This project examines how different species form and why some species are restricted to certain habitats and not others. While it is widely accepted that evolution by natural selection promotes adaptation to different environments over time, how exactly one species splits into two is highly controversial. This process, known as speciation, is the primary way Earth's diversity of life is formed. However, the mechanisms of speciation are in many ways mysterious. These investigations will use experimental hybridization of plants in the family Asteracea, along with detailed physiological measurements and field studies of hybrids and parental species growing in their natural habitats to determine how natural selection creates two species from one.
The broader impacts of this project are severalfold. First, the PIs will continue to use research assistants from rural communities near field sites in remote regions of Mexico and the southwest United States. These communities traditionally have been underserved by the scientific infrastructure of both countries and contain high proportions of minorities underrepresented in science. Secondly, this project will contribute to general knowledge of the diversity of life and how it is formed. Concrete examples of questions these studies can assist in answering include: Should different populations be considered endangered species? Are populations within a species evolutionary significant units? What will happen in the future if evolutionary different populations are preserved? While these questions cannot be directly answered by this project, the information gained by these investigations will provide policy makers with the scientific data they need to make intelligent, informed decisions.
What causes new species to form? This study addressed this question by examining two species in the process of speciating. Encelia ventorum and Encelia palmeri, though they look very different, can still reproduce and their offspring are fully fertile. This means that they are at an "intermediate" stage of generating new species, if you define species as entities that cannot interbreed any longer. Interestingly, though these species coexist within meters of each other throughout most of their range and hybridize freely wherever they contact each other, they maintain their uniqueness and show no evidence of blending. How is this possible given the propensity to interbreed? Our experiments showed that each species lives in different soil types, and each soil type requires different adaptations in order to survive. Some traits that are adaptations in one habitat are liabilities in another habitat, thus one species cannot be best in all habitats. Thus, natural selection creates a diversifying force that prevents species from fusing together due to interbreeding, and instead continually weeds out unfit hybrids and migrating parental species each generation, maintaining the two species intact and likely driving their further evolutionary divergence. In our study, we specifically found that E. ventorum is restricted to sand dunes because the other environment, flat desert, is too dry to support the succulent leaves of E. ventorum. However, the succulent leaves are a benefit when growing in the salt spray zone of the coastal sand dune, and so this is one example of a trait that is advantageous in one habitat but deleterious in another. Our study also suggests that several other factors contribute to divergent natural selection at this site. Herbivory from large nocturnal beetles strongly prevents E. ventorum from invading the desert habitats, and burial by moving sand strongly prevents E. palmeri from invading dune habitats. Thus it is clear that in this system multiple interacting factors contribute to (in this case) push these species in different directions, and it is likely these same pressures that led to their formation in the first place. This is signifiant because the magnitude of divergent natural selection measured is incredibly high, among the highest published levels. Natural selection this high is sufficient to cause speciation according to most theoretical models, and thus the wide-reaching intellectual significance of this study is to show that natural selection can be a potent force in speciation, at least as important as the effects of reduced gene flow. This is the scenario that Darwin first evisioned: the ecological interactions among plants and animals and their environment driving specialization and eventually creating a panoply of organisms each suited to different micro-environments. For many decades evolutionary biology relegated natural selection and "ecological" factors in speciation secondary to a broad class of factors relating to reproductive isolation (e.g. populations on islands gradually becoming different from their mainland relatives). However, research such as this shows that even species living five meters apart, that form fully fertile hybrids, and that have fully overlapping flowering times, can in fact diverge and adapt if the habitat differences are strong enough. During the course of the project a large number of high school and middle school students from communities surrounding the field site were actively involved in the project including seed collection, plant propagation, experimental maintenance, harvesting, and sample preparation. In addition, undergraduate students from the University of California Berkeley helped with processing samples and data analysis. Furthermore, officials from the federal natual protected area in which the study was conducted were involved intimately at the planning and execution stages and will be included in publications resulting from this research. Several informal conferences were also attended on conservation issues related to the desert environments where this research was conducted, and the results of this research presented to local governments and land management agencies.
