow can be challenging.

Examples often cited include plant lineages that arise via polyploidy within a shared region, and animal systems such as host shifts in certain insects (for instance, populations adapting to different host plants within the same area). Classic discussions also feature cichlid fishes in highly diverse lake communities, where rapid diversification may involve sympatric processes, though the exact mode of origin is frequently debated.

Related concepts include allopatry and parapatry, which describe alternative geographic relationships that influence gene flow and divergence.