Species splitting increases estimates of evolutionary history at risk

Changes in species concepts and the rapid advances in DNA-based taxonomy and phylogeny of the past decades have led to increasing splits of single species into several new species. The consequences of such splits include the delineation of post-split species that may have restricted ranges and potentially increased extinction risks. Species splitting also leads to a re-evaluation of phylogenetic trees, with post-split trees having more species, but species that are less evolutionarily distinctive compared to pre-split trees. Such changes in extinction risks and distinctiveness may influence strategies for the conservation of phylogenetic diversity (PD). In this study, we evaluated the effect of splitting a species into two sister species on two widely used measures to evaluate PD at risk: (i) the expected loss of phylogenetic diversity associated with a set of species and, (ii) for each species, the gain in the expected phylogenetic diversity if the species is saved from extinction. We developed theoretical predictions and then explored these in a real-world case study of species splitting in the Rhinocerotidae family. Species splitting increases both of our measures related to PD at risk, implying underestimation of PD at risk when valid species splitting is not recognised. This bias may lead to suboptimal conservation decisions: the subset of species or sites given priority for conservation may be different from the subset that actually deserves priority conservation attention. We discuss how our findings can be applied to more complex studies and the perspectives this highlights for accommodating new taxonomic knowledge in conservation strategies.