Overview and Intellectual Merit. Phenotypic plasticity is the ability of an individual to express developmental pathways in an environment-specific manner that produces environment-specific forms. When phenotypic plasticity exists for morphological traits, individuals will appear differently, depending on the environment in which they developed. Some evidence suggests that phenotypic plasticity can facilitate adaptation to local environments whereas other evidence suggests that phenotypic plasticity should inhibit such adaptive evolution. Our project was largely focused on determining the roles of phenotypic plasticity in morphological traits as a capacitor or inhibitor of adaptive change. As a biological model, we used the fruit fly, Drosophila melanogaster.

Drosophila, like many other insects, exhibit phenotypic plasticity in the size of their wings relative to the body. Flies reared at cool temperatures have wings that are much larger relative to their bodies as compared to flies from the same population reared at warmer temperatures. Moreover, flies from genetically differentiated populations from cooler localities constitutively express larger wings than do flies from populations in warmer localities. This variation in wing to body size ratio, be it the result of phenotypic plasticity or geographic origin, is hypothesized to be the product of natural selection for flight at each temperature. Low-temperature flight is believed to favor relatively large wings as this aids in cool-temperature take off. However, flight at warm temperature favors smaller wings, as this increases maneuverability. While these ideas about the adaptive nature of morphological pattern across developmental temperatures and geography are held widely, they have not been investigated thoroughly.

We conducted a series of experiments to determine the relative roles of genetic differentiation and developmental plasticity in the evolution of flight-related morphology in Drosophila. The primary focus of the work was to apply artificial selection on flight ability at low or warm temperature to flies reared under one of two experimental conditions. Some populations were reared at an intermediate temperature so they would express an 'average' morphology suited equally well (or equally poorly) to flight at cool and warm temperatures. Other populations of flies were reared at the cool or warm temperature at which flight and selection occurred; these flies expressed phenotypic plasticity that generated morphologies presumably well suited to flight at one temperature but not the other. We compared the response to artificial selection on flight performance between the intermediate temperature-reared flies and the warm / cool temperature-reared flies to determine if phenotypic plasticity acted primarily as a capacitor or inhibitor of adaptive evolution. Once we had artificial populations that had adapted to fly well at warm or cool temperatures, we used them in follow up studies to investigate (a) if there is a cost to specialization at one temperature such that flight performance at alternate temperatures is reduced, and (b) if differences in flight performance across temperatures translates into differences in reproductive success, as measured by survival in the presence of different kinds of predators.

Our results from the artificial selection experiments indicate a very strong, positive role of phenotypic plasticity in adaptive evolution. Moreover, we found that costs to adaptation, with respect to flight performance in alternate environments, accrue with time as specialization to one environment occurs. We also found that the morphological basis of the response to selection differs among performance temperatures but is generally consistent with geographic patterns of morphology observed in nature. Finally, we observed that selection on...

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