THE SKELETON SPACE: A FINITE SET OF ORGANIC DESIGNS.

The structures of animal skeletons converge repeatedly on a limited number of architectural designs that can be constructed by growing organisms and that are functionally viable, although often not optimal. Properties of materials, construction rules that determine patterns of development, and physical constraints exerted by the requirements of function suggest that organic structure must necessarily approach these recurrent elements of design. A set of potential designs for the elements of animal skeletons is derived in terms of geometric and construction rules and the properties of materials. Skeletons of actual living and extinct organisms are matched with the possibilities defined within this theoretical morphospace. This provides a metric of skeletal complexity and of the extent to which various groups of animals have been able to exploit the range of possibilities of organic structure. These analyses show that the most evolutionarily advanced animals within a given phylum do not have the most complex skeletons; that arthropods are less morphologically diverse than vertebrates and molluscs; that the physical constraints of life on land and in the air substantially limit the variety of skeletal structures suitable for life in these environments; and that overall the range of possible skeletal designs has been very fully exploited by living and extinct organisms. These results strongly support the hypothesis that the essential elements of organic design are inherent in the material properties of the universe. The organizational properties of animal skeletons suggest that their design elements are fixed point attractors, structures that we characterize as topological attractors that evolution cannot avoid. © 1993 The Society for the Study of Evolution.