Fig. 2. A scenario where the plastic response of one partner affects the fitness surface of an interacting partner; here an example between an insect herbivore population and its host plant. The red curves indicate the phenotypic distribution (middle) and fitness surface (top) of the insect herbivore. The green curve indicates the distribution (bottom) of host plant traits relevant to the interaction. (A) Prior to the plastic response of the plant, the traits of the herbivore mirror those of the plant. If the phenotypic distributions remain constant, stabilizing selection will occur, selecting against extreme phenotypes with lower relative fitness. (B) A plastic response of the plants shifts the distribution of phenotypes in the population. As a consequence, the fitness surface of the herbivore changes and directional selection occurs. Note that it is possible for the herbivore to have a complementary plastic response, which will change its phenotypic distribution and the shape of the fitness surface.