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  1. Abstract

    Numerous mechanisms can drive speciation, including isolation by adaptation, distance, and environment. These forces can promote genetic and phenotypic differentiation of local populations, the formation of phylogeographic lineages, and ultimately, completed speciation. However, conceptually similar mechanisms may also result in stabilizing rather than diversifying selection, leading to lineage integration and the long‐term persistence of population structure within genetically cohesive species. Processes that drive the formation and maintenance of geographic genetic diversity while facilitating high rates of migration and limiting phenotypic differentiation may thereby result in population genetic structure that is not accompanied by reproductive isolation. We suggest that this framework can be applied more broadly to address the classic dilemma of “structure” versus “species” when evaluating phylogeographic diversity, unifying population genetics, species delimitation, and the underlying study of speciation. We demonstrate one such instance in the Seepage Salamander (Desmognathus aeneus) from the southeastern United States. Recent studies estimated up to 6.3% mitochondrial divergence and four phylogenomic lineages with broad admixture across geographic hybrid zones, which could potentially represent distinct species supported by our species‐delimitation analyses. However, while limited dispersal promotes substantial isolation by distance, microhabitat specificity appears to yield stabilizing selection on a single, uniform, ecologically mediated phenotype. As a result, climatic cycles promote recurrent contact between lineages and repeated instances of high migration through time. Subsequent hybridization is apparently not counteracted by adaptive differentiation limiting introgression, leaving a single unified species with deeply divergent phylogeographic lineages that nonetheless do not appear to represent incipient species.

     
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  2. Abstract

    Significant advances have been made in species delimitation and numerous methods can test precisely defined models of speciation, though the synthesis of phylogeography and taxonomy is still sometimes incomplete. Emerging consensus treats distinct genealogical clusters in genome-scale data as strong initial evidence of speciation in most cases, a hypothesis that must therefore be falsified under an explicit evolutionary model. We can now test speciation hypotheses linking trait differentiation to specific mechanisms of divergence with increasingly large data sets. Integrative taxonomy can, therefore, reflect an understanding of how each axis of variation relates to underlying speciation processes, with nomenclature for distinct evolutionary lineages. We illustrate this approach here with Seal Salamanders (Desmognathus monticola) and introduce a new unsupervised machine-learning approach for species delimitation. Plethodontid salamanders are renowned for their morphological conservatism despite extensive phylogeographic divergence. We discover 2 geographic genetic clusters, for which demographic and spatial models of ecology and gene flow provide robust support for ecogeographic speciation despite limited phenotypic divergence. These data are integrated under evolutionary mechanisms (e.g., spatially localized gene flow with reduced migration) and reflected in emergent properties expected under models of reinforcement (e.g., ethological isolation and selection against hybrids). Their genetic divergence is prima facie evidence for species-level distinctiveness, supported by speciation models and divergence along axes such as behavior, geography, and climate that suggest an ecological basis with subsequent reinforcement through prezygotic isolation. As data sets grow more comprehensive, species-delimitation models can be tested, rejected, or corroborated as explicit speciation hypotheses, providing for reciprocal illumination of evolutionary processes and integrative taxonomies. [Desmognathus; integrative taxonomy; machine learning; species delimitation.]

     
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  3. null (Ed.)
    Jacob Green was born in 1790 to a prominent New Jersey family of scholars and theologians. He taught at the College of New Jersey (now Princeton University) from 1818 to 1822 before co-founding Jefferson Medical College (now Thomas Jefferson University) in 1825, where he taught Chemistry until his death in 1841. Between 1818 and 1831, he published a series of nine papers on lizards, salamanders, and snakes, authoring the original description of several well-known species of salamanders from the eastern United States. Many of his names are ambiguous; some have been adjudicated by the ICZN, while others are currently treated as nomina dubia. Here, we review all of Green’s publications, report on newly re-discovered or re-interpreted material from several major natural history collections, and resolve most if not all remaining issues through a series of taxonomic actions. In particular, we first designate a neotype for Salamandra nigra Green, 1818. We then place S. sinciput-albida Green, 1818 and S. frontalis Gray in Cuvier, 1831 in synonymy with S. scutata Temminck in Temminck & Schlegel, 1838 and invoke Reversal of Precedence under Article 23.9 to designate them nomina oblita. We also designate a lectotype for S. bislineata Green, 1818. Finally, we resurrect the name S. fusca Green, 1818 as the valid name for the species Desmognathus fuscus, assuming priority over Triturus fuscus Rafinesque, 1820, designating S. fusca Laurenti, 1768 a nomen oblitum, and placing S. nigra Green, 1818 in synonymy. While Green’s herpetological legacy is not as expansive as that of some of his successors such as Holbrook, he is nonetheless a foundational early worker in salamanders, having described some of the most-studied species in the world. 
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  4. null (Ed.)
    Dusky salamanders (Desmognathus) constitute a large, species-rich group within the family Plethodontidae, and though their systematic relationships have been addressed extensively, most studies have centered on particular species complexes and therefore offer only piecemeal phylogenetic perspective on the genus. Recent work has revealed Desmognathus to be far more clade rich—35 reciprocally monophyletic clades versus 22 recognized species—than previously imagined, results that, in turn, provide impetus for additional survey effort within clades and across geographic areas thus far sparsely sampled. We conceived and implemented a sampling regime combining level IV ecoregions and independent river drainages to yield a geographic grid for comprehensive recovery of all genealogically exclusive clades. We sampled over 550 populations throughout the distribution of Desmognathus in the eastern United States of America and generated mitochondrial DNA sequence data (mtDNA; 1,991 bp) for 536 specimens. A Bayesian phylogenetic reconstruction of the resulting haplotypes revealed forty-five reciprocally monophyletic clades, eleven of which have never been included in a comprehensive phylogenetic reconstruction, and an additional three not represented in any molecular systematic survey. Although general limitations associated with mtDNA data preclude new species delineation, we profile each of the 45 clades and assign names to 10 new clades (following a protocol for previous clade nomenclature). We also redefine several species complexes and erect new informal species complexes. Our dataset, which contains topotypic samples for nearly every currently recognized species and most synonymies, will offer a robust framework for future efforts to delimit species within Desmognathus. 
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