Nature vs Nurture: Quantifying Evolution and Ecophenotypic Variation in Pelicaria vermis

The fossil record provides the only direct evidence we have for the timing, rate, and nature of morphological evolution within particular lineages of complex animals or plants. Well preserved lineages, with high resolution phenotypic trait data, covering an evolutionarily relevant time frame, provide the best opportunity to investigate and understand fine-scale evolutionary trends and modes. Potential environmental drivers of such evolutionary change can similarly be inferred by comparisons with detailed environmental proxy data from the geological record. This study is a contribution to the study of timing,rate and nature of evolution, using data from a group of endemic fossil snails from NewZealand.

The Late Neogene-Quaternary, marine gastropod genus Pelicaria (Caenogastropoda: Stromboidea: Struthiolriidae) is common within the New Zealand fossil record and shows rich morphological diversity. Several different taxonomic interpretations exist due to the highly variable morphology of Pelicaria, but recent work suggests much of the morphological diversity is contained within the species P. vermis. In this study we quantify seven arithmetically independent morphological parameters, which include two coiling rate estimates, namely the rate of translation of successive apertures towards the anterior (Te) and rateof whorl expansion (increase in area, We), two measures of the exterior profile of the penultimate whorl, two measures of aperture shape of the penultimate aperture, and the centroid size of the final adult aperture. We use a variety of methods to capture the nature of morphological variation in a biologically meaningful way; each morphological parameter relates directly to how the animal grew and interacted with its environment. To the best of our knowledge this study provides the most thorough quantitative description ofgastropod shell morphology ever undertaken.

We quantify these morphological parameters in 296 Plio-Pleistocene Pelicaria specimens and analyse the evolutionary modes of each parameter within two geographically separate locations (85km apart) in the Wairarapa area, southern North Island, New Zealand. A sedimentary facies based approach is used to model environmental change throughout each sequence, and used to investigate potential correlations between morphology and environmental variability.

Three partially overlapping morphological groups were identified within Pelicaria morphospace based on the seven morphological parameters, but these groups do not correspond to previously identified Pelicaria forms, which we interpret to mean that much of the morphological variation shown within these forms is indeed contained within the single species, P. vermis. Time series of morphological parameters suggest patterns of evolutionary stasis and unbiased random-walk are common throughout the observed time period, indicating that different characters evolve in different ways, reflecting mosaic evolution. One parameter, representing overall whorl curvature and spiral cord height, shows apparent directional evolution at a single location. We attribute this pattern to ecophenotypy and not evolution. We found that environmental change, most likely related to changing water depth, showed differential effects on morphology, and was strongly correlated with two separate morphological parameters, which we suggest reflect ecophenotypic variation. We find little evidence of any continuous evolutionary trends in response to known trends in environmental change (for example, shallowing water depth and temperature). Thissuggests that much of the variation shown in Pelicaria evolution may be a result of population genetic differences, potentially through random drift, or in response to small-scale environmental changes that were not measured here.

The endemic snail species P. vermis is shown to morphologically evolve following stasis and random walk with little apparent response to environmental trends through time; however it does show short-term population specific variations in several shell characters which are correlated to local changes in environmental factors related to water depth.