<unk>  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<unk>  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.<unk>  was first explicitly explained by Brown and Wilson (1956): “Two closely related species have overlapping ranges. In the parts of the ranges where one species occurs alone, the populations of that species are similar to the other species and may even be very difficult to distinguish from it. In the area of overlap, where the two species occur together, the populations are more divergent and easily distinguished, i.e., they “displace” one another in one or more<unk> s. The<unk> s involved can be morphological, ecological, behavioral, or physiological; they are assumed to be genetically based.”
Brown and Wilson (1956) used the term<unk>  to refer to instances of both reproductive<unk> , or reinforcement of reproductive barriers, and ecological<unk>  driven by competition. As the term<unk>  is commonly used, it generally refers to morphological differences due to competition. Brown and Wilson viewed<unk>  as phenomenon involved in speciation, stating, “we believe that it is a common aspect of geographical speciation, arising most often as a product of the genetic and ecological interaction of two (or more) newly evolved, cognate species [derived from the same immediate parental species] during their period of first contact” (1956). While<unk>  is important in various scenarios of speciation, including adaptive radiations like the cichlid fish faunas in the rift lakes of East Africa (Meyer 1993), it also plays an important role in structuring communities. The results of numerous studies contribute evidence that<unk>  often influences the evolution of resource acquisition among members of an ecological guild (Dayan and Simberloff 2005).
Competitive release (Grant 1972), defined as the expansion of an ecological niche in the absence of a competitor, is essentially the mirror image of<unk> . It too was described by Brown and Wilson (1956): “Two closely related species are distinct where they occur together, but where one member of the pair occurs alone it converges toward the second, even to the extent of being nearly identical with it in some<unk> s.”
Character displacement
Disequilibrium causes
Dominance relationship
Quaternary structure
Special characters
Chemical polarity
Related changes
Resonance