Punctuated evolution of bill morphology in the largest family of songbirds (Thraupidae)

We assessed correlations between speciation rate and bill morphology
(size, shape, and evolutionary rate) in an ecologically diverse and
continental-scale songbird radiation, the tanagers (Thraupidae). Our
analyses showed that bill size, shape, and their evolutionary rates are
not correlated with speciation rate. However, we did find evidence that
each axis of variation in bill morphology diversifies at speciation events
interspersed with periods of gradual evolution, consistent with a
punctuated equilibrium model of character change. To determine
correlations, we incorporated a time-calibrated molecular phylogeny and
high-resolution three-dimensional (3D) surface scans of bill structure
from museum study skins. Overall, our findings suggest that bill size and
shape diversify in association with the speciation process, but not
through any effect on the rate of speciation. Previous studies have shown
other traits, such as song, ecological niche, and plumage do influence
speciation and, ultimately, species richness in tanagers.