Disparity: Morphological Pattern and Developmental Context

Abstract

The distribution of organic forms is clumpy at any scale from populations to the highest taxonomic categories, and whether considered within clades or within ecosystems. The fossil record provides little support for expectations that the morphological gaps between species or groups of species have increased through time as it might if the gaps were created by extinction of a more homogeneous distribution of morphologies. As the quantitative assessments of morphology have replaced counts of higher taxa as a metric of morphological disparity, numerous studies have demonstrated the rapid construction of morphospace early in evolutionary radiations, and have emphasized the difference between taxonomic measures of morphological diversity and quantitative assessments of disparity. Other studies have evaluated changing patterns of disparity across mass extinctions, ecomorphological patterns and the patterns of convergence within ecological communities, while the development of theoretical morphology has greatly aided efforts to understand why some forms do not occur. A parallel, and until recently, largely separate research effort in evolutionary developmental biology has established that the developmental toolkit underlying the remarkable breadth of metazoan form is largely identical among Bilateria, and many components are shared among all metazoa. Underlying this concern with disparity is a question about temporal variation in the production of morphological innovations, a debate over the relative significance of the generation of new morphologies vs. differential probabilities of their successful introduction, and the relative importance of constraint, convergence and contingency in the evolution of form.Key words: Morphological disparity, morphospace, evolutionary radiation, ecomorphology, theoretical morphology.The diversity of morphological form has been at the heart of much of organismal biology since the days of Cuvier, who used the discontinuities in shape and form as a basis for grouping animals into clusters. But this clustering itself raises a host of questions: Why is it that the morphology of arthropods is so disparate, but that of priapulids, or flamingoes, so limited? Why do biological forms exist in clumps, at a variety of scales? What can we learn about the evolutionary process by tracking the diversity of organic form (disparity) through time? Understanding the processes and conditions that influence differences in the range of form is critical for understanding the construction of biodiversity. Over the past two centuries, many approaches have been pursued to understand disparity. Considerable impetus to this effort was provided in 1989 by Stephen Jay Gould's 'Wonderful Life', in which he employed the exquisite fossils of the Middle Cambrian Burgess Shale to argue that the morphological disparity of arthropods at that single locality was greater than for all extant arthropods. Such early disparity was, in Gould's view, inconsistent with the predictions of an evolutionary theory that impli...

The origin and early radiation of dinosaurs

“…Morphological disparity and high rates of character change spiked early in dinosaur history, long before and out-of-step with taxonomic diversity. This decoupling of disparity and diversity, and the burst of disparity before diversity, is typical for many evolutionary radiations (Erwin, 2007). Likely competitors to early dinosaurs explored a larger range of body types throughout the Late Triassic and evolved at indistinguishable rates, and even the closest cousins of dinosaurs persisted alongside their better-known relatives for up to 20 million years.…”

“…The goal in each case is to represent the overall morphology of a set of organisms. These morphological measurements or characters are then subjected to multivariate statistical analysis, which ordinates taxa in a multidimensional space (a "morphospace": Raup, 1965;McGhee, 1999;Erwin, 2007). In essence, a morphospace is akin to a morphological "map," which graphically represents how similar and different taxa are from each other in their body plans.…”

Morphospaces and Databases: Diatom Diversification through Time

“…A thorough review of these different metrics is beyond the scope of this chapter (for more detailed treatments see Wills et al 1994, Ciampaglio et al 2001, and Erwin 2007, but a key point is that these metrics do not all describe the same aspects of morphospace occupation. To illustrate this notion, we present two disparity metrics here: convex hull volume and mean pairwise distance.…”

“…While there is broad consensus that theoretical morphospaces are preferable because their use of explicit, measurement-independent growth models that allow one to explore a wider range of unexplored as well as impermissible forms (e.g. Erwin 2007), their application is unfortunately not always possible (McGhee 1999, p. 26). Growth models for theoretical morphospaces are more readily devised for organisms with accretionary or branching growth (e.g.…”

“…The former, often referred to as taxonomic richness or simply as diversity, is the familiar measure that tallies numbers of taxa (commonly species). The latter, disparity for short, describes the variety of shapes or the "within-group variance of form" (Erwin 2007) by directly quantifying organismal morphology. In a sense, diversity and disparity both measure variety of form, because fossil taxonomy is, of course, itself based on morphology.…”