Testing for Burden in a Macroevolutionary System

ABSTRACT

The traits defining organismal body plans are expected to acquire burden, approaching invariance as they accumulate characters that depend on them, developmentally and functionally. Thus, the oldest features of a clade's body plan should decline in evolvability over time. With over 500 million years of evolutionary history, starting in the Cambrian Explosion of animal body plans, the marine bivalves are an ideal testing ground to evaluate the role of burden in macroevolution. We find substantial evolutionary changes in the novel elements constituting the foundation of the bivalve body plan—the ligament, hinge, and musculature that form the lever system that operates the animal's two shell valves. We observe instances of remodeling, translocation, and loss in each of these foundational structures of the bivalve body plan, and in the mineralized shell inherited from deeper ancestors. Our temporal inventory of these changes to the lever components and to the shell itself, including striking divergences from the ancestral bilateral symmetry, indicates that, contrary to expectations of burden increasing through time, most were not confined to the early evolutionary history of the class. Thus, despite their antiquity, these fundamental elements of the bivalve body plan have been surprisingly evolvable and remain so to the present day, underscoring a need to investigate these macroevolutionary outcomes in explicit developmental terms.