Forced evolution of dissimilar characters in related species where their ranges overlap. Elsewhere, where they exist on their own, their similarities are maintained.
A pattern in which two species with overlapping ecological requirements differ more when they co-occur than when they do not. The difference is usually in a morphological feature related to resource exploitation, as in the case of head size, which may be related to prey size.
variation of traits due to the presence of competing species. Example, three species of finches have different beak sizes on 4 different islands (A-D) where the other species are present (Lack, 1947).