Life history and morphological evolution

Morphological evolution is influenced by a wide variety of processes, at levels that likely range from molecules to communities, or even ecosystems. The relative contributions of these processes and other biological properties to morphological evolution in individual lineages, and their ultimate role in mediating the evolutionary success of these groups, remain fundamental problems in evolutionary biology (e.g., Miiller and Wagner, 1991; Nitecki, 1990; Roth and Wake, 1989). One underappreciated feature, at least with respect to its potential role in the evolution of morphology, is life history. Organisms with a complex life history comprising a series of discrete, free-living stages might be expected to possess more morphological adaptations than taxa with simple life histories, especially when successive stages occupy radically different environments. Also, the genetic and developmental mechanisms that underlie the sequential appearance of distinct phenotypes might be expected to allow, if not actually facilitate, morphological change at one stage without correlated effects at others to an extent not possible with a simpler life history (Elinson, 1990). On the other hand, the presence of fully differentiated, functional structures at one stage might be expected to constrain the morphology of structures that form earlier or later (D. Wake and Roth, 1989). Yet, ideas such as these remain largely unexplored in most groups. Here I briefly review aspects of the relation between life history and morphological evolution in Recent amphibians, an especially good group for such an analysis. Many taxa retain the ancestral complex life history, which comprises discrete aquatic larval and terrestrial adult stages bridged by a sharply defined transition, or metamorphosis (Fig. IA). In addition, there is a remarkable array of derived life history modes that can be used to test evolutionary hypotheses and illustrate phylogenetic patterns. When examined in this way, a complex life history can be seen to provide substantial opportunities for morphological evolution in these vertebrates. These opportunites are realized through a variety of evolutionary