Abstract
BackgroundQuantifying morphological diversity across taxa can provide valuable insight into evolutionary processes, yet its complexities can make it difficult to identify appropriate units for evaluation. One of the challenges in this field is identifying the processes that drive morphological evolution, especially when accounting for shape diversification across multiple structures. Differential levels of co-varying phenotypic diversification can conceal selective pressures on traits due to morphological integration or modular shape evolution of different structures, where morphological evolution of different modules is explained either by co-variation between them or by independent evolution, respectively.MethodsHere we used a 3D geometric morphometric approach with x-ray micro CT scan data of the skull and bones of forelimbs and hindlimbs of representative species from all 21 genera of the ancient Australo-Papuan myobatrachid frogs and analysed their shape both as a set of distinct modules and as a multi-modular integrative structure. We then tested three main questions: (i) are evolutionary patterns and the amount and direction of morphological changes similar in different structures and subfamilies?, (ii) do skulls and limbs show different levels of integration?, and (iii) is morphological diversity of skulls and limbs shaped by diet, locomotion, burrowing behavior, and ecology?.ResultsOur results in both skulls and limbs support a complex evolutionary pattern typical of an adaptive radiation with an early burst of phenotypic variation followed by slower rates of morphological change. Skull shape diversity was phylogenetically conserved and correlated with diet whereas limb shape was more labile and associated with diet, locomotion, and burrowing behaviour. Morphological changes between different limb bones were highly correlated, depicting high morphological integration. In contrast, overall limb and skull shape displayed semi-independence in morphological evolution, indicating modularity.ConclusionsOur results illustrate how morphological diversification in animal clades can follow complex processes, entailing selective pressures from the environment as well as multiple trait covariance with varying degrees of independence across different structures. We suggest that accurately quantifying shape diversity across multiple structures is crucial in order to understand complex evolutionary processes.
Highlights
Quantifying morphological diversity across taxa can provide valuable insight into evolutionary processes, yet its complexities can make it difficult to identify appropriate units for evaluation
With this method we were able to focus on the tempo and mode of morphological evolution in this old Gondwanan radiation by asking three main questions: (1) whether morphological evolutionary patterns are similar for different structures, (2) if the amount and direction of morphological change differs for each structure and clade, and (3) if morphological evolution is correlated to functional traits such as locomotion, burrowing, or diet
We discuss each of these topics in turn, and suggest that accurately quantifying shape diversity across multiple structures is crucial in order to understand complex evolutionary processes
Summary
Quantifying morphological diversity across taxa can provide valuable insight into evolutionary processes, yet its complexities can make it difficult to identify appropriate units for evaluation. While some clades display extensive ecological and morphological variation that is correlated with lifestyle, others retain ancestral environmental niches and conserved body shape patterns that are better explained by phylogenetic conservatism on a shared ancestral lifestyle [9, 10]. These differing patterns of diversification are best illustrated in related groups of species where one group might display more phenotypic diversification than another due to different selective pressures [4, 11]. Identifying the patterns of variation in each module, while accounting for integration among them, is crucial in order to study morphological evolution and the processes that might have driven it [23]
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