Incipient speciation and the impact on taxonomic decision: a case study using a sky island sister species pair of stag beetle (Lucanus; Lucanidae)

Species delimitation can be difficult when the divergence between focal
taxa falls in the incipient stage of speciation, because conflicting
results are expected among different data sets and the species limit may
differ depending on the applied species concept. We studied the speciation
history and investigated the impact on taxonomic decision when using
different data types in a Taiwanese endemic sister species pair of stag
beetle, Lucanus miwai and L. 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 may not be monophyletic. Our
multispecies coalescent based species delimitation result using five
nuclear loci supported the evolutionary independence of the two sister
species, but the calculated values of genealogical divergence index (gdi)
corresponded to the species delimitation ambiguous zone. We further showed
that post divergence gene flow is unlikely. Our study demonstrates
challenges in incipient species delineation, but shows the importance of
understanding the speciation history and integrative approaches to
reconcile seemingly conflicting results before making evolutionarily
relevant taxonomic decisions.