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Within populations of animals that have determinate growth, all fully grown adults, even of one sex, are not identical. Both genetic and environmental factors are responsible for this variation. In recent years the importance of this variation has received much attention (Mayr 1963; Van Valen 1965; Fretwell 1969; Soule and Stewart 1970; Rothstein 1973). The important question asked has been, What effect does this morphological variation have on niche width and the ecology of a population? Implicit in many of these works is the concept that differences in the morphologies of population members can result in differences in their niches. This kind of variation is called adaptive variation and the general concept is known as the niche variation hypothesis.
A considerable alteration of the classical view of a population is necessary if adaptive variation is important. The competitive interaction term, alpha, from the Lotka-Volterra equation is no longer unity between population members. Different phenotypes may occupy very different niches and so compete at a lower level than they would with similar phenotypes. Such competition could cause natural selection to increase the variation within a population by selecting for those phenotypes which occupy niches at the periphery of the populational niche. In populations with adaptive variation the total populational niche is made up by differences among phenotypes in their niche use. By adding or removing peripheral phenotypes the populational niche can be enlarged or reduced. Do patterns such as those suggested above exist in natural populations? It is impossible to assess without the quantification of adaptive variation. It is surprising, for this reason, that so little evidence has been presented to test such morphologicalecological relationships within populations.