Paraphyletic species no more – genomic data resolve a Pleistocene radiation and validate morphological species of the Melanoplus scudderi complex (Insecta: Orthoptera)

Rapid speciation events, with taxa generated over a short time period, are among the most investigated biological phenomena. However, molecular systematics often reveals contradictory results compared with morphological/phenotypical diagnoses of species under scenarios of recent and rapid diversification. In this study, we used molecular data from an average of over 29,000 loci per sample from RADseq to reconstruct the diversification history and delimit the species boundary in a short-winged grasshopper species complex (Melanoplus Scudderi group), where Pleistocene diversification has been hypothesized to generate more than 20 putative species with distinct male genitalic shapes. We found that based on a maximum likelihood molecular phylogeny that each morphological species indeed forms a monophyletic group, contrary to the result from a previous mitochondrial DNA sequence study. By dating the diversification events, the species complex is estimated to have diversified during the Late Pleistocene, supporting the recent radiation hypothesis. Furthermore, coalescent-based species delimitation analyses provide quantitative support for independent genetic lineages, which corresponds with the morphologically-defined species. Our results also showed that male genitalic shape may not be predicted by evolutionary distance among species, indicating that this trait is not only labile, but also implying that selection may play a role in character divergence. Additionally, our findings suggest that the rapid speciation events in this flightless grasshopper complex might be associated primarily with the fragmentation of their grassland habitats during the Late Pleistocene. Collectively, our study highlights the importance of integrating multiple sources of information to delineate species, especially for a species complex that diversified rapidly, and whose divergence may be linked to ecological processes that create geographic isolation (i.e., fragmented habitats), as well as selection acting on characters with direct consequences for reproductive isolation (i.e., genitalic divergence).

快速成种事件——即在短时间内产生多个类群——是当前备受关注的生物学现象之一。然而，在近期快速分化的类群中，分子系统学的研究结果往往与基于形态学/表型的物种鉴定结论存在冲突。本研究利用每样本平均超过29000个位点的限制性位点相关DNA测序（Restriction Site-Associated DNA sequencing, RADseq）分子数据，以短翅蝗虫物种复合体（Melanoplus Scudderi类群）为研究对象，重建其分化历史并界定物种边界。此前有假说认为，更新世的分化事件催生了该类群的20余个具有独特雄性外生殖器形态的疑似物种。研究结果显示，基于最大似然法构建的分子系统发育树显示，每个形态学物种均构成单系群，这与此前线粒体DNA序列研究的结论相悖。通过对分化事件进行定年分析，该物种复合体的分化时间被估算为更新世晚期，支持了近期辐射演化假说。此外，基于溯祖理论的物种界定分析为独立遗传谱系提供了定量支持，这与基于形态学定义的物种划分结果相符。本研究结果还表明，雄性外生殖器形态或许无法通过物种间的进化距离进行预测，这意味着该性状不仅具有易变性，还暗示选择可能在性状分化过程中发挥了作用。另外，我们的研究发现，这一无翅蝗虫复合体的快速成种事件或许主要与更新世晚期草原生境的片段化有关。综上，本研究强调了整合多源信息开展物种界定的重要性，尤其适用于那些快速分化的物种复合体——这类类群的分化可能与引发地理隔离的生态过程（如生境片段化），以及作用于影响生殖隔离性状的选择（如外生殖器分化）密切相关。