Analyzing disparity and rates of morphological evolution with model-based phylogenetic comparative methods

Understanding variation in rates of evolution and morphological disparity
is a goal of macroevolutionary research. In a phylogenetic comparative
methods framework, we present three explicit models for linking the rate
of evolution of a trait to the state of another evolving trait. This
allows testing hypotheses about causal influences on rates of phenotypic
evolution with phylogenetic comparative data. We develop a statistical
framework for fitting the models with generalized least-squares
regression, and use this to discuss issues and limitations in the study of
rates of evolution more generally. We show that the power to detect
effects on rates of evolution is low in that even strong causal effects
are unlikely to explain more than a few percent of observed variance in
disparity. We illustrate the models and issues by testing if rates of
beak-shape evolution in birds are influenced by brain size, as may be
predicted from a Baldwin effect in which presumptively more behaviorally
flexible large-brained species generate more novel selection on themselves
leading to higher rates of evolution. From an analysis of morphometric
data for 645 species we find evidence that both macro- and microevolution
of the beak are faster in birds with larger brains, but with the caveat
that there are no consistent effects of relative brain size.