Delimiting the rare, endangered and actively speciating

Species delimitation is a contentious topic. The genomics revolution initially brought hope that identifying and classifying species would be easier through better methods and more data, but genomics has also brought complexity and controversy to delimitation. One solution can be to collect a larger sample of individuals at a finer geographic scale. But what if taxa are rare and collecting more samples is difficult or detrimental to the organisms at hand? In this issue of Molecular Ecology Resources, Opatova et al. (2023) tackle the ambiguity of species delimitation in rare and endangered trapdoor spiders (genus Cyclocosmia). The authors propose a framework for delimiting species when samples are hard to come by, such as in these rare and cryptic spiders. The authors combine extensive genomic sampling with statistical approaches that consider both the genetic distinctiveness of each population of spiders and how much gene flow occurs between these populations. Their proposed taxonomy balances two opposing signals, structure and gene flow, to count eight lineages of Cyclocosmia, and to point the way for future taxonomic studies of the rare or difficult to obtain.