THE TIMING OF INSECT PLANT DIVERSIFICATION - MIGHT TETRAOPES (COLEOPTERA, CERAMBYCIDAE) AND ASCLEPIAS (ASCLEPIADACEAE) HAVE CO-EVOLVED/

Ehrlich and Raven's essay on coevolution has stimulated voluminous wor k on the mechanisms of insect/plant interaction, but few explicit test s of their model's prediction that the evolutionary success of entire insect and plant clades is governed by their putative reciprocal adapt ations. This paper begins an inquiry into possible coevolutionary dive rsification for North American milkweeds of the genus Asclepias and on e of their few major herbivores, the longhorn beetle genus Tetraopes, focusing first on the historical duration and continuity of the intera ction. A phylogeny for Tetraopes and relatives, estimated from morphol ogy and allozymes, shows evident similarity to a morphology based host plant cladogram synthesized from the literature, though the significan ce of the correspondence under heuristic statistical tests depends on the treatment of one beetle species reported (without certainty) from multiple host species. Fossils and biogeography support the interpreta tion that cladogram correspondence reflects synchronous diversificatio n of these two clades, hence opportunity for coevolution, rather than beetle 'host-tracking' of previously-diversified plants. Cladogram cor respondence is more evident at higher than at lower levels, as expecte d under Ehrlich and Raven's model. An apparent phylogenetic progressio n in the potency and location of milkweed cardenolides: seemingly rela ted to species diversity of both Asclepias and Tetraopes subclades, pr ovides further suggestive evidence for that model. The phylogeography of the Tetraopes/Asclepias assemblage suggests that extant species evo lved largely in their current, often quite localized biomes, facilitat ing potential experimental tests for hypotheses of adaptation and coun teradaptation and their importance to diversification. (C) 1998 The Li nnean Society of London.