THE EVOLUTIONARY INTERACTION AMONG SPECIES - SELECTION, ESCALATION, AND COEVOLUTION

The hypothesis of escalation states that enemies-competitors, predator s, and dangerous prey-are the most important agents of natural selecti on among individual organisms, and that enemy-related adaptation and r esponses brought about long-term evolutionary trends in the morphology , behavior, and distribution of organisms over the course of the Phane rozoic. In contrast to this top-down view of the role of organisms in determining the directions of evolution, the hypothesis of coevolution holds that two interacting species or groups of species change in res ponse to each other. I review and evaluate these hypotheses in the lig ht of criticisms about the existence of evolutionary trends and the ro le of interactions of species in evolution. Models describing the evol utionary effects organisms have on each other have been based largely on population dynamics and on cost-benefit analyses of the net outcome of interactions between species. Yet, the hypotheses of escalation an d coevolution are statements about the nature, frequency, causes, and role of selection. Although these models have provided valuable insigh ts and have forced some modifications in the hypotheses of escalation studies seeking to distinguish between escalation and coevolution will require empirical observations and cost-benefit evaluations of the di screte events of interaction that collectively constitute organism-cau sed selection.