UC Santa Barbara

Until recently, the sub-fields of ecology and evolution have existed as separate entities in the broader field of biology. Ecologists focused their attention on community dynamics, species interactions, abiotic contributions to the biological system, and conservation of these systems. Evolutionary biologists preferred to devote their studies to population genetics, selective pressures and the definition, identification and mechanism of evolutionary events, including speciation. When viewing these topics from a broader vantage point, it is clear there is discernible overlap between these topical niches. Community dynamics are the phenotypic, multi-species versions of population genetics and species interactions and abiotic contributions are, frequently, the selective agents of evolution and speciation. Competition between and among species is one field that is overtly shared by ecologists and evolutionary biologists. Competitive pressures both shape the species composition of a community and the gene frequency of a population. Coexistence, the process by which two or more species evade competitive exclusion of one of the species through implementation of a coexistence mechanism, is an important component of the study of competition; however, the mechanisms described in coexistence theory are not as readily accepted as both ecological and evolutionary mechanisms. In this dissertation, I aim

to reconcile the disconnect of these fields with respect to coexistence theory, illustrating that not only are coexistence mechanisms vital in preventing competitive exclusion of one species or deme, so too are they facilitators of evolutionary events such as divergence

or even sympatric speciation. I first attempt to exemplify this by reviewing the literature that discusses coexistence mechanisms (both ecologically and molecularly derived) that also show evidence of facilitating evolutionary events. Next, I attempt to substantiate

these two outcomes of coexistence and divergence mechanisms empirically, by evaluating one known coexistence mechanism, the competition-colonization trade-off, in microbial communities. Using the yeast Saccharomyces cerevisiae as model of a species that exhibit different competitive behavioral strategies, and the fruit fly Drosophila melanogaster as an exemplary insect which participates in the gut-vectoring of yeasts and other microbes, describe how behavioral trade-offs of the yeast can allow two or more different phenotypes to coexist. Further, I illustrate that this same behavioral trade-off can promote change in the differing populations and may lead to further divergence of these two populations. By expanding on Tilman’s competition-colonization trade-off model, we can evaluated whether one possible coexistence mechanism functions in the model organism Saccharomyces cerevisiae, using parameters derived from empirical work with the species S. cerevisiae and D. melanogaster. Results from this dissertation will help to formulate a mathematical model of the yeast-insect dispersal system based on Tilman’s competitioncolonization model and may offer breadth to the new field of eco-evolutionary dynamics and evaluate coexistence theory’s applicability to species divergence and sympatric speciation.