Data from: Correlation of shell phenotype and local environment suggests a role for natural selection in the evolution of Placostylus snails

The giant edible Placostylus snails of New Caledonia occur across a wide range of environmental conditions, from the dry southwest to the wetter central and north-eastern regions. In large, slow-moving animals such as Placostylus, speciation could be assumed to be largely driven by allopatry and genetic drift as opposed to natural selection. We examined variation in shell morphology using geometic morphometrics and genetic structure within two species of Placostylus (P. fibratus, P. porphyrostomus), to determine the drivers of diversity in this group. Despite the current patchy distribution of snails on New Caledonia, both mtDNA and nuclear SNP datasets (>3,000 loci) showed weak admixing between populations and species. Shell morphology was concordant with the genetic clusters we identified and had a strong relationship with local environment. The genetic data, in contrast to the morphological data, did not show concordance with climatic conditions, suggesting the snails are not limited in their ability to adapt to different environments. In sympatry P. fibratus and P. porphyrostomus maintained genetic and morphological differences, suggesting a genetic basis of phenotypic variation. Convergence of shell shape was observed in two adjacent populations that are genetically isolated but experience similar habitat and climatic conditions. Conversely, some populations in contrasting environments were morphologically distinct although genetically indistinguishable. We infer that morphological divergence in the Placostylus snails of New Caledonia is mediated by adaptation to the local environment.

新喀里多尼亚的巨型可食用扁蜗牛属（Placostylus）螺类，其栖息环境跨度极大，涵盖从干燥西南部到湿润的中部及东北部区域。对于扁蜗牛属这类体型庞大、移动缓慢的动物而言，物种形成通常被认为主要由异域分化（allopatry）和遗传漂变驱动，而非自然选择。本研究针对该类群的两种螺类（P. fibratus、P. porphyrostomus），利用几何形态测量学（geometric morphometrics）分析其壳形变异，并探究种群遗传结构，以明确该类群多样性的驱动因素。尽管目前新喀里多尼亚的扁蜗牛分布呈斑块状，但线粒体DNA（mtDNA）与核单核苷酸多态性（SNP）数据集（含超3000个位点）均显示，不同种群及物种间的基因交流程度极低。壳形形态与本研究鉴定出的遗传聚类结果一致，且与当地环境存在显著关联。与形态学数据不同的是，遗传数据并未与气候条件呈现出一致性关联，这表明扁蜗牛对不同环境的适应能力并未受到限制。在同域分布（sympatry）区域内，P. fibratus与P. porphyrostomus始终保持遗传与形态上的差异，这表明表型变异存在遗传基础。在两个遗传上相互隔离但栖息环境与气候条件相似的相邻种群中，观察到了壳形的趋同演化。反之，部分栖息环境差异显著的种群，尽管遗传上难以区分，但形态上却存在明显差异。本研究推断，新喀里多尼亚扁蜗牛属螺类的形态分化，是由对当地环境的适应性演化所介导的。