A COMPARISON OF TWO STICKLEBACKS.

We present results of an experiment designed to address fundamental issues in the ecology and evolution of plastic trophic morphology: (1) Is observed plasticity adaptive? (2) How much interspecific morphological variation is the result of plasticity? (3) Have different selective regimes resulted in the evolution of different degrees of plasticity? (4) Is genetic variation for phenotypic plasticity present in contemporary populations? We raised fish from two recently diverged species of freshwater threespine sticklebacks on two different diets representative of the natural prey of the two species. Both species exhibited morphological plasticity in an adaptive direction: each species more closely resembled the other when raised on the latter's diet. Dietreversal reduced the natural morphological gap between these two species, -1% to 58%, depending on the trait. One species is known to have a more variable diet in the wild than the other species, and we found that it also exhibited the greater amount of morphological plasticity. Given that the two species have recently diverged, this result is compelling evidence that diet variability is important in the evolution of plastic trophic morphology. Finally, by using a full-sib experimental design, we demonstrated that genetic variation for morphological plasticity exists in contemporary populations, thus confirming that plasticity has evolutionary potential. © 1994 The Society for the Study of Evolution.