A MOLECULAR PHYLOGENETIC PERSPECTIVE ON THE ORIGINS OF A LOWLAND TROPICAL SALAMANDER FAUNA. II. PATTERNS OF MORPHOLOGICAL EVOLUTION.

The patterns and mechanisms of morphological transformation in natural lineages remain largely obscure and a source of much controversy (Bock, 1979; Stanley, 1979). A major factor in the process of morphological transformation is the relationship between morphological change and adaptive evolution, the acquisition of features whose biological role facilitates a favorable response to environmental selective forces (see Bock, 1979 p. 45). This relationship theoretically may fall into one of four contexts: (1) the morphological change is neutral with respect to organismal adaptation, (2) the morphological change is related to a relatively minor change in ecology, (3) the morphological change may facilitate the invasion of a novel adaptive zone (sensu Simpson, 1953) or (4) the morphological change may improve the efficiency of the organism's functioning within an adaptive zone (an increase or change of adaptive peak; Wright, 1932). Presumably, a morphological transformation that is primarily detrimental to organismal adaptation will not proceed in nature. An appropriate neontological approach to the resolution of evolutionary patterns and mechanisms is to characterize populations or species that demonstrate a distinct morphological transition, and to analyze their morphologies in light of ecology and phylogenetic relationships. Phylogenetic inferences from independent morphological or molecular data are essential when using inter-populational or interspecific morphoclines or transformation series to analyze a morphological transition (Lauder, 1981). If the morphological transition can be placed within an ontogenetic context, the resolution of evolutionary patterns and mechanisms may be strengthened. My objective here is to analyze patterns and mechanisms of morphological transformation for a lineage in which the morphological change may facilitate the functioning of the organism within a given adaptive zone. I also compare and contrast these patterns with morphological transformations in two additional adaptive contexts, one involving a minor ecological shift and the other, a change of adaptive zone. Morphological changes associated with an increase of adaptive peak, specifically, increased efficiency within an arboreal adaptive zone, are illustrated by invasions of the lowland tropics by the salamander genus Bolitoglossa (Amphibia, Plethodontidae) (see Wake, 1966, for a discussion of plethodontid adaptive zones). Because the terrestrial adaptive zone in the lowland tropics is occupied by frogs and also possibly by lizards and snakes, access to tropical lowlands by salamanders has been gained only by arboreal and fossorial forms (Wake, 1966; Wake and Lynch, 1976). The increased arboreality of the lowland tropical Bolitoglossa is accompanied by increased interdigital webbing and flattening of the hands and feet (Wake, 1966 p. 78). The experimental observations of Alberch (1981) support the interpretation that these limb modifications may facilitate attachment to substrates with low surface energy, such as those encountered on tropical forest vegetation. The arboreal microhabitat also is utilized to a considerable degree by less highly modified species of Bolitoglossa at high and intermediate elevations, demonstrating that the foot