Department of Biological Sciences, The George Washington University, Washington, District of Columbia, USA.
Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, USA.
Mol Ecol. 2024 Jan;33(2):e17219. doi: 10.1111/mec.17219. Epub 2023 Nov 28.
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.
许多机制可以驱动物种形成，包括适应、距离和环境导致的隔离。这些力量可以促进当地种群的遗传和表型分化，形成系统发育谱系，并最终完成物种形成。然而，概念上相似的机制也可能导致稳定而不是多样化的选择，导致谱系整合和遗传上凝聚力物种内种群结构的长期存在。促进地理遗传多样性形成和维持的过程同时促进了高迁移率和限制表型分化，从而导致种群遗传结构不伴随生殖隔离。我们建议，当评估系统地理学多样性时，该框架可以更广泛地应用于解决“结构”与“物种”的经典困境，统一种群遗传学、物种划分和物种形成的基础研究。我们在美国东南部的渗出蟾（Desmognathus aeneus）中展示了这样一个例子。最近的研究估计线粒体的分歧高达 6.3%，并且在地理杂种区内存在四个系统发育谱系，这些谱系可能代表着由我们的物种划分分析支持的不同物种。然而，尽管有限的扩散促进了显著的距离隔离，但微生境特异性似乎对单一、统一、生态介导的表型产生了稳定选择。结果，气候循环促进了谱系之间的反复接触和时间上的高迁移率。随后的杂交显然没有被限制基因渐渗的适应性分化所抵消，留下了一个单一的统一物种，具有深度分化的系统地理学谱系，但这些谱系似乎并不代表初期物种。
