Testing environmental correlates of clines in clades: an example from cassidine beetles

Phenotypic change across environmental gradients has been an important topic in evolutionary biology. Members of the tortoise beetle tribe Dorynotini are characterised by an elytral suture adorned with either a tubercle or a large, vertical spine. Overall spine height across species had previously been posited to exhibit a latitudinal gradient of increasing height and decreasing width towards the southern extreme of the tribe's range, and this pattern had been linked to environmental variation. We explore the evidence behind such a cline by testing associations between climate and morphology across the clade's geographic distribution using an approach based on ecological niche modelling (ENM) and morphological and environmental hypervolumes. The degree of overlap between the respective hypervolumes was assessed, and the correlation of matrix overlap values was quantified using Mantel tests. Degrees of niche similarity and conservatism at the genus level were also assessed using both Schoener's index and Hellinger distances. Overall, we observed that characters defining our morphological hypervolumes were informative, and capable of grouping taxa into discrete units in morphospace. In contrast, environmental hypervolumes were largely homogeneous across the tribe, with high overlap among taxa. No significant correlations were found between environmental and morphological hypervolumes. Our results indicate that morphological divergence occurs along with high levels of environmental overlap; perhaps historical biogeographic factors along with sexual selection may have promoted its diversification. Our approach based on ENM and statistical comparisons between environmental and morphological hypervolumes can provide a useful approach to testing the existence of gradients and clines.