Covariance Sets and Ordered Geographic Variation in Cerion from Aruba, Bonaire and Curaçao: A Way of Studying Nonadaptation

-The islands of Aruba, Bonaire and Curacao are Cerion's geographic outlier, inhabited by the single species C. uva. I studied geographic variation in C. uva as recorded in 135 samples of 20 snails each, based on 19 measures selected to show covariance in patterns of ontogenetic allometry. The four regions of eastern Curacao, western Curacao, Bonaire and Aruba are clearly distinguished by consistent differences displayed in covariance sets. Snails from western Curacao reach a constrained final size with few large whorls, those from Aruba with more and smaller whorls. Each mode of growth entails, through its covariance set, a host of subsidiary consequences for the shell's basic form. Snails from Bonaire grow their apertures with more intense allometry. Nonadaptation must be an important theme in this variation; for even if the basic regional differences have an adaptive trigger (which is, in itself, far from demonstrated), the large set of allometric consequences, forming most of the noted and measured differences, must follow as automatic sequelae, not as selected traits. Covariance sets therefore offer a way of studying nonadaptation with positive evidence. Although each region can be distinguished by covariance sets, all regions display large and probably ecophenotypic differences in shell size as a response to moisture and vegetation of immediate habitats. This large variation in size within each region conceals the subtler but consistent differences among regions and has prevented their proper identification in past studies. An allometric and multivariate approach must be used to identify regional variation. [Cerion; nonadaptation; geographic variation; covariance sets; allometry.] Jan Swammerdam, the great 17th-century embryologist, wrote of the apparent complexity and diversity of molluscan shell form: Omnis enim quae inter eas animadvertitur differentia ex sola nascitur diversitate gyrationum-for all the differences that are noted among them originate only from the diversity of spirals. He had recognized one approach to a central problem of morphology-the reduction of complexity to a smaller set of generating factors in ontogeny. In this deductive strategy, we attempt to model a complex form by specifying the generating factors and constructing an array of realistic results by varying them within reasonable, restricted ranges. D'Arcy Thompson (1942), using the logarithmic spiral, developed such a system for molluscs as a thought experiment. With computers as architects, Raup (1966) built an array of realistic molluscs by varying just four parameters: the shape of the generating curve, its rate of increase in size, and translation down and away from the axis of coiling. Cerion is a land snail renowned for a diversity of form unparalleled within its group-dwarfs, giants, pencil thin shells, even squares and golf balls occupy its realm of realized shapes (Gould et al., 1974: fig. 1). Previous naturalists responded to this diversity by naming each nuance as a separate species, and producing an unrealistic array of 600 taxa. (Even the mosi disparate forms interbreed and few of the names represent true species.) The search for simplified causes in development that must underlie this maximal diversity may be designated as "the Cerion problem." As an initial approach, Woodruff and I have shown that Cerion's geographic distribution in the field is generally not the "crazy quilt" advocated by previous writers caught in an older taxonomy (Clench, 1957), but a well ordered distribution by region and environment (Gould and Woodruff, 1978, 1984; Woodruff and Gould, 1980). But the second question has generally eluded solution. How are the remarkable differences in