Evolutionary Rates Standardized for Evolutionary Space: Perspectives on Trait Evolution

Characterization of evolutionary radiations benefits from describing the temporal patterns of trait disparification. Comparative methods attempt this by evaluating the statistical fit of trait distributions to a phylogenetic hypothesis under assumed evolutionary models. However, it can be challenging to differentiate between models, with discriminatory power depending on the modes of evolution underlying trait distributions. We suggest rates of 'trait space saturation', standardized for limits to evolutionary change, as an additional tool to distinguish between modes of trait evolution. We evaluate this approach using simulations and show that trait space saturation can identify the true model of trait evolution in cases where traditional comparative methods can fail. We illustrate our approach using diverse empirical studies that represent contrasting scenarios of evolutionary radiation.