Abstract
Accurate delineation of lineage diversity is increasingly important, as species distributions are becoming more reduced and threatened. During the last century, the subspecies category was often used to denote phenotypic variation within a species range and to provide a framework for understanding lineage differentiation, often considered incipient speciation. While this category has largely fallen into disuse, previously recognized subspecies often serve as important units for conservation policy and management when other information is lacking. In this study, we evaluated phenotypic subspecies hypotheses within shovel-nosed snakes on the basis of genetic data and considered how evolutionary processes such as gene flow influenced possible incongruence between phenotypic and genetic patterns. We used both traditional phylogenetic and Bayesian clustering analyses to infer range-wide genetic structure and spatially explicit analyses to detect possible boundary locations of lineage contact. Multilocus analyses supported three historically isolated groups with low to moderate levels of contemporary gene exchange. Genetic data did not support phenotypic subspecies as exclusive groups, and we detected patterns of discordance in areas where three subspecies are presumed to be in contact. Based on genetic and phenotypic evidence, we suggested that species-level diversity is underestimated in this group and we proposed that two species be recognized, Chionactis occipitalis and C. annulata. In addition, we recommend retention of two subspecific designations within C. annulata (C. a. annulata and C. a. klauberi) that reflect regional shifts in both genetic and phenotypic variation within the species. Our results highlight the difficultly in validating taxonomic boundaries within lineages that are evolving under a time-dependent, continuous process.
Highlights
Scientific debates over species concepts persist, the notion that species exist as separately evolving lineages, presumably under various stages of speciation, is central to discussions related to intraspecific diversity
C. occipitalis was composed of three geographically distinct mitochondrial DNA (mtDNA) lineages that were similar to Wood et al [37], the increased geographic sampling in this study provided added resolution of clade boundaries and increased posterior probability support for a third lineage (Fig. 2; Fig. S1-3)
Individuals belonging to clade A occupied the range of C. o. occipitalis and C. o. talpina throughout the Mojave Desert, with the exception of samples taken from eastern Mojave Desert in California and Arizona
Summary
Scientific debates over species concepts persist, the notion that species exist as separately evolving lineages, presumably under various stages of speciation, is central to discussions related to intraspecific diversity. Intraspecific diversity garnered early interest in the field of evolutionary biology, as Darwin [2] makes clear: ‘‘those forms which possess in some considerable degree the character of species, but which are so closely similar to some other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank them as distinct species, are in several respects the most important for us.’’ This sentiment extended through the 20th century, where researchers often used the subspecific rank to fulfill two roles: (1) to denote the phenotypic variation within a species range, and (2) to provide a framework for understanding the heirarchical levels of lineage differentiation, often considered incipient speciation [3,4,5,6,7]. Accurate delineation of lineage diversity and population genetic structure remains an important task, as species distributions are becoming ever more reduced and threatened [12,13,14,15]
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