THE EVOLUTIONARY INTERACTION AMONG SPECIES: Selection, Escalation, and Coevolution

The hypothesis of escalation states that enemies-competitors, predators, and dangerous prey-are the most important agents of natural selection among individual organisms, and that enemy-related adaptation and responses brought about long-term evolutionary trends in the morphology, behavior, and distribution of organisms over the course of the Phanerozoic. 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 response to each other. I review and evaluate these hypotheses in the light of criticisms about the existence of evolutionary trends and the role of interactions of species in evolution. Models describing the evolutionary 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 and coevolution are statements about the nature, frequency, causes, and role of selection. Although these models have provided valuable insights 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 discrete events of interaction that collectively constitute organism-caused selection.