Abstract

Delineating species limits is central to understanding how species evolve and diversify. However, species delimitation is challenging for newly emerging species. Beetle species in the genus Ceroglossus are diverse, presenting geographically structured colour races that have challenged taxonomists over the years. Here, we use RADSeq data and coalescent species delimitation analyses (BPP and gdi) to test whether color differences between races represent species limits in Ceroglossus species. Results from the multispecies coalescent method showed high support for most color races as species, even under conservative methodological choices (e.g. adding a well-recognized species from the same genus as outgroup). However, when multiple sites within a single colour race were tested as hypothetical species, the results also supported most of them as distinct, suggesting that the method was sensitive to population structure rather than species limits. In contrast, the gdi estimations were more conservative and revealed that most putative species were within population-level divergence. We call for caution when inferring species limits using genomic data and the multispecies coalescent approach due to its power to detect slight levels of population structure as species divergence and encourage the use of complementary approaches, control tests, and a well-established sister species as outgroups, to substantially reduce over-splitting. © The Author(s) 2025. Published by Oxford University Press on behalf of The Linnean Society of London. All rights reserved.