Extensive gene flow suggests lack of reproductive barriers between the two subspecies of the green whip snake, Hierophis viridiflavus (Squamata: Colubridae)

Speciation is the fundamental process that underlies the diversification of species. Different steps of speciation, such as reproductive isolation or ecological niche separation, can evolve in different orders and over different time scales. The two subspecies of the green whip snake, Hierophis viridiflavus viridiflavus and Hierophis viridiflavus carbonarius, sometimes considered as separate species, are ideal to study speciation because they diverged in isolated glacial refugia. So far, little is known about whether these two subspecies are reproductively isolated from each other. Furthermore, it is unknown whether the common melanism of the subspecies H. v. carbonarius might be genetically encoded. We quantified gene flow along two transects across the contact zone of the two subspecies using double-digest restriction site-associated DNA. Our analyses implicated the existence of two well-defined clades that experience substantial gene flow along a contact zone that is similar to 300 km wide. We detected a clear association between the genetic assignment and the quantitative extent of melanism within the contact zone, which is likely to be genetically polygenic. Given the level of gene flow, it seems that strong postzygotic reproductive barriers have not evolved. We conclude that the two subspecies represent evolutionarily significant units for conservation and suggest treating them taxonomically as subspecies.