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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. [ABSTRACT FROM AUTHOR]

Besides studies on mental glands, research regarding cranial integumentary glands potentially involved in courtship of plethodontid salamanders is scarce. In this study, we examined the cranial glands of Eurycea chamberlaini. Scattered amongst typical mucous and serous glands of the integument, we found simple alveolar glands that reacted positively with the periodic acid-Schiff procedure along the lateral sides of the heads of only male E. chamberlaini during the putative mating season. These glands were identical to the lateral head courtship glands previously described in E. bislineata. External nasal glands of E. chamberlaini were also identical in morphology and histology to those previously described in E. bislineata, with the bulk of the secretory terminals residing behind the orbital cavities and extending anteriorly to where their excretory ducts empty through the integument dorsal to the external nares. We measured external nasal gland length, width, height, and volume and found that the means were greater in males when compared to females except for width, which was not sexually dimorphic. We compared external nasal gland tubular diameter to mental gland tubular diameter (a known secondary sexual structure) throughout the year and found a direct correlation that was significant in males but not for females; thus, hypertrophy of external nasal glands occurs at the same time pheromone production increases in mental glands, supporting a previously proposed hypothesis that external nasal glands are also secondary sexual structures in Eurycea. In culmination, Eurycea from two different complexes possess the same suite of cranial integumentary glands that may be involved in courtship, providing evidence that these glands are possibly ubiquitous across Eurycea. [ABSTRACT FROM AUTHOR]