Adaptive Plasticity and Plasticity as an Adaptation: A Selective Review of Plasticity in Animal Morphology and Life History

During the last decade there has been a rapidly growing interest in the study of phenotypic plasticity in animals. Confused terminology in this field of research may be one reason why the focus of many studies is not as clear as it could be. The field of plasticity highlights the general problem of demonstrating adaptation. We discuss the terminology and methodology of plasticity studies, with particular reference to the question of which patterns should be considered evidence for plasticity as an adaptation to the environment, and how to find such evidence. We suggest a terminology where plasticity can be adaptive (i.e. beneficial, and maintained by selection) with respect to a function without strictly being an adaptation for it (evolutionary origin linked to this function), and vice versa. Modifications of the original reaction norm, seen today as differences in plasticity between populations and species, can be adaptations for a function even when the plasticity itself is not (it may follow from constraints or from selection for another function). We selectively review cases reported as evidence of adaptive plasticity in animal morphology and life history, choosing examples from a wide range of taxa to illustrate our criteria for what should be called adaptive and adaptation when applied to plasticity.