Macroevolutionary theory predicts high rates of evolution should occur early in a clade's history as species exploit ecological opportunity. Evidence from the fossil record has shown a high prevalence of early bursts in morphological evolution, but recent work has provided little evidence for early high rates in the evolution of extant clades. Here I test the prevalence of early bursts in extant data using phylogenetic comparative methods. Existing models are extended to allow a shift from a background Brownian motion (BM) process to an early burst process within sub-clades of phylogenies, rather than an early burst being applied to an entire phylogenetic tree. This nested early burst model is compared to other modes of evolution that can occur within sub-clades, such as evolution with a constraint (Ornstein-Uhlenbeck model) and nested BM rate shift models. These relaxed models are validated using simulations and then are applied to body size evolution of three major clades of amniotes (mammals, squamates, and aves) at different levels of taxonomic organisation (order, family). Applying these unconstrained models greatly increases the support for early bursts within nested sub-clades, and so early bursts are the most common model of evolution when only one shift is analysed. However, the relative fit of early burst models is worse than models that allow for multiple shifts of the BM or OU process. No single shift or homogenous model is superior to models of multiple shifts in BM or OU evolution, but the patterns shown by these multi-rate models are generally congruent with patterns expected from early bursts.
- early bursts
- body size
- phylogenetic comparative methods