A 2011 research paper published in Molecular Biology concludes, based on genetic evidence, that the Mexican populations of Narrow-headed Garter Snakes represent distinct evolutionary lineages — that is to say, separate species. Populations in southwestern Durango had already been classed as a separate species (the Southern Durango Spotted Garter Snake) on morphological grounds in 1995; this study affirms that conclusion. But it also moves the remaining Mexican populations of Narrow-headed Garter Snake into a new species: the Madrean Narrow-headed Garter Snake, Thamnophis unilabialis. This leaves Arizona and New Mexico as the only place where Narrow-headed Garter Snakes (in the new, more limited sense) may be found. It’s in trouble in both states.