Incipient speciation and its impact on taxonomic decision: a case study using a sky island sister-species pair of stag beetles (Lucanidae: Lucanus)

Species delimitation can be difficult when the divergence between focal taxa is in the incipient stage of speciation, because conflicting results are expected among different data sets, and the species limits can differ depending on the species concept applied. We studied speciation history and investigated the impact on taxonomic decision-making when using different types of data in a Taiwanese endemic sister-species pair of stag beetles, Lucanus miwai and Lucanus yulaoensis, from sky island habitats. We showed that the two geographical taxa can be diagnosed by male mandibular shape. We found two mitochondrial co1 lineages with pairwise sequence divergence > 3%; however, L. miwai might not be monophyletic. The result of our multispecies coalescent-based species delimitation using five nuclear loci supported the evolutionary independence of the two sister species, but the calculated values of the genealogical divergence index (gdi) corresponded to the ambiguous zone of species delimitation. We also showed that post-divergence gene flow is unlikely. Our study demonstrates challenges in the delineation of incipient species, but shows the importance of understanding the speciation history and adopting integrative approaches to reconcile seemingly conflicting results before making evolutionarily relevant taxonomic decisions.