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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.
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