Spermatophyta Molecular Clock: Time Drift and Recent Acceleration

Angiospermae radiation is widely recognized as a mid-Cretaceous event, but the adaptive radiation of <i>Asarum</i> and <i>Viola</i> as spring ephemerals also occurred during the Quaternary. To better understand the evolution of Angiospermae through geological time, a robust and well-calibrated timetree for Spermatophyta was constructed. The Angiospermae topology was aligned with the APG (Angiosperm Phylogeny Group) system, although node dates tend to be overestimated in recent mega-trees compared to those in this study. An exponential increase in the base substitution rate in recent geologic time was also revealed through the use of an alternative dating function in BEAST (Bayesian Evolutionary Analysis Sampling Trees) v1.10.4, followed by a presumed increase in the mid-Cretaceous. These events are thought to correspond to the Angiospermae radiations at the species level during the Quaternary and at the order level during the mid-Cretaceous. One possible cause of the recent increase in substitution rates and subsequent radiations, including those of <i>Asarum</i> and <i>Viola</i>, could be the proliferation of C₄ grasses, a reduction in atmospheric CO₂, and the onset of the Quaternary glacial period. Plant evolution has drastically altered Earth's environments, which, in turn, have influenced evolutionary processes. The mid-Cretaceous event may have been driven by co-radiation with herbivorous beetles, although beetles also include species with non-herbivorous habits.