Hierarchy, Morphology, and Adaptive Radiation: a Test of Osborn’s Law in the Carnivora

Abstract

AbstractHenry Fairfield Osborn’s law of adaptive radiation was intended to explain the early proliferation of morphological and functional variation in diversifying clades. Yet, despite much theoretical development and empirical testing, questions remain regarding the taxonomic levels at which adaptive radiation occurs, the traits involved, and its frequency across the tree of life. Here, we evaluate support for this “early burst” model of adaptive radiation in 14 ecomorphological traits plus body mass for the extant mammalian order Carnivora. Strong support for an early burst adaptive radiation is recovered for molar grinding area, a key proxy for diet. However, we find no evidence for early burst–like dynamics in body mass or multivariate trait data, suggesting a decoupling of evolutionary modes among traits driven by dietary specialization. Furthermore, the signal of an early burst is only recovered for Carnivora, and not in family–level clades. The lack of support for the early burst model of morphological adaptive radiation in previous phylogenetic studies may be a consequence of focusing on the wrong traits at the wrong taxonomic levels. Osborn’s law predicted that adaptive radiation should be hierarchically structured, and the search for its signature and understanding of its prevalence will require a renewed focus on functional traits and their evolution over higher-level clades.