Relictual ant lineages (Hymenoptera: Formicidae) and their evolutionary implications

Due to their widespread distribution, range of adaptations, local abundance, and species diversity, ants are a major component of terrestrial communities worldwide. However, such high ecological prevalence is not shared among all ant lineages, with four ant genera - Camponotus, Pheidole, Strumigenys, and Crematogaster - Accounting for a disproportionately large share of ant diversity. As a consequence, the study of variation in ant diversity has hitherto focused on testing whether more species-rich clades or regions are associated with disproportionately higher speciation and / or lower extinction rates. In this study, we investigate an intriguing but largely overlooked phenomenon in ants: the existence of ancient, species-poor lineages. We used both analytical and simulation results to assess evolutionary scenarios that could lead to current levels of ant diversity. We found that such relictual lineages are highly unlikely given constantrate models of speciation and extinction. In particular, such old, species-poor lineages are much more likely to either go extinct, or to generate more species than currently observed. The possible mechanisms leading to such relictual lineages are discussed, particularly with respect to current scenarios of early ant evolutionary history.