Natural Selection on Continuous Reaction Norms: Thermal Sensitivity of Feeding in Pieris Catepillars

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For most organisms, how well an individual performs vital tasks- eating, growing, moving, etc-varies with that individual's body temperature. The PIs' call the relationship between performance and body temperature a thermal performance curve. How do performance curves evolve? How does natural selection act on performance curves for populations in the field? Is infrequent exposure to extreme low or high temperatures more important than frequent exposure to 'good' temperatures for natural selection on performance curves? The proposed research addresses these questions, by studying thermal performance curves for growth and feeding rates of caterpillars of the Cabbage White, a common and widespread butterfly species that feeds on a variety of native and agricultural crucifers (including cabbage, collards and related vegetables). The PIs' will measure patterns of genetic variation in thermal performance curves in the lab, and determine how variation in performance curves affects survival, generation time and fecundity in experimental collard gardens. By simultaneously characterizing caterpillar body temperatures in the field, the PIs can assess how patterns of temperature variation relate to selection on thermal performance curves for growth and feeding. The PIs can then directly test this relationship by manipulating temperature variation (using partial shade cloths) in the garden and measuring the changes in selection on thermal performance curves. These studies will provide the first studies of selection on thermal performance curves in the field. Because Cabbage Whites are an agricultural pest with unusually large geographic range (e.g. they occur throughout most of North America, Europe, Japan and Australia), these studies will also add to our understanding of temperature adaptation in this widespread pest species: How is it able to tolerate and adapt to the wide range of temperatures experienced in these different geographical areas?