Glossary of Ecology (6th ed.) by Krebs (2009)
- the divergence in morphology between similar species in the region where the species both occur, but this divergence is reduced or lost in regions where the species' distributions do not overlap; presumed to be caused by competition.
Glossary of "Evolution (4th ed.)" by Futuyma & Kirkpatrick (2017)
- As originally used, a pattern of geographic variation in which a character differs more greatly between sympatric than between allopatric populations of two species; now often used for the evolutionary process of accentuation of differences between sympatric populations of two species as a result of the reproductive or ecological interactions between them.
- Krebs, C. J. 2009. Ecology, 6th ed. -xvi + 655 pp. Pearson Benjamin Cummings, San Francisco.
- Futuyma, D. J. & Kirkpatrick, M. 2017. Evolution, 4th ed. -xviii + 602 pp. Sinauer Associates, Inc., Sunderland.