Previous theory has shown that assortative mating for plastic traits can maintain genetic divergence across environmental gradients despite high gene flow. Yet, these models did not examine how assortative mating affects the evolution of plasticity. We here describe patterns of genetic variation across elevation for plasticity in a trait under assortative mating, using multiple-years observations of bud burst date in a common garden of sessile oaks. Despite high gene flow, we found significant spatial genetic divergence for the intercept, but not for the slope, of reaction norms to temperature. We then used individual-based simulations, where both the slope and intercept of the reaction norm evolve, to examine how assortative mating affects the evolution of plasticity, varying the intensity and distance of gene flow. Our model predicts the evolution of, either suboptimal plasticity (reaction norms with a slope shallower than optimal), or hyperplasticity (slopes steeper than optimal) in the presence of assortative mating, when optimal plasticity would evolve under random mating. A co-gradient pattern of genetic divergence for the intercept of the reaction norm (where plastic and genetic effects are in the same direction) furthermore always evolves in simulations with assortative mating, consistently with our observations in the studied oak populations.< Leer menos