The directionality and consistency of evolution has long been a subject of contention among evolutionary biologists since the days of Darwin. However, it is unknown how much can be quantified and how much results from more complex variables. It is also unknown whether evolution is consistent or whether it occurs differently in each system. My study focuses on predation and habitat as ecological gradients that may create convergent evolution in livebearing fishes. In Chapter I, I focus on predation as a mechanism for driving convergent evolution in Gambusia affinis. A suite of 7 microsatellite markers was used in order to determine independence of morphological evolution. Mantel tests were used to compare genetic, phenotypic, geographical and environmental distances among the six focal populations. These tests showed that there was a significant correlation between genetic and geographic distance but no significant correlation between genetic and phenotypic distances, which may indicate that phenotypic divergence has arisen independently in multiple instances. The second chapter focuses on a unique form of convergence that arose during speciation of three livebearing fishes, which we termed "convergent speciation." I focus on habitat type as a selective pressure in the lake system of Lake Catemaco, Mexico and the surrounding rivers. Lake Catemaco has been isolated from the surrounding rivers for approximately 1.2 million years and during that time several endemic species have evolved in the lake. This provides an excellent study system for studying convergent divergence. To test the theory of convergent speciation in this system, a MANOVA was conducted. The effect of habitat was an important source of variance in the system, indicating that habitat is a likely driving force responsible for convergent speciation in the system. Using discriminant functions I was able to correctly predict the habitat of fish of six different species between 68% and 71% of the time. This may indicate that evolutionary response to habitat is consistent across taxa (i.e., convergent divergence is taking place).

Competition has long been recognized as a central force in shaping evolution, particularly through character displacement. Yet research on character displacement is biased as it has focused almost exclusively on pairs of interacting species while ignoring multispecies interactions. Unfortunately, communities are seldom so simple that only pairs of species interact, and it is not clear if inferences from pairwise interactions are sufficient to explain patterns in nature. A more realistic approach is to ask how traits evolve when multiple species interact. Here I explore the importance of multispecies competitive interactions on trait evolution in four congeneric species of livebearing fishes in the genus Poeciliopsis (P. prolifica, P. viriosa, P. latidens, and P. presidionis). These species are found co-occurring throughout northwestern Mexico: My first chapter builds a framework for multispecies character displacement research by hypothesizing three effects that an unconsidered competitor, termed a hidden competitor, can have on pairwise interactions and the resulting pattern of character displacement. I show through these effects that research focused solely on pairwise interactions can be misleading for character displacement. I also provide suggestions on how to address character displacement research that incorporates more complexity. In chapter two, I test for character displacement in body shape in the four congeneric species. I found evidence for convergent character displacement in populations of P. prolifica, P. viriosa, and P. latidens. I also found that the convergence in body shape was not consistently in the same direction, meaning that when more than two species co-occurred I did not find a more extreme body shape that when only two species co-occurred. On the contrary, body shape when more than two competitors co-occurred seemed to be intermediate between the shape of two competitors and no competitor. This intermediate shape suggests that evolution in multispecies communities may occur in response to several competitors, rather than pairwise interactions. Finally, in chapter three, I test the effect of several hypothetical selective pressures on life history of P. prolifica, including intraspecies and interspecies competition, factors not often considered in life history evolution. I found that competition, both intraspecific and interspecific, was the most important factor in explaining variation in life history. I also found that the best models were those that included these selective pressures as direct effects as opposed to indirect effects through resource availability. However, it is not clear why competition was supported as a direct effect and future studies are needed to fully understand this aspect. Overall, my research suggests that competition plays an important role in shaping trait evolution, even in traits where it has not been considered. Thus, competition should be included in future studies as it may be an important factor in shaping several traits. I also found that competition in multispecies interactions is more complex than in a simple pairwise interactions, and can be harder to detect due to confounding effects acting in conjunction with competition. My study highlights the importance of competition and of considering multispecies competition to better understand the effects of competition.