KURPIUS, James W., Museum of Paleontology and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780
Modern morphometric techniques and phylogenetic hypotheses provide a powerful toolkit to discover potential developmental patterns correlated with morphological divergence. How variation evolves is a fundamental evolutionary question, where the underlying developmental changes leading to shell shape change are poorly understood. Two broad classes of developmental change produce variation in morphology, heterochrony and heterotopy. By altering the timing or rate of events during ontogeny, snails have the potential to develop novel morphologies by heterochrony. The evolution of new forms by heterotopy, where changes in location and direction of a morphology occurs during development, has been ignored in gastropods. Recent advances in modern morphometric techniques provide a means to examine and differentiate these two classes of developmental change.
Gastropod shells are particularly useful and well suited to investigations of developmental change through time because the entire ontogeny of an adult snail is preserved by the shell. Also, gastropods are well known for their wide range in shell variation, where large differences occur intraspecifically and within single populations. Ecotypic variation is common in some groups, however, it is not known whether processes producing ecotypic variation are population specific, species specific, or clade specific. Are developmental pathways constrained or are closely related taxa converging on similar end morphologies through different developmental trajectories? To address this question, shape change in the Neritrema clade within the genus Littorina is quantified using partial warp scores of the thin-plate spline. Relative warps are used to reveal significant shape variation among ecotype classes and between species. By identifying developmental patterns, questions of whether shape change occurs through similar modes within an ecotype, within species, and across clades can be answered. This has profound implications for incorporating the fossil record into cladistic analyses where character sets based on shell morphology are largely ignored.