Integrative ichthyological species delimitation in the Greenthroat Darter complex (Percidae: Etheostomatinae)

Species delimitation is fundamental to deciphering the mechanisms that generate and maintain biodiversity. Alpha taxonomy historically relied on expert knowledge to describe new species using phenotypic and biogeographic evidence, which has the appearance of investigator subjectivity. In contrast, DNA-based methods using the multispecies coalescent model (MSC) promise a more objective approach to describing biodiversity. However, recent criticisms suggest that under some conditions the MSC may over-split lineages, identifying species that do not reflect biological reality. Here, we reconcile these approaches using empirical data for the Greenthroat Darter complex (Etheostoma lepidum), a small freshwater fish species with a disjunct distribution in Texas and New Mexico, USA. We demonstrate that MSC methods recognizes all nine sampled populations as distinct species, sometimes splitting specimens from a single locality into multiple species. However, environmental, phenotypic, and biogeographic evidence do not corroborate the nine species supported by the MSC. Instead, collective evidence indicates that E. lepidum is comprised of just three species that are consistent with the molecular phylogeny: Etheostoma lepidum (Greenthroat Darter) in rivers draining the eastern Edwards Plateau, Etheostoma cf. lepidum (Texas Darter) in the Concho and San Saba rivers, and Etheostoma cf. lepidum (Pecos Darter) in the Pecos River. The Pecos Darter is likely highly imperiled due to its localized distribution and reliance on vanishing spring-fed stream habitats. The impending biodiversity crisis makes integrative and swift species delimitation more necessary than ever. Our study exemplifies how classic taxonomic expertise combined with molecular phylogenetics can produce a more robust description of threatened biodiversity.

Automated summary

Species delimitation is crucial for understanding the mechanisms behind biodiversity. Traditional alpha taxonomy relied on expert knowledge to describe new species, while DNA-based methods like the multispecies coalescent model (MSC) offer a more objective approach, though recent criticisms question their accuracy in certain situations.