DATING THE HAWAIIAN SILVERSWORD RADIATION IN THE FACE OF PHYLOGENETIC, BIOGEOGRAPHIC, AND PALEOGEOGRAPHIC UNCERTAINTY

The Hawaiian silversword alliance is an iconic adaptive radiation of roughly thirty species. However, owing to the poor fossil record as found in many island plant lineages, the age of this clade, and thus the rate of the radiation, is not known precisely. In lieu of fossils, paleogeography-conditioned biogeographic processes may inform speciation times since a lineage can only colonize an island after it comes into existence. We date the silversword radiation by modeling interactions between species relationships, molecular evolution, biogeographic scenarios, divergence times, and island origination times using the Bayesian phylogenetic framework, RevBayes. Our approach is the first to inferentially disentangle the time of a lineage's arrival on an island with the time of the lineage diversifying upon that island, providing a richer understanding of the biogeography and timing of diversification of silversword lineages than previously obtained. The ancestor of living silverswords most likely colonized the modern Hawaiian islands once from the mainland approximately 5 Mya, with early surviving silversword lineages first appearing approximately 4 Mya. We reject alternative colonization scenarios that involve older Hawaiian islands and those that require multiple long-distance dispersal events. Lineages tend to colonize younger rather than older islands in concurrence with the progression rule of island biogeography. As a point of contrast, implausible scenarios are recovered under a paleogeography-naive analysis, such as silverswords colonizing and radiating on Kauai before it had emerged from the Pacific. This work serves as a general example for how diversification studies benefit from incorporating biogeographic and paleogeographic components.