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Description
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Morphological evolution is often explosive, producing visually spectacular adaptive radiations like Caribbean anoles and African Rift Lake cichlids. Yet morphological stasis, the long-term retention of a conserved body plan, is equally common across evolutionary radiations, presenting a more challenging pattern to explain. Woodland salamanders (*Plethodon*) are a classic example of such ‘non-adaptive’ radiation, characterized by prolific speciation alongside morphological stasis, often attributed to phylogenetic conservatism in their climatic and microhabitat niches. However, the multidimensional nature of phenotypes and the niche means that adaptive evolution in less apparent traits can occur even when morphology appears static. We investigated whether woodland salamanders exhibit adaptive divergence in a less conspicuous phenotypic axis—specifically, physiology—and compared patterns and rates of trait evolution to those of morphological traits. We found that physiological traits, particularly water-loss physiology (skin resistance to water loss), metabolic rate, and cold tolerance, are associated with climatic variation and exhibit elevated rates of evolution, strong trait disparity, and a high number of adaptive optima, consistent with evolutionary signatures of adaptive radiation. By contrast, morphological traits largely exhibit evolutionary stasis consistent with non-adaptive radiation. Biological systems, as illustrated by woodland salamanders, are not universally ‘conserved’ or ‘labile’ in their evolution, and this system illustrates that morphological stasis may belie physiological lability. Woodland salamanders exemplify how adaptive radiations can unfold beneath a veneer of morphological stasis.
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