The Evolution of Morphological Diversity

The diversity of organismic form has evolved nonuniformly during the history of life. Quantitative morphological studies reveal profound changes in evolutionary rates corresponding with the generation of morphological disparity at low taxonomic diversity during the early radiation of many clades. These studies have also given insight into the relative importance of genomic and ecological factors in macroevolution, the selectivity of extinction, and other issues. Important progress has been made in the development of morphological spaces that can accommodate highly disparate forms, although this area still needs more attention. Other future directions include the relationship between morphological and ecological diversification, geographic patterns in morphological diversity, and the role of morphological disparity as a causal factor in macroevolution. MORPHOLOGICAL DIVERSITY IN SYSTEMATIC BIOLOGY How has the diversity of organic form, living and extinct, come to be? This question, perennially at the heart of systematic biology, has been addressed in paleobiology with a focus on quantitative approaches and large-scale evolutionary questions. The goals of this review are to discuss some recent developments, mainly paleobiological, in the study of morphological diversity [disparity (55, 56, 57, 112, 172)] as opposed to taxonomic richness, and to outline some promising future directions in disparity studies. Many of the pressing questions, such as the pattern of divergence early in clade history and the filling of morphological space, have a long pedigree (54, 108, 116, 122). Although