The spectacular diversity of insects has often been attributed to accelerated radiation of groups acquiring specialized trophic habits. In accord with this hypothesis, a previous study demonstrated consistently greater diversification in clades attacking higher plant\, as contrasted to their predaceous or saprophagous sister groups. Faster diversification of phytophagous insects could represent radiation in an unsaturated adaptive zone or result from the population fragmentation and diversifying selection imposed by ecological specialization per se. The latter effect underlies the hypothesis that rapid diversification characterizes "parasitic" insects in a broad sense including most phytophages, contrasting with the classical view of parasitic specialization as an evolutionary "dead end." To test these hypotheses, we catalogued the origins and effect on diversification of animal parasitism by insects. Of 15 carnivorous parasitic insect clades with estimated relationships, six were more diverse than their predaceous or saprophagous sister groups, and nine less diverse (Wilcoxon T = 28, P < .LO). The parasitic lifestyle in the broad sense is by itself unlikely to be a dominant explanation of variable insect diversification rate, while the hypothesis that parasitism in the strict sense is an evolutionary dead end remains plausible. Carnivorous parasitism and phytophagy have significantly different effects on diversification. We found no evidence for ascribing either this difference or the heterogeneity of rates among carnivorous parasitic clades to clade age, mode of parasitism, diversity of host clade, or host specificity. Greater diversification by phytophages than by other trophic levels might reflect simply greater average abundance of the resource used by primary consumers.

Simpson's postulate that rapid diversification follows entrance into a new "adaptive zone" is frequently invoked a posteriori for groups of unusual diversity. The postulate can be tested more rigorously by defining an adaptive zone according to ecological criteria, independent of particular groups of organisms. The adaptive-zone hypothesis predicts that if multiple lineages have invaded a new adaptive zone, they should be consistently more diverse than their (equally old) sister groups, when the latter retain the more primitive way of life. Higher-plant feeding among insects is an independently defined, repeatedly invaded adaptive zone, to which a profound acceleration of diversification rate has been attributed. We have quantified the evidence for this hypothesis by comparing sister groups and species diversity of as many phytophagous insect groups as current taxonomic evidence allows. A sign test showed significant association of diversification rate with the adoption of phytophagy. The possible artifactual bases for this trend are discussed and provisionally rejected. We discuss several possible biological explanations for the association, including models of phylogenesis either dependent on or independent of ecological role.