Speciation is an extended process in which two populations lose the capacity to interbreed and exchange genetic material. At the beginning, restrictions on genetic exchange between the populations is minimal; at the end it is complete. Along the way, the degree of genetic restriction, i.e. reproductive isolation, increases over time. This project uses pairs of populations and species of a group of herbivorous stick insects that vary in their history of isolation and opportunity for gene flow. By evaluating reproductive isolation among dozens of these population/species pairs with respect to two major causes of speciation, either ecological divergence associated with host plant use or geographic separation, this study will investigate the speciation process in unprecedented detail.
Speciation is the fundamental evolutionary process that generates biodiversity. Given increased public awareness of, and interest in, biodiversity conservation, understanding speciation should be of public, as well as academic, concern. Findings from this study will provide insights into the conditions under which reproductive isolation is low, a situation that can promote the loss of biodiversity through interbreeding and genetic fusion of previously distinct populations. Additionally, this project will train two graduate students and multiple undergraduates in the conduct of science through participation in this research. Finally, collaboration with a professor of education will provide understanding about how students do (and do not) learn evolutionary concepts, an important issue in contemporary society.
This project was a study of the relationship between divergent ecological adaptation and species formation. Specifically, this project treated many pairs of herbivorous and host-plant-specific stick insect populations, from which three primary forms of data were collected: (1) The ecological divergence of each pair of populations was evaluated by quantifying the degree to which the host plant preferences of the populations differed. (2) The reproductive isolation (i.e., reproductive incompatibility) of each pair was quantified as the degree to which individuals preferred to mate with others from their own population rather than individuals of the other population. (3) The baseline genetic divergence of each pair was quantified as mean DNA sequence divergence. Genetic divergence was quantified as a surrogate for time since population divergence, to which it is directly related. The project statistically evaluated whether a positive relationship existed between ecological divergence and reproductive isolation across population pairs, while holding constant the confounding effects of time. The 30+ pairs of populations in this study allow for the first such experimental evaluation of potential "ecological speciation" in an evolutionarily diversifying group of organisms. Previous analyses based on data from the literature had found the predicted relationship across several unrelated groups of organisms, providing the motivation for the project. The answer that this project provides to this question is, however, as yet unaddressed. This is because all forms of data for all pairs of populations were required in order to initiate such analyses. Now that we have finally completed data collection, data analysis can begin. This project involved multiple undergraduate students in each year of its conduct. These included one student each year that was funded through NSF's Research Experiences for Undergraduates (REU) program. Additionally, three graduate students played major roles, with one of these continuing to work on the project as a post doc. During the years of support for this project, I collaborated on pedagogically oriented research that sought to investigate how undergraduate students learned to understand evolutionary trees. That work has thus far yielded three publications. I also worked with the School for Science and Math at Vanderbilt (SSMV) in hosting a high school student from Nashville's challenged public school system. This student conducted full time summer research and will be a coauthor on a publication deriving from it. Funding for this project has thus had diverse impacts on student education and pedagogy.
