WHEN IS A SPECIES NOT A SPECIES? QUANTIFYING REGIONAL AND EVOLUTIONARY DIFFERENCES IN MORPHOLOGY FOR THE GENUS CUCULLAEA [BIVALVIA]

Variation in morphology within species arises primarily because of changes in the timing and rate of developmental events (ontogeny), and from adaptations to local environmental conditions (ecophenotypy). Over time, these processes may lead to speciation, as existing morphological features are modified, leading to new species diverging into separate regions of morphospace. The bivalve genus Cucullaea has a globally extensive fossil record dating back 200 million years, providing an opportunity to test i) how shape variation within species relates to ontogenetic and ecophenotypic effects, and ii) how morphological difference among species evolves through time in concert with changes in geographic distribution and diversity. Within a broader, ongoing investigation of the synergistic roles of geographic distribution, species richness and morphological variability within Cucullaea, this study focuses on quantifying intra- and inter-specific variation in shape. To assess morphological disparity, numerous well-preserved museum specimens from various geographic areas and stratigraphic intervals were digitized for morphometric analyses. Landmark techniques, in addition to traditional and outline methods, were then used to recognize i) regional, ecophenotypic variation within the present day, Indo-Pacific species, C. labiata, and ii) variation among geographically and temporally distinct cucullaeid species from the present-day and fossil record. Combining morphometric methods provides the opportunity to explore the significance of multiple morphological characteristics, ranging from those features that represent phylogenetic relationships and best discriminate species as taxonomic units, to those that evolve primarily in response to environmental conditions and life strategies. This multifaceted, quantitative approach provides insight into the dominant selective pressures operating on individual shell characteristics, separate populations or on entire species. Because cucullaeid species often exhibit wide morphological ranges, taxonomic designations must be made with consideration of intraspecific disparity throughout the genus, allowing for rigorous analyses of regional differences within species and evolutionary differences among species.