Philosophiae Doctor - PhD / Variation in sympatric and allopatric populations is believed to be a precursor to eventual speciation. The dispersion of genes from one gene pool into another is prevented by various processes, including the founder effect, sexual selection, ecological differences and random genetic divergence. Examining patterns of intraspecific variation in phenotypic and genotypic traits may thus provide valuable insights into the processes that govern species origination. Bladder grasshoppers (Orthoptera; Pneumoroidea) are an ideal model system to investigate patterns of geographic and ecological divergence due to their high host plant specificity, low dispersal and distinctive acoustic signals. This dissertation investigates intraspecific diversification in the bladder grasshopper Bullacris unicolor (Orthoptera: Pneumoroidea). Recent research on this species has shown significant intra- and inter-population variation in male advertisement calls and morphological characters. However, the exact cause of this variation has remained unclear. Furthermore, a previous study showed that the calls of B. unicolor from one particular population are highly differentiated from other populations, possibly due to the effects of anthropogenic noise. Here we aim to examine the drivers of diversity within Bullacris unicolor by embracing a multidisciplinary approach that encompasses the effects of environmental factors, genetics, anthropogenic noise and host plant associated divergence.

The biological diversity on planet Earth is astounding. Understanding the origins of this diversity, and how it is maintained, are the twin goals of ecology and evolutionary biology. An early and oft-repeated insight in this investigation is that that similar organisms cannot coexist indefinitely. Theory predicts that individuals and species will compete for limited resources and whichever has even a slight advantage will drive all others extinct in a process known as ‘competitive exclusion’. By diversifying, species avoid competition, thereby ‘stabilizing’ their coexistence. Yet natural systems often display levels of diversity that are surprisingly high, given this theory and investigations of how the similarity of coexisting species is maintained have received much less attention. Using a combination of field studies and experiments I demonstrate that highly similar species of freshwater amphipods may compete for resources without resulting in competitive exclusion. These findings suggest that there exist a range of interactions among Hyalella amphipods, ranging from strong stabilizing effects due to ecological trade-offs, to weakly stabilizing effects, to a total lack of stabilizing effects among various pairs of species in this system. These findings demonstrate how the relative strength of stabilizing forces may vary among coexisting species. Although much effort has been dedicated to enumerating and classifying the ways in which ecological and evolutionary forces promote diversity among species, there has been far less attention paid to mechanisms such as convergent evolution, habitat filtering, competition for non-substitutable resources, and non-ecological speciation, among others. I surveyed current theory that may explain the high levels of similarity among species often found in natural systems. I describe how several ecological and evolutionary mechanisms may operate to promote the coexistence of similar species and present results from new theoretical combinations of mechanisms to demonstrate how they may further act in concert with one another. / text

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Sympatric speciation has received much attention both empirically and theoretically. However, the contribution of sympatric speciation to biodiversity remains unclear. One piece missing from the speciation puzzle is the plausibility of sympatric ecological divergence of species through adaptation in polygenic traits. I consider an environment consisting of two niches, where one value of the trait is advantageous in only one niche, and vice versa. The selection regime is described by a trade-off in viabilities between the niches. These polygenic traits can, and often do, involve epistatic interactions among and between loci, so that the contribution of the alleles to viability deviates from additivity. Epistasis then also affects the curvature of the trade-offs: predominant less-than-additive epistasis turns the curve towards concavity and predominant more-than-additive towards convexity. The curvature of the trade-off plays a crucial role in the evolution of populations. With a convex trade- off, extreme values of the trait are favored and the population tends to diverge, but relatively stringent symmetry in strength of selection within the niches and the niche proportions is necessary to maintain polymorphism. In this study I use two and three- locus haploid versions of Levene's model to...