Ancestral plasticity and mating system evolution in the stickleback radiation

Clark University, Worcester MA

Investigators

Abstract

Adaptive radiation occurs when an ancestral population invades many novel habitats, and the daughter populations evolve in response to differences in the patterns of natural selection across habitats. Initially, shifts in behavior may buffer populations from the changing conditions, and differences in the responses to different environments can determine which behavior patterns are exposed to selection in the ensuing years. Thus, the patterns of ancestral plasticity could predispose populations to evolve in particular directions, and could lead to the repeated evolution of divergent patterns of expression across habitats as is seen in a number of recent adaptive radiations. In the post-glacial radiation of the threespine stickleback fish (Gasterosteus aculeatus), ancient oceanic populations have repeatedly given rise to freshwater populations that possess morphological and behavioral features that cause them to be specialized for feeding on plankton (limnetic type) or bottom-dwelling invertebrates (benthic type). Prior research has shown that female mating preferences covary with male color and behavior expression during breeding, and that benthic and limnetic populations differ predictably. However, data also offer evidence of phenotypic plasticity in the expression of these traits in the oceanic ancestor that parallels divergent patterns of plasticity of the two types in the freshwater radiation. This research will involve rearing sticklebacks from three ancestral, oceanic populations, and from three derived freshwater benthic and limnetic populations. Male nuptial behavior and color expression will be compared with female mating preferences before and after exposure to cannibalistic foraging groups, as these groups are the trigger for plastic responses in these traits. Such groups are formed only by oceanic and benthic fish. Plastic expression in the oceanic forms will be used to infer the ancestral condition for comparison with that in the derived types. This research should offer insight into the role of plasticity in adaptive radiation - an exciting and fundamentally important issue in evolutionary biology. Another question that will be evaluated is the intriguing possibility that ancestral phenotypic plasticity could, under certain scenarios, lead to the decoupling of female mating preferences and the norms of reactions of the traits original favored by this selection, a suggestion at odds with the view that signal and receiver characteristics should be tightly coupled. Broader Impact: The research will involve training at least 15 undergraduate students in scientific theory and practice. Clark University offers an especially rich research experience that, for those entering the fifth year masters program, can last as long as five years and result in one or more publications. Data collected from the project will also be used in laboratory exercises that focus on behavior, statistics and evolution. Secondary and college teachers will be able to download exercises and data from a web site free of charge. Finally, this research may help conservationists understand the loss of a very unusual pair of stickleback species that inhabited a single lake in British Columbia, Canada. This pair has been lost to hybridization, and one of the possible explanations is that increased turbidity decreased the effectiveness of visual signals critical to assortative mating. This research will allow us to evaluate the plausibility of this hypothesis.