The influence of multiple functional demands on morphological diversification: A test on turtle shells

Organismal parts are often involved in the performance of more than one function. The role of trade-offs in influencing phenotypic evolution of such parts is well-studied; less well-understood is their role in influencing phenotypic diversity. Increases in the number of functions a part is involved in may inhibit subsequent diversification, as the number of trade-offs increases. Alternately, such an increase might promote phenotypic diversification, by increasing adaptive landscape complexity and promoting specialization for different roles. We compare these predictions by testing whether aquatic turtle shells, which resist loads, act as hydrodynamic elements, facilitate self-righting, and exchange heat with the environment, differ in phenotypic diversity from those of terrestrial species, which perform all the same functions except for hydrodynamics. We used 53 3D landmarks digitized on 2722 specimens of 274 hard-shelled turtle species to quantify shell shape variation, and a set of phylogenetic hypotheses to examine evolutionary patterns. Terrestrial turtles consistently had higher phenotypic diversity than aquatic species. Differences are not due to differences in the rates of evolution between the two groups, but rather differences in evolutionary mode. Thus this study supports the traditional view of the role of multiple functions in determining phenotypic diversity.