Species are defined by their inability to reproduce successfully with other species. Darwin originally argued that new species arose as a consequence of distinct populations adapting to different environments. Despite the widespread acknowledgment that natural selection plays a role in the formation of new species, the question of whether adaptation to different versus similar environments accelerates the evolution of reproductive isolation remains controversial. This study addresses the role of divergent adaptation in speciation by characterizing the degree to which reproductive isolation correlates with adaptive divergence across the genus Cakile, a coastal dune plant that spans a range of environmental variation and geographical isolation throughout Eastern North America, the Caribbean, and the Mediterranean.
In addition to addressing long-standing questions concerning the role of the environment and local adaptation in speciation, this research is relevant to conservation initiatives, since Cakile is endemic to the threatened coastal habitat. The project will share information with local and national conservation groups in the Gulf Coast and Caribbean region. The project also has an international component. Undergraduate students will acquire training in experimental design and analysis. Each student will be given the opportunity to develop a component of the project independently. In addition, minority high school students will be recruited to participate in the research during the summers.
The research funded by grant #1011329 focused on how the important coastal strand plant, sea rocket (Cakile sp.), has adapted to different climates, and how, in turn, this adaptation lead to the formation of new species. Whether plants exhibit trade-offs when grown in different climates or are able to thrive across a wide range of climates is an important question in the face of ongoing climate change. Based on a series of greenhouse experiments, we found within the genus Cakile, we found evidence for expanded climatic tolerances in species that occur in northern latitudes. Based on a large-scale comparative study of reproductive barriers across the genus Cakile, however, we found little evidence that historical adaptive divergence to new climates influenced the formation of new species. These results have important implications for both conservation and fundamental evolutionary theory. With regard to conservation, the lack of an adaptive trade-off between northern and southern species suggests that, all else being equal, these important coastal plants will be able to tolerate rising temperatures under climate change without having to migrate or adapt. With regard to evolutionary theory, these results provide important negative evidence for the role adaptive divergence in the evolution of reproductive barriers, in particular, intrinsic reproductive barriers, like incompatible genes, that have received a lot of theoretical attention, but little empirical attention. This grant contributed to the completion of the dissertation of the co-PI, Charlie Willis. It will also result in at least three publications, currently in preparation, in peer-reviewed journals. Findings from this grant were also presented at the 14th Natural History Conference of the Bahamas at San Salvador, Bahamas 2011 [Title: The diversification of the genus Cakile (Brassicaceae)]. This grant funded several undergraduate and high school research assistances. Duke University undergraduates Wendy Lin, Timothy Wang, and Laura Brodhead participated as student researchers and worked on several aspects of the grant including experiment design, plant care, hybridization, data collection, and data analysis. Additional two high school students, Kalli Bunch and Andrew Johnson, participated in grant funded research under two separate programs. Andrew Johnson participated for a summer as a volunteer through a program offered by his school, Peddie School. Andrew designed, executed and publicly presented an entire research project on the Cakile mating system response to climate. Kalli Bunch participated as part of the Project SEED Program of North Carolina for two full summers. During this time Kalli designed and executed two experiments on the influence of climate on pollen-pistil interactions between Cakile species. She presented this work twice in public science fairs, one year winning 1st place in a statewide competition.
