The Trials and Tribulations of Priors and Posteriors in Bayesian Timing of Divergence Analyses: the Age of Butterflies Revisited

The need for robust estimates of times of divergence is essential for downstream analyses, yet assessing this robustness is still rare. We generated a time-calibrated genus-level phylogeny of butterflies (Papilionoidea), including 994 taxa, up to 10 gene fragments and an unprecedented set of 12 fossils and 10 host-plant node calibration points. We compared marginal priors and posterior distributions to assess the relative importance of the former on the latter. This approach revealed a strong influence of the set of priors on the root age but for most calibrated nodes posterior distributions shifted from the marginal prior, indicating significant information in the molecular dataset. We also tested the effects of changing assumptions for fossil calibration priors and the tree prior. Using a very conservative approach we estimated an origin of butterflies at 107.6 Ma, approximately equivalent to the Early Cretaceous–Late Cretaceous boundary, with a credibility interval ranging from 89.5 Ma (mid Late Cretaceous) to 129.5 Ma (mid Early Cretaceous). This estimate was robust to alternative analyses changing core assumptions. With 994 genera, this tree provides a comprehensive source of secondary calibrations for studies on butterflies.