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).