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Magnitude versus direction of change and the contribution of macroevolutionary trends to morphological disparity

Hopkins, Melanie J.
Biological journal of the Linnean Society 2016 v.118 no.1 pp. 116-130
Echinodermata, animal morphology, arthropods, evolution, fossils, taxonomy
Considerable research has been devoted to measuring rates of morphological evolution and identifying when rate differences or rate shifts have led to differences in morphological diversity (i.e. disparity) among clades. Although less frequently investigated, differences in the direction of change may also contribute to differences in disparity. These same components, namely magnitude (a function of rate and time) and direction of change, have also been used to describe morphological trends, although the direction of change is more frequently the focus of the latter. Increases in disparity are often considered as a diffusive process of volume‐filling, whereas disparity patterns may just as frequently represent the sum total of trends among subclades. As a result, different trend dynamics across subclades can produce different patterns of morphological diversification at different taxonomic scales. For example, although a subclade undergoing a driven trend will not increase in disparity over time, it may contribute to an increase in disparity at more inclusive taxonomic levels. This is demonstrated in the present study using examples from post‐Paleozoic echinoids and Cambrian trilobites.