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Adaptive radiations are characterized by an increase in species and/or phenotypic diversity as organisms fill open ecological niches. Often, the putative adaptive radiation has been studied without explicit comparison to the patterns and rates of evolution of closely related clades, leaving open the question whether notable changes in evolutionary process indeed occurred at the origin of the group. Anolis lizards are an oft-used model for investigating the tempo and mode of adaptive radiations. Most of the prior research on the diversification of Anolis morphology has focused on the post-cranium because of its significance towards subdivision of the arboreal habitat. But the remarkable diversity in head shape in anoles has not been as thoroughly investigated. It remains unknown whether the tempo or mode of head shape diversification changed as anoles diversified. We performed geometric morphometric analysis of skull shape across a sample of 12 Iguanian families (110 species), including anoles. Anolis lizards occupy a unique area and a wider region of morphological space compared to the 11 other families examined. We did not find a difference in the evolutionary rate of head shape diversification between anoles and their relatives. Rather, the extraordinary amount of skull diversity arose through a distinct mode of evolution; anoles moved into novel regions by relatively large morphological transitions across morphological space compared to their relatives. Our results demonstrate that traits not directly tied to the adaptive shift of a lineage into unique ecological spaces may undergo exceptional patterns of change as the clade diversifies. 

Abstract Morphological variation among the viviparous sea snakes (Hydrophiinae), a clade of fully aquatic elapid snakes, includes an extreme “microcephalic” ecomorph that has a very small head atop a narrow forebody, while the hind body is much thicker (up to three times the forebody girth). Previous research has demonstrated that this morphology has evolved at least nine times as a consequence of dietary specialization on burrowing eels, and has also examined morphological changes to the vertebral column underlying this body shape. The question addressed in this study is what happens to the skull during this extreme evolutionary change? Here we use X-ray micro-computed tomography and geometric morphometric methods to characterize cranial shape variation in 30 species of sea snakes. We investigate ontogenetic and evolutionary patterns of cranial shape diversity to understand whether cranial shape is predicted by dietary specialization, and examine whether cranial shape of microcephalic species may be a result of heterochronic processes. We show that the diminutive cranial size of microcephalic species has a convergent shape that is correlated with trophic specialization to burrowing prey. Furthermore, their cranial shape is predictable for their size and very similar to that of juvenile individuals of closely related but non-microcephalic sea snakes. Our findings suggest that heterochronic changes (resulting in pedomorphosis) have driven cranial shape convergence in response to dietary specializations in sea snakes. 

Analyses of morphological disparity have been used to characterize and investigate the evolution of variation in the anatomy, function and ecology of organisms since the 1980s. While a diversity of methods have been employed, it is unclear whether they provide equivalent insights. Here, we review the most commonly used approaches for characterizing and analysing morphological disparity, all of which have associated limitations that, if ignored, can lead to misinterpretation. We propose best practice guidelines for disparity analyses, while noting that there can be no ‘one-size-fits-all’ approach. The available tools should always be used in the context of a specific biological question that will determine data and method selection at every stage of the analysis.