GRASSHOPPER MANDIBLES AND THE NICHE VARIATION HYPOTHESIS.

Abstract

Evolutionary biology consists, in large measure, of attempts to understand biotic diversity. For this reason, the hypothesis (Van Valen, 1965) of adaptive variation occupies a central position among evolutionary theories. The hypothesis attempts to explain biotic diversity at both genetic and phenetic levels, postulating a causal relationship between genetic or morphological variation and ecological habits. Under the conditions of the hypothesis, populations that occupy broader ecological niches are expected to be more variable, in phenotype and genotype, than more narrow-niched populations (Van Valen, 1965). In the past, both continuous (unimodal) and discontinuous (polymorphic) variation have been considered as facets of a single process leading to adaptive variation. A variety of studies have examined the niche variation hypothesis. Tests of the hypothesis typically consist of estimates of genetic (i.e., electromorphic) or morphological variability in comparison with suitable estimates of ecological amplitude. In general, the results of these studies are ambivalent. While the electromorphic analyses of Shugart and Blaylock (1973) and Steiner (1977) generally support the niche variation model, others by Sabath (1974), Soule and Yang (1973), Powell and Wistrand (1978), Mitter and Futuyma (1979), and Smith (1981) provide conflicting, occasionally contradictory, results. Similarly, morphological analyses by Van Valen (1965), Grant et al. (1976), Davidson (1977, 1978), and Bernstein (1979) provide supporting evidence, while those of Soule and Stewart (1970), Soule (1972),