| One of the best arguments for the ubiquity of parasitism is that it has evolved independently several times in plants. In spite of this, the ecology and evolutionary biology of parasitic plants has been treated from a "plant-centered" perspective, and they have escaped the notice of most parasitologists and epidemiologists. As parasites, the parasitic plants offer a unique glimpse into the evolution of parasitism because contemporary species display a wide range of ecological intermediates between autotrophic ancestor and extremely derived, obligate parasites. In this dissertation, a theoretical basis for the evolutionary ecology of the facultative root hemiparasitic plants has bridged the gap between two categories that have been widely regarded as ecological opposites.;The theory provides a framework for understanding the ecology and evolutionary biology of parasitism in the parasitic plants. Parasitism in facultative root hemiparasitic plants can be regarded primarily as a trait that mediates competition for resources and gives parasitic plants a unique niche, but simple ecological pressures can influence the evolution of parasitic traits and lead to the evolution of increased dependence. The evolution of life-history traits in the parasitic plants is influenced by parasitism, and models for the evolution of intermediate virulence may apply to the evolution of growth form, seed size and number, rate of growth, and physiology as well as organs devoted directly to parasitism.;The theory offers several concrete predictions. Where limiting resources are more available, parasitic plants should become relatively more abundant. However, in richer resource environments, selection favors the evolution of competitive traits over parasitic traits. On hosts that are better at competing for soil resources, parasitic plants should evolve to become more parasitic. Where the length of the growing season varies, parasitic plants should evolve to become less exploitative of hosts where the growing season is longer. This is also consistent with the observation that water use efficiency in perennial parasitic plants is lower than in annual parasitic plants. The spatial distribution of plants and the clustering of related individuals can lead to the evolution of kin cooperation, but long-distance dispersal or outcrossing with unrelated individuals will increase exploitation. Competition between seeds for limited attachment sites will lead to increased exploitation, suggesting that seed number may drive the correlation between degree of parasitism and seed number. |
