Character displacement was first explicitly explained by William L Brown and EO Wilson $(1956);$ Two closely related species have overlapping ranges. In the parts of the ranges where one species occurs alone, the population of that species are similar to the other species and may even by 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 characters. The characters involved can be morphological, ecological, behavioral or physiological; they are assumed to be genetically based

Competitive release (Grant; $1972$ ), defined as the expansion of an ecological niche in the absence of a competitor, is essentially the mirror image of character displacement. 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 characters