Data from: Ecological speciation in an island snail: evidence for the parallel evolution of a novel ecotype and maintenance by ecologically dependent postzygotic isolation

Speciation is the process by which reproductive isolation evolves between populations. Two general models of speciation have been proposed: ecological speciation, where reproductive barriers evolve due to ecologically based divergent selection, and mutation-order speciation, where populations fix different mutations as they adapt to similar selection pressures. I evaluate these alternative models and determine the progress of speciation in a diverse group of land snails, genus Rhagada, inhabiting Rosemary Island. A recently derived keeled-flat morphotype occupies two isolated rocky hills, while globose-shelled snails inhabit the surrounding plains. The study of one hill reveals that they are separated by a narrow hybrid zone. As predicted by ecological speciation theory, there are local and landscape level associations between shell shape and habitat, and the morphological transition coincides with a narrow ecotone between the two distinct environments. Microsatellite DNA revealed a cline of hybrid index scores much wider than the morphological cline, further supporting the ecological maintenance of the morphotypes. The hybrid zone does not run through an area of low population density, as is expected for mutation-order hybrid zones, and there is a unimodal distribution of phenotypes at the centre, suggesting that there is little or no prezygotic isolation. Instead, these data suggest that the ecotypes are maintained by ecologically dependent postzygotic isolation (i.e. ecological selection against hybrids). Mitochondrial and Microsatellite DNA indicate that the keeled-flat form evolved recently, and without major historical disruptions to gene flow. The data also suggest that the two keeled-flat populations, inhabiting similar rocky hills, have evolved in parallel. These snails provide a complex example of ecological speciation in its early stages.

物种形成（speciation）是种群间演化出生殖隔离（reproductive isolation）的过程。目前已提出两类通用的物种形成模型：生态物种形成（ecological speciation），即生殖隔离机制因基于生态的歧化选择而演化；以及突变顺序物种形成（mutation-order speciation），即种群在适应相似选择压力的过程中固定了不同的突变。本研究对这两种替代模型进行了评估，并确定了栖息于罗斯玛丽岛（Rosemary Island）的一类多样性陆生蜗牛，隶属于Rhagada属（Rhagada）的物种形成进程。一类新近演化出的龙骨状扁平形态型（keeled-flat morphotype）占据了两处孤立的岩质丘陵，而球壳型（globose-shelled）蜗牛则栖息于周边的平原地带。对其中一处丘陵的研究显示，二者之间存在狭窄的杂交带（hybrid zone）。正如生态物种形成理论所预测的那样，在局域和景观尺度上均存在壳形与栖息地的关联，且形态过渡恰好位于两种截然不同环境之间的狭窄生态交错带（ecotone）。微卫星DNA（microsatellite DNA）分析显示，杂交指数分值（hybrid index scores）的渐变群宽度远大于形态渐变群，这进一步支持了生态因素对两种形态型的维持作用。该杂交带并未如突变顺序物种形成模型所预测的那样，穿过种群密度较低的区域，且表型在杂交带中心呈单峰分布，这表明几乎不存在或完全不存在合子前生殖隔离（prezygotic isolation）。相反，这些数据表明，两种生态型是由依赖于生态环境的合子后生殖隔离（postzygotic isolation）——即对杂交个体的生态选择——所维持的。线粒体DNA（mitochondrial DNA）与微卫星DNA数据显示，龙骨状扁平形态型为新近演化而来，且历史上未出现过基因流（gene flow）的重大扰动。研究数据还表明，两处栖息于相似岩质丘陵的龙骨状扁平种群经历了平行演化。这些蜗牛为我们提供了早期生态物种形成的一个复杂案例。