The costs of paraphyletic classification in the world's most endangered fishes

Taxonomy is the organizing principle underlying how scientists, policymakers, and the public engage with biological diversity. Yet, the ongoing transition from Linnaean ranks to explicitly phylogenetic, rank-free taxonomy remains controversial. The tension between phylogenetic accuracy and nomenclatural stability is nowhere more consequential than in clades facing imminent extinction. In these cases, taxonomy directly shapes legal protections, conservation prioritization, and the identification of evolutionarily significant units. This means that taxonomic revisions carry consequences for the survival of lineages that extend well beyond their position in the catalog of biodiversity. Here, we argue that taxonomic revision using phylogenetic rank-free nomenclature provides a much-needed quantitative basis for conservation biology. We focus on the case of sturgeons (Acipenseridae), a clade of Holarctic ray-finned fishes in which all species are at risk of extinction. Recently, the IUCN Sturgeon Specialist Group (SSG) voted unanimously to reject a phylogenetic rank-free taxonomy of this clade published in 2025. A conservative alternative classification proposed by the SSG sustains a paraphyletic concept of Acipenser that fails on the group’s stated terms of conserving names that reflect morphology and other aspects of natural history. We argue that the SSG rejection exemplifies a recurring response in which the practical costs of taxonomic change are invoked to resist classifications that are demonstrably more reflective of evolutionary history. This has direct consequences for conservation through the misrepresentation of evolutionary diversity, the distortion of conservation prioritization metrics, and the misidentification of evolutionarily significant units. Taxonomies based on empirically reconstructed evolutionary relationships do not inhibit conservation but inform the protection of Earth’s biological heritage.