Two modes of host–enemy coevolution

The process of coevolution between host and enemy has traditionally been viewed as an evolutionary arms race between resistance and counterresistance. The arms-race metaphor of coevolution is widely accepted because it explains the evolution of many characters in species involved in host–enemy interactions. However, molecular work in plant–pathogen systems suggests a coevolutionary interplay between plant recognition of an attacking pathogen and pathogen evasion from recognition. We refer to this process as information coevolution, and contrast this with arms race coevolution to show that these two processes result in very different patterns of host resistance and enemy virulence at the population level. First, information coevolution results in a lower proportion of hosts that are susceptible to enemy attack within a population. Second, information coevolution produces a pattern of local maladaptation of enemy on host, a naturally occurring phenomenon that is difficult to explain under arms race coevolution. We then conduct a literature review to survey the empirical support for either mode of coevolution using 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 invertebrate–parasite systems.