Integrative systematic structuring of the widespread psammophiid snakes (Psammophiidae) : a multi-evidence species delineation approach
Authors:   , Keates Chad 
Published: October 2022

Species form the foundations upon which we build our understanding of the natural world. Although a focus of much scientific attention, our understanding of species is stunted by the intrinsic ‘fuzziness’ of boundaries within nature. Due to the complexity of the evolutionary process, coupled with an ever-changing abiotic landscape, species are hard to delineate, even at the best of times. Whilst various species concepts and sophisticated delimitation methods have helped scientists tease apart species, many species complexes persist. This is because taxonomy is a discrete ordering system imposed upon the continuous and intercalated structure of life. To improve our understanding of a wide-ranging family of snakes, I investigated the taxonomy and evolutionary structuring within Psammophiidae using both molecular and morphological approaches, employing phylogenetic, phylogeographic, and morphometric analyses on the group. The systematic complexity of the family (as evidenced by past research) coupled with the group’s widespread distribution and ecological importance, made the taxon an ideal candidate for a broad-sweeping multi-level systematic analysis using multiple species delimitation methods. Additionally, in this thesis I attempted to build on the ground-breaking work of Christopher Kelly by addressing several knowledge gaps identified within the family, and in so doing, produce the most thorough evolutionary and taxonomic study of Psammophiidae possible. Given the taxonomic uncertainty associated with the family, Chapter Two used a representative sampling from every available species (near complete taxon sampling approach) in the family. The chapter used both standard and time-calibrated phylogenetic modelling and distance/threshold-based species delimitation, to elucidate the finer-level structuring within the family. Geometric morphometrics was used to determine whether there were diagnosable differences in head structure between the different genera. The final phylogenetic tree incorporated 320 samples, representing the most comprehensive phylogenetic reconstruction of the family to date. By using a near-complete taxon sampling approach, I was able to resolve previously unsupported relationships within the family whilst also identifying several novel instances of an under- and over-appreciation of species diversity within the family. Geometric morphometrics also identified clear distinctions between genera based on head shape (head width and ‘beakedness’). This chapter showcased the importance of complete taxon sampling and robust methodology for species delimitation and the deleterious effect of species concepts when implemented in isolation. In Chapter Three, I narrowed the scope of the study to focus on the genus level. Psammophylax (Fitzinger 1843) is an abundant, yet poorly studied genus of grass snakes, endemic to Africa. The generalist nature of the genus and wide-spanning distributions of the constituent species has given rise to several subspecies and a poor understanding of the taxonomic structuring within the genus. The overlapping distributions (sympatry) of many of Psammophylax species, coupled with the potential for cryptic speciation via mechanisms such as convergent evolution, made the group the ideal candidate for a broad-sweeping systematic study (as evidenced in Chapter Two). By applying the suite of analyses used in Chapter Two to the generic level, we aimed to determine the effectiveness of a multi-evidence species delineation approach when tackling systematic problems at lower taxonomic levels. A genetic phylogeny of six of the seven species was estimated using multiple phylogenetic and distance/ threshold-based species delimitation methods. To support the molecular analyses, we conducted morphological analyses on the body (traditional morphology) and head (geometric morphometrics) separately. Phylogenetic analyses recovered a similar topology to past studies, but with better resolution and node support. I found substantial genetic structuring within the genus, supported by significantly different head shapes between Ps. a. acutus and other Psammophylax species. Psammophylax a. acutus was recovered as sister to its congeners, and sequence divergence values and morphometrics supported its recognition as a new genus. Increased sampling in East Africa (Tanzania, Kenya, and Ethiopia) revealed that Psammophylax multisquamis is polyphyletic, necessitating the description of a new, morphologically cryptic, species from northern Tanzania. The distribution of Ps. multisquamis sensu stricto is likely restricted to Kenya and Ethiopia. Within this chapter, taxon-specific phylogenetic analyses yielded stronger intrageneric support as compared to Chapter Two, allowing for more defensible conclusions about taxonomical amendments. Geometric morphometrics proved similarly useful (as compared to Chapter Two) in teasing apart genera within the family but lacked the robustness to delineate species within Psammophylax with confidence, highlighting the apparent convergence of form within the genus. In Chapter Four, I investigated the evolutionary structuring within the Southern African endemic Psammophylax rhombeatus. The structural and environmental heterogeneity within the region has given rise to many morphological forms distributed throughout the country, with previous studies neglecting the associated molecular significance of these forms. Irrespective of their small sample sizes, both Chapter Two and Three identified substantial phylogenetic structuring within the species, making Ps. rhombeatus the ideal candidate for a multi-faceted systematic review, using a combination of phylogenetics, geometric morphometrics and, for the first time in this species, phylogeographic analyses. By investigating a single species, in detail, I was able to assess the effectiveness of the methodologies implemented in previous chapters on systematic sorting using the multi-evidence species delineation approach. Phylogenetic and haplotype analysis retrieved four well-supported clades: southeast South Africa (SESA), southwest South Africa (SWSA), north-eastern South Africa (NESA) and western South Africa (WSA). Although not variable enough to warrant taxonomic re-evaluation, the clades represented important genetic hotspots, with relatively high intraspecific genetic divergence values separating them, irrespective of the small geographic distances separating populations. This is likely a product of the taxon’s habitat-generalist lifestyle, enabling them to bypass vicariant barriers that might otherwise cause speciation in less versatile species. The clades are also geographically distinct, with little overlap, indicating previous vicariance, a finding that is supported by the split of Ps. rhombeatus from Ps. ocellatus in the mid-Pliocene, followed by the diversification of Ps. rhombeatus into four clades throughout the Pleistocene. The genetic structuring observed in Ps. rhombeatus may be a product of population expansion following ancient refugial isolation (potentially Last Glacial Maximum [LGM]). The molecular distinctiveness of the clades was not replicated in the morphological component of this chapter, with neither dorsal nor lateral geometric morphometric analyses of head shape showing any discernible distinctiveness based on geography. Whilst head shape has not been shown to be an effective delineator of evolutionary units at the species level (within this taxon), body colour, scalation, and snout-vent length has been linked to morphotypes within the species based on the work of Broadley (1966). These morphological groupings are loosely attributable to the molecular clades identified in the phylogenetic analyses, highlighting the complex interplay of genetic and morphological characteristics in the process of speciation, and their representation in systematic accounts. This thesis represents the most thorough evolutionary and systematic study of the family currently possible. In addition to identifying and describing both a new genus and species, this thesis also highlighted several instances of an over- and under-appreciation of species diversity within Psammophiidae. By applying a multi-evidence species delineation approach to this thesis, I show the intricacy of the evolutionary process (at various taxonomic levels) and showcase the ease to which species boundaries can be confounded when species concepts are implemented in isolation. These findings also highlighted the importance of sample size, sample range, species delimitation method on the outcome of taxonomic analyses, and their interpretation. Lastly, this thesis addressed the knowledge gaps left by Christopher Kelly’s PhD work and investigated the findings of recent papers that attempted to do the same. Whilst this study answers the questions of old, the taxon-intensive focus revealed several new knowledge gaps within the family, highlighting how much we know about snake systematics, and furthermore, how much we still need to learn about evolutionary structuring.