Meeting Abstract
Do structures that perform a large number of functions diversify more than those which perform fewer functions? There is currently no theoretical consensus on this question, and little empirical study. Classically, additional functions were thought to impose additional functional constraints and decrease phenotypic diversification. More recent theoretical work suggests that additional functional demands can expand the range of optimal morphologies, facilitating diversification. Turtle shells have proven an excellent system for addressing this question. It has been shown that terrestrial clades show greater levels of morphological diversification than their aquatic relatives, lending support for the classical view. Here, I explore whether this increased diversification can be tied to differences in the functional landscape experience by shells of terrestrial species (which must optimize mechanical strength and righting ability and aquatic species (which additionally must optimize shell hydrodynamics). I assessed the functional performance of theoretical shapes located at evenly-spaced intervals throughout shell morphospace, and constructed performance surfaces for each function. Many-to-one mapping was observed for all functions. Some measures, such as mechanical strength and righting ability, showed similar performance surfaces; others, such as strength and hydrodynamics, were nearly perfectly opposed. Multiple optimization methods were used to predict patterns of variation in both groups. Although the terrestrial turtles generally conformed to expectations, aquatic turtles did not show patterns of variation that were predicted based on their shell functions. Either the assumptions of the optimization methods are violated for aquatic turtles, or additional shell functions, beyond those studied, could influence turtle shell evolution.
