Natural selection and the joint evolution of toleranceand resistance as plant defenses

Plants can defend themselves against the damaging effects of herbivory in a t least two ways. Resistant plants avoid or deter herbivores and are theref ore fed upon less than susceptible plants. Tolerant plants are not eaten le ss than plants with little tolerance, but the effects of herbivore damage a re not so detrimental to a tolerant plant as they are to a less tolerant pl ant. Biologists have suggested that these two strategies might represent tw o alternative and redundant defenses against herbivory since they appear to serve the same function for plants. I explore the relationship between res istance and tolerance, particularly with regards to how the joint evolution of these two traits will influence the evolution of plant defense. Althoug h I briefly review some of the contributions of theory to the study of tole rance, I concentrate on an empirical, ecological genetic approach to the st udy of the evolution of these characters and the coevolution of tolerance a nd herbivores. In order to understand the evolution of any trait, we must u nderstand the evolutionary forces acting on the trait. Specifically, we mus t understand how natural selection acts on tolerance. I review several stud ies that have specifically measured the form of selection acting on toleran ce and tested the hypothesis that resistance and tolerance are alternative strategies. I also present a statistical analysis that does not support the hypothesis that herbivores are selective agents on tolerance. Finally, I c onsider a variety of constraints that possibly restrict the evolution of to lerance.