Studying Tempo and Mode in Evolutionary Radiations: An Integrated, Quantitive Approach

Patterns of morphological change during an evolutionary radiation can yield important information on mechanisms of speciation and the nature of radiations. In particular, if evolutionary radiations hold a common pattern, then it is possible that general processes govern the rapid cladogenesis and morphological differentiation of a clade. One such pattern, high early morphological change in a clade undergoing evolutionary radiation, has been found in a number of groups, suggesting early opportunistic ecological differentiation. Here, this hypothesis was tested on a classic case of an adaptive radiation in the fossil record involving Devonian calmoniid trilobites of the Malvinokaffric Realm using geometric morphometrics to quantify morphological change within the clade. A phylogeny of the Metacryphaeus group calmoniids exists, providing a species-level framework in which to constrain the analyses of the morphological change. Landmark data on trilobite cephala were digitized from photographs of appropriately preserved specimens. Resulting warp scores of terminal nodes were used to infer ancestral node values within the phylogeny using square-change parsimony for continuous character reconstructions. Using Euclidean distance of warp scores between descendent and ancestor, a measure of morphological change could be determined at each speciation event. The tempo of the evolutionary radiation showed typical high initial speciation rates followed by no speciation at later stages of the group. Morphological change plotted against cladogenetic rank showed no correlation, supporting a uniform morphological diversification throughout the period of high speciation rates. There is little support for a model of high early morphological diversification, which is usually interpreted as high initial ecological differentiation due to available opportunity as typical of adaptive radiations. The results suggest other possible mechanisms may play a role in the evolutionary radiation of the calmoniids.