Abstract
Evolutionary theory predicts that selection in distinct microhabitats generates correlations between morphological and ecological traits, and may increase both phenotypic and taxonomic diversity. However, some microhabitats exert unique selective pressures that act as a restraining force on macroevolutionary patterns of diversification. In this study, we use phylogenetic comparative methods to investigate the evolutionary outcomes of inhabiting the arboreal microhabitat in salamanders. We find that arboreality has independently evolved at least five times in Caudata and has arisen primarily from terrestrial ancestors. However, the rate of transition from arboreality back to terrestriality is 24 times higher than the converse. This suggests that macroevolutionary trends in microhabitat use tend toward terrestriality over arboreality, which influences the extent to which use of the arboreal microhabitat proliferates. Morphologically, we find no evidence for an arboreal phenotype in overall body proportions or in foot shape, as variation in both traits overlaps broadly with species that utilize different microhabitats. However, both body shape and foot shape display reduced rates of phenotypic evolution in arboreal taxa, and evidence of morphological convergence among arboreal lineages is observed. Taken together, these patterns suggest that arboreality has played a unique role in the evolution of this family, providing neither an evolutionary opportunity, nor an evolutionary dead end.
Highlights
Understanding how taxa respond evolutionarily to the microhabi‐ tats they utilize is of major interest in evolutionary biology
That differing microhabitats exert unique ecological selection pressures is axiomatic in biology, and species inhabiting distinct microhabitats often display phenotypic or functional differences resulting from adaptive diversification in those niches (Irschick, Meyers, Husak, & Galliard, 2008; Kaliontzopoulou, Carretero, & Adams, 2015; Marques & Nomura, 2015; Price, Friedman, & Wainwright, 2015)
How organisms respond to selection pressures across differing mi‐ crohabitats is a major topic in evolutionary biology
Summary
Understanding how taxa respond evolutionarily to the microhabi‐ tats they utilize is of major interest in evolutionary biology. McEntire (2016) recently revealed that nearly 100 species (over 40% of the family) utilize arboreal and vegetative habitats, over 60 of which use these habitats as their pri‐ mary microhabitat type, emphasizing the potential ecological impor‐ tance of this understudied microhabitat These species were found in over 20 genera across the family, prompting McEntire (2016) to hypothesize that arboreality is either an ancestral trait in pletho‐ dontids or has evolved multiple times across the phylogeny. Given the frequency with which plethodontids exploit arboreal microhabitats, the macroevolutionary consequences of arboreality have been surprisingly understudied in this group To fill this void, we investigated the history of microhabitat use across salamanders in order to evaluate the extent to which use of the arboreal micro‐ habitat influences patterns of morphological diversification.
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