DIVERSIFICATION OF CARNIVOROUS PARASITIC INSECTS - EXTRAORDINARY RADIATION OR SPECIALIZED DEAD-END

The spectacular diversity of insects has often been attributed to acce lerated radiation of groups acquiring specialized trophic habits. In a ccord with this hypothesis, a previous study demonstrated consistently greater diversification in clades attacking higher plants, as contras ted to their predaceous or saprophagous sister groups. Faster diversif ication of phytophagous insects could represent radiation in an unsatu rated adaptive zone or result from the population fragmentation and di versifying selection imposed by ecological specialization per se. The latter effect underlies the hypothesis that rapid diversification char acterizes ''parasitic'' insects in a broad sense including most phytop hages, contrasting with the classical view of parasitic specialization as an evolutionary ''dead end.'' To test these hypotheses, we catalog ued the origins and effect on diversification of animal parasitism by insects. Of 15 carnivorous parasitic insect clades with estimated rela tionships, six were more diverse than their predaceous or saprophagous sister groups, and nine less diverse (Wilcoxon T = 28, P < .10). The parasitic lifestyle in the broad sense is by itself unlikely to be a d ominant explanation of variable insect diversification rate, while the hypothesis that parasitism in the strict sense is an evolutionary dea d end remains plausible. Carnivorous parasitism and phytophagy have si gnificantly different effects on diversification. We found no evidence for ascribing either this difference or the heterogeneity of rates am ong carnivorous parasite clades to clade age, mode of parasitism, dive rsity of host clade, or host specificity. Greater diversification by p hytophages than by other trophic levels might reflect simply greater a verage abundance of the resource used by primary consumers.