Ancestral plasticity may have played an important role in guiding the repeated evolution of benthic and limnetic ecotypes. In shallow, relatively eutrophic lakes, stickleback forage for large food in the benthos and are characterized by deep bodies, large mouths, and relatively small eyes. Stickleback populations in deeper, more oligotrophic lakes spend their lives eating plankton in the water column, and have more fusiform bodies, small, upturned mouths, and relatively large eyes. As ancestral stickleback repeatedly encountered either of these two environments, developmental plasticity may have differentially shaped their morphology, initiating the evolution of parallel patterns of divergence. Currently, we are experimentally testing whether, and to what degree, threespine stickleback from these morphologically distinct ecotypes exhibit plasticity of trophic morphology when forced to feed on alternative food types. We predict that the pattern of diet-induced plasticity in the ancestral population will mirror the pattern of phenotypic divergence between the two derived ecotypes. Furthermore, we are interested in how evolution subsequently shaped patterns of plasticity in derived populations (e.g, genetic accommodation, genetic assimilation; Wund, Baker, Clancy, Golub).