Meeting Abstract
Evolutionary biologists have considered understanding the relationship between organismal morphology and functional performance to be fundamental for understanding phenotypic diversification. Numerous studies have investigated performance and morphological evolution within lineages. However, far less attention has been paid to the relationship between performance and morphological diversification among lineages. This study develops the turtle shell as a model system for studying morphological and performance diversification within a comparative context. Original data consisted of 3D landmark coordinates digitized on 1962 turtle shells representing 254 separate species. Data were aligned using a Generalized Procrustes fit and ordinated with principal component (PC) analysis. High scores on PC1 and 2 indicate taller, more domed shells. To explore the functional implications of this variation, theoretical shell shapes corresponding to 117 evenly-spaced points in morphospace were extracted. Finite element models were built for all theoretical shapes to assess mechanical performance. Heat transfer ability was quantified using surface area to volume ratios (SA/V), and three shape indices were used to assess righting ability. Turtle shells with high PC1 and 2 scores were stronger, possessed greater righting ability, and had lower SA/V. Shells with low PC1 and 2 scores were more streamlined, and had higher SA/V. Terrestrial and aquatic turtles did not differ in shell shape (see part I), but terrestrial turtles showed a tendency to evolve towards higher PC1 and 2 scores. Similar values for all performance measures could be found in large areas of morphospace, suggesting many-to-one mapping of form onto function; thus, turtle shells can diversify morphologically without necessarily sacrificing performance for all shell functions.
