Integrative approaches to delimiting species and taxonomy: lesson learned form highly structured arthropod taxa

Advances in molecular techniques over the past decades have significantly shaped the field of systematics and taxonomy. Approaches to species delimitation that employ methods like DNA barcoding and generalized mixed Yule coalescent models are seemingly attractive because they present a more simplified solution to a putative “crisis” in taxonomy; that is, thereare simply too few taxonomists to document biodiversity on our planet within a reasonabletime period using traditional, often morphology-based taxonomic approaches. Nevertheless,the vast majority of spider species are, and continue to be, described on the basis of morphological features (often single character systems). Given the importance of species discovery to all fields within the biological sciences and to addressing the global biodiversity crisis, it is disturbing that the notion that taxon-based scholars can be replaced by mere technicians is being promulgated. Our molecular-based studies of arthropod taxa (e.g., mygalomorph spiders) have demonstrated that these taxa are often highly structured from a population genetic or phylogeographic perspective despite being relatively morphologically homogenous.Consequently, molecular approaches to species delimitation often grossly overestimate diversity whereas morphology-based species delineation underestimates it. It is clear that neither of these approaches taken alone is optimal and thus the field of taxonomy must employ a broader perspective that entails species hypotheses based on multiple lines of evidence. Through a process of reciprocal illumination, studies of mygalomorph species demonstrate that integrating morphological, ecological, and molecular-based approaches is the only way to ensure that our taxonomy reflects evolutionary diversity.