A PHYLOGENETIC ANALYSIS OF PHYSIOLOGICAL-ECOLOGICAL CHARACTER EVOLUTION IN THE LIZARD GENUS COLEONYX AND ITS IMPLICATIONS FOR HISTORICAL BIOGEOGRAPHIC RECONSTRUCTION

Lizards of the genus Coleonyx exhibit diversity in standard metabolic rate (SMR), evaporative water-loss rate (EWLR), and temperature preference (TP). We conducted a phylogenetic analysis of these characters to reconstruct the historical sequence of character evolution and to address the adaptive significance of particular character states. Four equally parsimonious hypotheses of derived-state acquisition are presented. Our preferred hypothesis was selected by considering additional information on the natural history, distribution, and biogeography of several species. Our analysis indicates that the North American ancestor of Coleonyx was a mesophilic forest dweller characterized by a low SMR, a high EWLR, and a low TP. In one stage of character transformation, both low EWLR and high TP evolved independently in two xerophilic clades: C. switaki and variegatus-brevis-fasciatus. Coleonyx reticulatus, a xeric-habitat species that occupies a somewhat mesic microhabitat, retained the ancestral states of high EWLR and low TP. A second stage of character transformation was the evolution of high SMR in the ancestral lineage of the variegatus-brevis-fasciatus clade. We tested the hypothesis that low SMR represents an adaptation in Coleonyx for inhabiting an energy-poor microhabitat. Our analysis reveals that low SMR is the ancestral state for this genus. Furthermore, within the genus low SMR is not historically correlated with energy-poor microhabitats. Although two species with low SMR, C. switaki and C. reticulatus, inhabit microhabitats in which a reduced SMR might be functional, there is no causal relationship between low SMR and energy-poor microhabitat. Thus, this adaptive hypothesis is rejected for the genus Coleonyx. We present a testable hypothesis for the evolution of high SMR in the variegatus-brevis-fasciatus clade. A historical biogeographic scenario based on phylogenetic relationships within the genus, the geological and climatological history of North and Central America, and physiological-ecological character evolution is presented. This study illustrates the importance of the phylogenetic approach to the study of physiological-ecological character evolution, a methodology that can distinguish between contemporary versus historical causal factors and produce testable adaptive hypotheses.