| Both nature and nurture contribute to the adaptation of any organism to its environment. This dissertation quantifies the relative contributions of nature, or local adaptation by natural selection, and nurture, or adaptive phenotypic plasticity, to the fitness of the seep monkeyflower Mimulus guttatus in contrasting flooding environments.; The classic tests of local adaptation (reciprocal transplant) and of adaptive plasticity (switching experiment) are in fact subsets of a single overall design involving three sets of environmental conditions: those in which a population evolves (origin), in which an individual develops (growth), and in which performance and fitness are assessed (final). To test the adaptive plasticity hypothesis, we performed a switching experiment: phenotypes were induced by moist and flooded growth environments and their fitness realized in moist and flooded final environments. To test local adaptation to soil flooding, plants from three pairs of moist and flooded population origins were tested under constant moist and flooded environments in the greenhouse. A third experiment contained all combinations of the three environmental conditions, including combinations not present in the classic experiments. These elucidate the potential role of plasticity in enhancing fitness and thus facilitating future natural selection.; Switching plants from moist growth environments into flooded final environments caused steep declines in maximum photosynthetic rate, stomatal conductance, and midday water potential. Plants in flooded environments deployed proportionally more roots to the soil surface and developed hollow stems. Plants from flooded population origins had higher instantaneous water use efficiency and higher root to shoot ratio.; The switching experiment demonstrated that phenotypic plasticity in response to flooding is adaptive: within each final environment, plants whose growth environment corresponded to their final environment had a higher lifetime fitness than those that had developed in the other environment. In the local adaptation experiment, two of the population pairs differed in the direction consistent with an adaptive response to flooding, although costs of this adaptation were not observed in the moist environment. The integrated design quantified the fitness benefits of plasticity and of local adaptation to flooding, consistent with the Baldwin effect and a role for phenotypic plasticity in facilitating evolutionary change. |
