ADAPTIVE RADIATION IN INSECTS AND PLANTS - TIME AND OPPORTUNITY

Insects and their hostplants represent the major part of terrestrial d iversity, yet we are just beginning to understand why there are so ver y many species. By far the most influential model of insect/plant dive rsification has been Ehrlich and Raven's (1964) hypothesis of insect/p lant coevolution. While the coevolution model was based on macroevolut ionary patterns in plant defenses and hostplant affiliations, most of the subsequent work has been on its possible ecological and genetic me chanisms, with relatively little systematic scrutiny of the evolutiona ry patterns Ehrlich and Raven described. We explore the possible roles insect/plant interactions may play in the long-term evolution of inse ct and plant lineages, and review some of the evidence on whether or n ot insects and plants have exerted reciprocal influences on each other 's diversification. Insects and plants have diversified over roughly t he same time intervals, and many insect host-affiliations are evolutio narily conserved, thus reflecting long-term, phylogenetic history. Rat her than accumulating herbivores at a rate proportional to their geogr aphic area of distribution or biomass, some plant groups pose apparent chemical barriers to potential herbivore colonists, and seem accessib le to relatively few insect lineages, possibly preadapted by use of ch emically similar or related hostplants. Evolutionary innovations in pl ant defenses and in insect feeding habits seem to have spurred their r espective adaptive radiations, thus ecological opportunity may influen ce long-term evolutionary success. The greater diversity of insects an d plants in the tropics, compared to the temperate zone, probably refl ects the greater age of tropical habitats as well as climatic barriers that limit successful invasion of the temperate zone to just those pr imitively tropical groups able to evolve strategies for both over-wint ering and use of temperate resources. Though evidence is still sparse, successful invasion of the temperate zone may promote subsequent radi ations of both insects and plants. We conclude that much of the availa ble evidence from systematics is consistent with Ehrlich and Raven's s uggestion that much of insect and plant diversification has been spurr ed by a series of ecological opportunities over evolutionary time.