Sympatric speciation

Definition

The formation of a new species from an ancestral population without geographic isolation, with diverging lineages remaining in overlapping or contiguous ranges throughout the speciation process. Distinguished from allopatric speciation by the absence of extrinsic barriers to gene flow; instead, reproductive isolation evolves through intrinsic mechanisms such as host or habitat shifts, disruptive selection, or sexual selection.

Etymology

From Greek syn- 'together' + patris 'fatherland'; coined by Edward Bagnall Poulton in 1904

Example

Rhagoletis pomonella, the apple maggot fly, is a classic case: a population that historically bred on hawthorn (Crataegus) shifted to domestic apples (Malus) introduced to North America in the 19th century. The two host-associated populations now exhibit partial allochronic isolation (different fruiting times), reduced gene flow, and emerging reproductive barriers, despite occupying the same geographic region.

Synonyms

• sympatric divergence

Related Terms

• Allopatric speciation
• parapatric speciation
• reproductive isolation
• disruptive selection
• host race
• Sibling species
• cryptic species

Usage Notes

Often contrasted with allopatric speciation; the distinction is operational rather than always clear-cut, as 'sympatric' spans cases from fine-scale microgeographic overlap to broad sympatry with microhabitat segregation. True sympatric speciation (without any spatial component) remains debated; many proposed cases involve some degree of micro-allopatry or host-associated isolation. The term applies to the mode of speciation, not merely the current distribution of species.