Running head: INVESTIGATING RELATIVE TIMING ON PHYLOGENETIC TREES A method for investigating relative timing information on phylogenetic trees

In this paper we present a new way to understand the timing of branching events in phylogenetic trees. Our method explicitly considers the relative timing of diversification events between sister clades; as such it is complimentary to existing methods using lineages-through-time plots which consider diversification in aggregate. The method looks for evidence of diversification happening in lineage-specific “bursts”, or the opposite, where diversification between two clades happens in an unusually regular fashion. In order to be able to distinguish interesting events from stochasticity, we propose two classes of neutral models on trees with timing information and develop a statistical framework for testing these models. Our models substantially generalize both the coalescent with ancestral population size variation and the global-rate speciation-extinction models. We end the paper with several example applications: first, we show that the evolution of the Hepatitis C virus appears to proceed in a lineage-specific bursting fashion. Second, we analyze a large tree of ants, demonstrating that a period of elevated diversification rates does not appear to occurred in a bursting manner.