Developmental plasticity and the origin of novel forms: unveiling cryptic genetic variation via "use and disuse"

Natural selection eliminates phenotypic variation from populations, generation after generation-an observation that haunted Darwin. So, how does new phenotypic variation arise, and is it always random with respect to fitness? Repeated behavioral responses to a novel environment-particularly those that are learned-are typically advantageous. If those behaviors yield more extreme or novel morphological variants via developmental plasticity, then previously cryptic genetic variation may be exposed to natural selection. Significantly, because the mean phenotypic effect of "use and disuse" is also typically favorable, previously cryptic genetic variation can be transformed into phenotypic variation that is both visible to selection and biased in an adaptive direction. Therefore, use-induced developmental plasticity in a very real sense "creates" new phenotypic variation that is nonrandom with respect to fitness, in contrast to the random phenotypic effects of mutation, recombination, and "direct effects" of environment (stress, nutrition). I offer here (a) a brief review of the immense literature on the effects of "use and disuse" on morphology, (b) a simple yet general model illustrating how cryptic genetic variation may be exposed to selection by developmentally plastic responses that alter trait performance in response to "use and disuse," and (c) a more detailed model of a positive feedback loop between learning (handed behavior) and morphological plasticity (use-induced morphological asymmetry) that may rapidly generate novel phenotypic variation and facilitate the evolution of conspicuous morphological asymmetries. Evidence from several sources suggests that handed behaviors played an important role both in the origin of novel forms (asymmetries) and in their subsequent evolution.