Investigating evolutionary lag using the species-pairs evolutionary lag test (SPELT)

For traits showing correlated evolution, one trait may evolve more slowly than the other, producing evolutionary lag. The species-pairs evolutionary lag test (SPELT) uses an independent contrasts based approach to detect evolutionary lag on a phylogeny. We investigated the statistical performance of SPELT in relation to degree of lag, sample size (species pairs), and strength of association between traits. We simulated trait evolution under two models: one in which trait X changes during speciation and the lagging trait Y catches up as a function of time since speciation; and another in which trait X evolves in a random walk and the lagging trait Y is a function of X at a previous time period. Type I error rates under “no lag” were close to the expected level of 5%, indicating that the method is not prone to false-positives. Simulation results suggest that reasonable statistical power (80%) is reached with around 140 species pairs, although the degree of lag and trait associations had a...