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

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Abstract
The islands of Aruba, Bonaire and Curacao are Cerion''s geographic outlier, inhabited by the single species C. uva. Geographic variation was studied in C. uva as recorded in 135 samples of 20 snails each, based on 19 measures selected to shown covariance in patterns of ontogenetic allometry. The 4 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.