the distribution of two or more similar species or, sometimes, intraspe-cific forms or genera encountered on the same territory; the opposite of the allopatric distribution of organisms.
The sympatric distribution of even those organisms that are most similar and most morphologically difficult to differentiate is usually a reliable indication of their specific independence in the absence of interbreeding under natural conditions, that is, if the organisms are reproductively isolated. In particular, closely related species differing in one or more characteristics may jointly inhabit large territories, often occurring in close proximity. Such species include the gray microtines Microtus arvalis and Microtus subarvalis, which differ in chromosome number (46 and 54, respectively) and spermatozoid shape but inhabit the same area.
The most complete form of a sympatric distribution of organisms is the total overlapping of the home range of two or more species. For example, the home range of the sand cat (Felis margarita) lies entirely within that of Felis lybica. Most frequently, the areas inhabited by similar species only partly overlap. The sympatric distribution of organisms is important in analyzing the process of species formation.