Interesting study on the roundtail chub - 'Holistic species delimitation supports recognition of multiple lineages in the 𝐺𝑖𝑙𝑎 𝑟𝑜𝑏𝑢𝑠𝑡𝑎 complex of the American Southwest', with researchers making the argument for the recognition of two (possibly three) distinct evolutionary lineages within. In the study, a great discussion ensues regarding species delimitation, and the authors support for a holistic, integrative approach, which advocates considering all previous information and seeking to arrive at a conclusion based on consensus. "Definitive species delimitation is a difficult problem in some cases, as we describe here. In a letter to Joseph D. Hooker, Charles Darwin noted, “It is really laughable to see what different ideas are prominent in various naturalists' minds, when they speak of ‘species’; in some, resemblance is everything and descent of little weight—in some, resemblance seems to go for nothing, and Creation the reigning idea—in some, descent is the key—in some, sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying to define the undefinable” (Darwin Correspondence Project, 2025). Meanwhile, unique and irreplaceable biodiversity continues to be lost at a precipitous rate." 𝗥𝗲𝘀𝗲𝗮𝗿𝗰𝗵 𝗧𝗶𝘁𝗹𝗲 Holistic Species Delimitation Supports Recognition of Multiple Lineages in the 𝐺𝑖𝑙𝑎 𝑟𝑜𝑏𝑢𝑠𝑡𝑎 Complex of the American Southwest Open-access - bioone.org/journals/ichthyol… 𝗖𝗶𝘁𝗮𝘁𝗶𝗼𝗻 Thomas F. Turner, Thomas E. Dowling, Paul C. Marsh, Robert W. Clarkson, David L. Propst, Steven P. Platania, Megan J. Osborne, Guilherme Caeiro-Dias "Holistic Species Delimitation Supports Recognition of Multiple Lineages in the 𝐺𝑖𝑙𝑎 𝑟𝑜𝑏𝑢𝑠𝑡𝑎 Complex of the American Southwest," Ichthyology & Herpetology, 114(2), 190-203, (15 May 2026) 𝗔𝗯𝘀𝘁𝗿𝗮𝗰𝘁 Challenges associated with species delimitation of the 𝐺𝑖𝑙𝑎 𝑟𝑜𝑏𝑢𝑠𝑡𝑎 species complex in the Gila River basin arise because of convergence of multiple evolutionary processes that result in perplexing patterns of morphological, genetic, and ecological variation. As currently recognized, the complex comprises three nominal species: 𝐺𝑖𝑙𝑎 𝑟𝑜𝑏𝑢𝑠𝑡𝑎, 𝐺𝑖𝑙𝑎 𝑖𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎, and 𝐺𝑖𝑙𝑎 𝑛𝑖𝑔𝑟𝑎, each with overlapping traits and disjunct geographic distributions. Key challenges for delimitation include limited morphological divergence, strong local genetic differentiation that obscures broader patterns of lineage diversification, interspersed geographic distributions of nominal taxa, and phylogenetic analyses complicated by admixture. We review and synthesize ecological, morphological, geological, and genetic data to infer evolutionary processes that shaped diversity in the complex. Historical events, including tectonically-driven drainage isolation and integration and cyclic climatic fluctuations strongly influenced the distribution and diversification of Gila. Morphological analyses revealed overlap among traits traditionally used for species identification, while genetic studies indicated strong population-level differentiation with limited gene flow. Independent phylogenomic analyses yielded conflicting interpretations that hinge upon methodologies employed. To reconcile these complexities, we advocate a holistic approach that integrates evolutionary processes, geologic history, and contemporary morphological and genetic patterns. We suggest these, in tandem, argue against a single polytypic species and support instead the presence of two (possibly three) distinct evolutionary lineages. Importantly, our review underscores the importance of integrating diverse data sources to evaluate taxonomic ambiguities in cryptic species complexes, especially those that evolved in highly dynamic geological and climatic settings. 𝗣𝗵𝗼𝘁𝗼 𝗖𝗿𝗲𝗱𝗶𝘁 (A) Roundtail Chub 𝐺𝑖𝑙𝑎 𝑟𝑜𝑏𝑢𝑠𝑡𝑎 (photo by R. R. Belnap and D. L. Ward), (B) Gila Chub 𝐺𝑖𝑙𝑎 𝑖𝑛𝑡𝑒𝑟𝑚𝑒𝑑𝑖𝑎 (photo by B. Gratwicke), (C) Headwater Chub 𝐺𝑖𝑙𝑎 𝑛𝑖𝑔𝑟𝑎 (photo by R. W. Clarkson). © 2026 the Author(s). Published in the journal Ichthyology & Herpetology. This paper is released under a Creative Commons Attribution 4.0 International (CC-BY-4.0) licence. #Gila #AmericanSouthwest #Ichthyology #Taxonomy #Biodiversity #USA #America #SpeciesDelimitation #Systematics #Leuciscidae #Chub #RoundtailChub #GilaRiver #Darwin