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).

A measurable physical difference between two species which has arisen by natural selection as a result of the selection pressures on one or both from competition with the other.

The increased difference between two closely related species where they live in the same geographic region ( sympatry) as compared with where they live in different geographic regions ( allopatry). Explained by the relative influences of intra- and inter-specific competition in sympatry and allopatry.

Character displacement refers to the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur but are minimized or lost where the species’ distributions do not overlap. This pattern results from evolutionary change driven by competition among species for a limited resource (e.g. food). The rationale for character displacement stems from the competitive exclusion principle, also called Gause's Law, which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche; without differentiation, one species will eliminate or exclude the other through competition.