Evolution of plant resistance to multiple herbivores: Quantifying diffuse coevolution
Studies of coevolution in plant-herbivore systems have typically focused on tight, pairwise interactions between one herbivore and one host plant species. Diffuse coevolution, by contrast, has received much less empirical attention, presumably because imprecise definitions of diffuse coevolution have hindered the development of experimental approaches for distinguishing between pairwise and diffuse coevolution. Here we provide a definition of diffuse coevolution that leads to three criteria for the operation of pairwise coevolution: susceptibilities (resistances) to different herbivores are genetically uncorrelated, the presence/absence of one herbivore does not affect the amount of damage caused by other herbivores, and the impact of one herbivore on plant fitness does not depend on the presence/absence of other herbivores. All three criteria must be satisfied for coevolution to be pairwise; if any of them fail, coevolution is diffuse. We then describe an experimental design and statistical analysis that permit the partitioning of the total selection imposed on a plant by a set of herbivores into components representing pairwise and diffuse selection, thus allowing determination of whether coevolution is pairwise or diffuse.
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