I used a Hierarchical approach to study historical biogeography in a group of colubrid snakes found in western North America. I combined small regions of mtDNA sequence data from a large number of individuals, with complete mt-genomic data. First, I investigated the relationships among leptodeirines-a presumed subgroup of dipsadines includeng nightsnakes (Pseudoleptodeira, Eridiphas, and Hypsiglena) - using ~1.5 Kb of data (cob and nad4). The relationships differed among parsimony, likelihood, and bayesian analyses. All analyses supported the monophyly of the nightsnakes; however, none supported the monophyly of the leptodeirines. Instead, these data supported a new hypothesis that the dipsadines were ancestrally rear-fanged and preyed on small vertabrates (frogs and lizards), such as the nightsnakes, while the more derived lineages have modified anterior maxillary dentition and prey strictly on invertabrates. Secondly, using an evolutionary species concept, I test species-subspecies boundaries in the wide-ranging hypsiglena, which has over 17 forms described, by collecting ~800 bp of sequence data (nad4 and tRNA) from ~ 175 individuals. Six major clades, concordant with geography, were recognized as species: Chihuahuan Destert (H. jani); central-western Mexico (H. torquata); upland Jalisco (H. Affinis); central California-Cape of Baja ("Coast," H. ochrorhyncha); Sonoran, Mojave, and Great Basin deserts ("Desert" H. chlorophaea), and an undescribed form from the Sonoran-Chihuahuan desert transition zone ("cochise"). The relationships among the major clades were not well resolved. Lastly, I collected complete mt-genome sequence data from 15 individuals including Eridiphas, Pseudoleptodeira, each of the major clades of Hypsiglena, and Sibon and Imantodes. All combined genomic-level analyses contained overwhelming support for a single phylogeny. These data, in conjunction with the phylogeographic data, supported my hypothesis that vicariance associated with the Miocene seperation of the Cape of Baja from mainland Mexico formed the Baja endemic Eridiphas, followed by subsequent range expansion and dispersal of Hypsiglena onto the northern portion of the peninsula and an even later vicariance event associated with the northern inundation of the gulf of California during the Pliocene. Hysiglena later dispersed down the Baja California Peninsula, coming into secondary contact with Eridiphas, forming a ring-like distribution around the Gulf of California.