COUMARINS AND CATERPILLARS: A CASE FOR COEVOLUTION

Abstract

Ehrlich and Raven (1964) were among the first to focus on coevolution as a distinct evolutionary process. In their formulation, insect-plant coevolution is a five-step sequence: 1. by mutation and recombination, angiosperms produce novel secondary substances; 2. by chance, these new secondary substances alter the suitability of the plant as food for insects; 3. the plants, released from the restraints imposed by herbivory, undergo evolutionary radiation in a new adaptive zone; 4. by mutation or recombination, insects evolve mechanisms of resistance to the secondary substances; 5. able to exploit a plant resource hitherto excluded from herbivores, the adapted insects enter a new adaptive zone and undergo their own evolutionary radiation. This scenario was inspired by broad patterns of hostplant utilization among families of butterflies (Rhopalocera). Although the schema gained widespread acceptance, to date no specific example demonstrates most or all of the steps in the sequence. This lack of empirical evidence has been the subject of considerable criticism (e.g., Jermy, 1976; Janzen, 1980). Recent experimental work on associations between various insects and plants containing furanocoumarins and related compounds (Berenbaum, 1978, 1980, 1981a, 1981b, 1981c), however, provides a case study with either direct or circumstantial evidence for each part of the coevolutionary process.