Evolutionary And Ecological Consequences Of Natural Selection By Herbivores On Solidago Spp.
Herbivory has been shown to be a selective agent on plant secondary chemistry, and has been hypothesized to limit plant growth in an evolutionary sense by selecting for plants that allocate resources into defense at a cost to growth. Various resistance tra its may be effective against different herbivores, and may be traded off with one another within a plant or over evolutionary time. The following series of studies address microevolutionary aspects of herbivore resistance and the evolution of resistance to different herbivores. The first paper analyzes the effects of a long-term herbivore exclusion treatment on herbivore resistance and plant growth rate. It investigates the ecological and physiological costs of herbivore resistance through correlations between resistances to two herbivores and between resistance and growth. The major findings of this study are that the correlation between growth and resistance is context dependent, and that resistance to different herbivores may evolve indepe ndently. The second paper investigates the variation of herbivore resistance along an elevation gradient, and measures the natural selection on secondary metabolites that vary with elevation. It finds that (1) the level of herbivory decreases with increasing elevation, (2) production of most secondary metabolites detected decreases with increasing elevation, although production of a few increase, and (3) there is natural selection in a low-elevation common garden to increase production of two metabolites that correlate negatively with elevation and to decrease production of a metabolite that correlates positively with elevation. These findings strongly suggest that herbivory can be a primary agent of selection, and that variation in herbivore pressure drives variation in secondary metabolism in this system. The third paper presents a method for analyzing two types of protease inhibitors as plant anti-herbivore resistance traits, and argues that although both are induced by herbivory, only one functions agains t a given herbivore. The findings of this experiment suggest that a common mechanism controls the induction of two types of protease inhibitors. The last paper addresses several issues in the evolution of plant resistance, and presents the argument that op posing selective pressures can constrain evolution of increases in a given resistance trait. It further argues that tolerance and chemical diversity may not be under opposing selective pressures like some resistance traits are, so that increased tolerance or increased chemical diversity may evolve without changing the selective pressures on herbivores .
Natural selection; Evolution; Herbivore
Gan, Susheng; Thaler, Jennifer S.
Ph.D. of Plant Biology
Doctor of Philosophy
dissertation or thesis