THE INTERACTION OF RATE, MAGNITUDE, AND DIRECTION OF CHANGE IN DETERMINING LONG-TERM PATTERNS OF MORPHOLOGICAL EVOLUTION

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. Differences in the direction or magnitude of change may also contribute to differences in disparity. These same three descriptors—rate, magnitude, and direction of change—have also been used to describe morphological trends. While we often think of increases in disparity as a diffusive process of volume-filling, disparity patterns may just as frequently represent the sum total of active trends among subclades. Here, I use these three descriptors and a phylogenetic framework to link long-term morphological trends with morphological disparity. I focus on post-Paleozoic echinoids, recently shown to have extremely heterogenous rates of morphological evolution, whose interpretation depends on temporal and taxonomic scale. Regular echinoids have shown low rates of evolution and appear to have been constrained in their long-term morphological evolution. In contrast, irregular echinoids have shown faster rates of evolution and directed accrual of morphological disparity. However, potential drivers of long-term trends may differ among irregular echinoid subclades. Neognathostomates have not been biased in the direction of evolution but the magnitude of change has been greater when it occurs in the direction of the long-term trend. The opposite is true for acelostomates where there has been no bias in the magnitude of change in particular directions, but evolution has occurred more frequently in the direction of the long-term trend.