What limits the morphological disparity of clades?

Abstract

The morphological disparity of species within major clades shows a variety of trajectory patterns through evolutionary time. However, there is a significant tendency for groups to reach their maximum disparity relatively early in their histories, even while their species richness or diversity is comparatively low. This pattern of early high-disparity suggests that there are internal constraints (e.g. developmental pleiotropy) or external restrictions (e.g. ecological competition) upon the variety of morphologies that can subsequently evolve. It has also been demonstrated that the rate of evolution of new character states decreases in most clades through time (character saturation), as does the rate of origination of novel bodyplans and higher taxa. Here, we tested whether there was a simple relationship between the level or rate of character state exhaustion and the shape of a clade's disparity profile: specifically, its centre of gravity (CG). In a sample of 93 extinct major clades, most showed some degree of exhaustion, but all continued to evolve new states up until their extinction. Projection of states/steps curves suggested that clades realized an average of 60% of their inferred maximum numbers of states. Despite a weak but significant correlation between overall levels of homoplasy and the CG of clade disparity profiles, there were no significant relationships between any of our indices of exhaustion curve shape and the clade disparity CG. Clades showing early high-disparity were no more likely to have early character saturation than those with maximum disparity late in their evolution.

Cope's rule and the adaptive landscape of dinosaur body size evolution

“…Slater ; Oyston et al . ). Dinosaurs have well‐resolved phylogenies compared to many fossil groups, and so provide a model system for addressing this question using phylogenetic comparative methods.…”

Diversity and morphological evolution of Jurassic belemnites from South Germany

“…5b). As described in numerous studies (Foote, 1997;Hughes et al, 2013;Oyston et al, 2015), the temporal discrepancy between morphological and taxonomical diversifications is frequent in the early history of clades, and it can be explained in two complementary ways: 1) ecological opportunities (and thus morphological adaptation) are usually more important in the early history of groups, while they gradually disappear through ecological saturation of environments; and 2) developmental pathways are much less canalized by genetic legacy in the early evolution of clades. However, the low disparity of Passaloteuthidae was balanced by the rapid diversification and disparification of Hastitidae (Fig.…”

Journeys through discrete‐character morphospace: synthesizing phylogeny, tempo, and disparity

“…This is logically related to the limits of disparity (Oyston et al . ) and hence is directly relevant to our understanding of phylomorphospaces. For example, these phenomena might serve to explain some distance‐from‐root patterns generated in an earlier version of this manuscript, where the root morphology is rapidly left behind with subsequent evolution apparently constrained at the ‘edges’ of the hyperdimensional space (see Lloyd ).…”