Phylogeography and species boundaries of the western North American Nightsnake (Hypsiglena torquata): Revisiting the subspecies concept Daniel G. Mulcahy 1 Department of Biology, Utah State University, Logan, UT 84322-5305, USA Received 27 July 2007; revised 26 November 2007; accepted 4 December 2007 Available online 23 December 2007 Abstract The subspecies concept has received considerable debate throughout the past century. Subspecies were originally used to delineate potential incipient species, but were later employed to simply capture geographical variation. There is a recent trend to eliminate the trinomial in light of new evidence. Discrete, diagnosable lineages are elevated to specific status, while those that show clinal variation and/or appear to represent ecological pattern classes are placed in synonymy with the parent species and the subspecific epithets are dis- regarded. Here, I examine the species boundaries of nightsnakes (Hypsiglena torquata) using standard phylogeographic methods and mtDNA data from 178 individuals. Previously, seventeen subspecies of H. torquata were described. In this study, I recognize six species in what was previously considered H. torquata: one is novel, two were previously recognized subspecies, while the remaining three are wide-spread, polymorphic lineages, composed of multiple subspecies. I make the case to maintain the subspecific lineages in these wide- ranging species because they are geographically cohesive, morphologically discrete, and may represent incipient species within each com- plex, which have not yet achieved speciation. These subspecies are maintained, not only pending future investigations, but because they provide a useful identity for the taxonomy of this diverse lineage. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Baja California; Biogeography; Deserts; Eridiphas; tanzeri; Pseudoleptodeira 1. Introduction The process of speciation is a fundamental evolutionary concept inspiring extensive deliberation (Darwin, 1859; Dobzhansky, 1937; Mayr, 1942; Moritz et al., 1992; Coyne and Orr, 2004; Wake, 1997, 2006). Identifying the point at which diverging lineages have achieved speciation has often proven to be a challenging task. Part of this task is choos- ing a widely accepted species concept, while another is selecting appropriate criteria to delimit species boundaries (Sites and Marshall, 2004). Recently, these challenges have to a certain extent been reconciled. de Queiroz (1998, 2005, 2007) proposed that most contemporary species concepts share a common element in the conceptualization of what constitutes a species and their incompatibilities are often in the criteria used to determine species boundaries. Most contemporary species concepts are consistent with the notion that species are segments of separately evolving metapopulation lineages, which de Queiroz (1998) coined as the ‘general lineage concept of species.’ Challenges remain in determining at what point in this gradual process of a diverging lineage has speciation been achieved. From a taxonomic perspective, the interface of diverging lineages and secondary contact is often at the subspecific level, an area that has long been controversial among systematic biologists (Darwin, 1859; Wilson and Brown, 1953; Frost and Hillis, 1990). Historically, many vertebrate lineages at the species– subspecies boundary have been described based on minor differences in morphology, including color patterns. Rep- tiles are no exception, and many subspecies described on the basis of color patterns and scalation were typically con- 1055-7903/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ympev.2007.12.012 1 Present address: Department of Biology, 401 WIDB (Widtsoe), Brigham Young University, Provo, UT 84602-5181, USA. E-mail address: dmulcahy@byu.edu www.elsevier.com/locate/ympev Available online at www.sciencedirect.com Molecular Phylogenetics and Evolution 46 (2008) 1095–1115