Two modes of host-enemy coevolution

The process of coevolution between host and enemy has traditionally been vi ewed as an evolutionary arms race between resistance and counterresistance. The armsrace metaphor of coevolution is widely accepted because it explain s the evolution of many characters in species involved in host-enemy intera ctions. However, molecular work in plant-pathogen systems suggests a coevol utionary interplay between plant recognition of an attacking pathogen and p athogen evasion from recognition. We refer to this process as information c oevolution, and contrast this with arms race coevolution to show that these two processes result in very different patterns of host resistance and ene my virulence at the population level. First, information coevolution result s in a lower proportion of hosts that are susceptible to enemy attack withi n a population. Second, information coevolution produces a pattern of local maladaptation of enemy on host, a naturally occurring phenomenon that is d ifficult to explain under arms race coevolution. We then conduct a literatu re review to survey the empirical support for either mode of coevolution us ing the predicted patterns of host resistance and enemy virulence. Evidence supports both modes of coevolution in plant-enemy interactions, whereas no support is found for information coevolution in vertebrate-parasite and in vertebrate-parasite systems.