Integrating phylogenomic and morphological data to assess candidate species-delimitation models in brown and red-bellied snakes (Storeria)

Abstract

Systematics at the species level is still marked by theoretical and empirical tensions amongst the desires to identify geographical lineages, delimit species, and estimate their relationships. These goals are often confounded because each relies, at least to some extent, on the others being known. However, recently developed methods can simultaneously address all three. Furthermore, next-generation genomic sequencing allows us to generate large-scale molecular data sets to examine variation within species at a fine scale. Finally, a renaissance in morphological species validation allows us to integrate historical species definitions with coalescent models for species delimitation. Here, we investigate the applicability of these methods in an empirical case, in the Nearctic snake genus Storeria. Integrating trait data into species delimitation reduces the number of species delimited from molecular data alone. Whereas molecular data support eight distinct species-level lineages, including morphological data reduces this to four. The taxa Storeria dekayi, Storeria occipitomaculata, Storeria storerioides, and Storeria victa are considered distinct, monotypic species, with no subspecies recognized. We highlight the need for careful assessment of species delimitation, combining both computational genetic methods as well as traditional character-based descriptions. It is now possible to identify phylogeographical lineages, delimit species using molecular and morphological data, and estimate their relationships in a single coherent set of analyses. Moving forward, this will allow for more rapid and objective assessments of cryptic diversity at the species level.