sympatric

Sympatric describes populations or species whose geographic ranges overlap, allowing individuals from the groups to encounter each other and interbreed. In evolutionary theory, the term is especially used with sympatric speciation, the process by which new species originate from a single ancestral species while inhabiting the same geographic area, without geographic isolation as a prerequisite. This concept is contrasted with allopatric speciation, where a geographic barrier separates populations, and with parapatric speciation, where neighboring populations diverge with only limited contact along a border.

Key mechanisms proposed for sympatric speciation include disruptive selection that favors extreme phenotypes within the same area, together with assortative mating that reduces interbreeding between diverging groups. Ecological differentiation—such as exploiting different resources or microhabitats—can reinforce isolation. In plants, polyploidy (chromosome duplication) can create instant reproductive isolation and promote sympatric speciation. In animals, host shifts or pronounced sexual selection can lead to reproductive isolation among sympatric lineages.

Well-known examples cited in textbooks include the apple maggot fly Rhagoletis pomonella, which has diverged populations associated with hawthorn versus cultivated apples, and various cichlid fishes in the African Great Lakes, where rapid diversification occurs in largely sympatric settings. The extent and frequency of true sympatric speciation remain topics of debate, with many cases likely involving some geographic structure or cryptic historical separation.