Collaborative Research: RUI: Phenotypic Plasticity and the Origin of Species in Threespine Stickleback Fish

This research will investigate the mode of adaptation of threespine stickleback fish when juveniles from the same species develop in distinct environments. When organisms encounter a new environment, they can either adapt by changing themselves in a non-genetic way (“phenotypic plasticity,”) or evolve through natural selection by passing useful mutations from one generation to the next. While these two adaptive processes operate at different levels (individuals versus populations) and timescales (within versus across generations), it is hypothesized that they may interact with each other in complex ways. Understanding these interactions is important not only for explaining existing biodiversity, but also for predicting organisms’ responses to a rapidly changing world. This research will investigate whether juveniles change their appearance so much that they fail to recognize each other as potential mates later in life. Natural selection will also occur, driving the individuals who do not interbreed to become increasingly genetically distinct. Over many generations, this process can cause new species to form. This speciation scenario has been predicted by mathematical models, and this research represents the most extensive test of these predictions to date. Undergraduate students will contribute to every aspect of the research, and students majoring in Biology Education will work with the PIs to develop K-12 curricular modules based upon the findings. This research will examine four recently-established lake populations of threespine stickleback fish (Gasterosteus aculeatus) near Anchorage, Alaska. Each spring, marine stickleback enter the lakes to breed. In the past few decades freshwater stickleback have emerged that diverge physically and genetically from their marine ancestors, presenting a unique opportunity to study how phenotypic plasticity and natural selection interact in the early stages of the formation of new species. Marine stickleback are much larger than freshwater stickleback, likely representing individual phenotypic plasticity from growing up in either a lake or the ocean. Stickleback prefer mates of their own body size, which could reduce mating between the freshwater and marine types, and reinforce genetic divergence driven by different natural selection pressures in the freshwater and marine habitats. A combination of field observations and behavioral experiments will be used to assess levels of mating between freshwater and marine stickleback, and to determine whether individual phenotypic plasticity of body size is a contributing factor. A genomic assessment of interbreeding between the two types will complement the behavioral work. Finally, genomic tools will be used to determine which genetic types remain in the lake year-round versus which types migrate between the lakes and the ocean. The project is being co-funded by the Behavioral Systems program in the Division of Integrative Organismal Systems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.