The ecological genetics of plant resistance to herbivory: Evolutionary constraints imposed by a multiple-herbivore community

Despite the impressive variety of physical and chemical traits that help plants to avoid or resist being eaten, individuals of virtually every species of plant are still attacked by numerous species of herbivores that are able to overcome their defenses. The obvious fitness disadvantage of being eaten and the near ubiquity of genetic variation for resistance in natural plant populations lead to the expectation that plants should evolve to become completely resistant to herbivory. Figuring out why plant resistance is almost universally incomplete in nature is a central question in the field of plant-herbivore evolutionary ecology.; The thesis of this dissertation is that the presence of multiple species of herbivores in the same host plant population can lead to ecological and genetic constraints on the evolution of resistance not apparent from the examination of a single herbivore species alone. This research involved a field experiment on the native herbaceous plant Solanum carolinense (horsenettle) and eleven herbivore species. Plant damage and fitness measures were taken to study the genetic architecture of resistance and the magnitude and direction of natural selection acting on resistance to each of the herbivores. Statistical methods described by Lande and Arnold (1983) were employed and modified to look for evidence of multiple-herbivore constraints and to quantify the relative strength of these constraints on the rate of evolution of resistance to horsenettle's herbivores.; Evidence of three types of multiple-herbivore constraints on the evolution of resistance was discovered in this study. First, negative genetic correlations in resistance to different herbivores were found to be a strong constraint on the evolution of resistance to two of the most common herbivores. Second, the fitness impact on plants of damage by one species of herbivore was often altered by the presence of damage by another species such that feeding by one species decreased the selective benefit of resistance to other species. This sort of nonadditivity of fitness impact appreciably decreased the predicted evolution of resistance to six different herbivores. Third, competitive interactions among species of herbivores resulted in strong indirect selection against resistance to at least six herbivores in the horsenettle community.