We assessed correlations between speciation rate and bill morphology 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 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.
LAY SUMMARY
Bill morphology is widely considered to play a role in avian speciation and diversification as a result of ecological specialization
Previous studies on the relationship between bill morphology and speciation have mostly focused on closely related species often in island settings
We quantify correlations between speciation rate and bill morphology in a diverse and broadly distributed songbird radiation, the tanagers
We find that bill size, shape, and their evolutionary rates are not correlated with speciation rate
We did find support for mixed punctuated and gradual evolution of bill morphology
Our results suggest that bill size and shape diversify in association with the speciation process, but not through any effect on the rate of speciation in the tanagers
Evaluamos las correlaciones entre la tasa de especiación y la morfología del pico (tamaño, forma y tasa evolutiva) en una radiación diversa y ampliamente distribuida de aves canoras, los tangaras (Thraupidae). Nuestros análisis mostraron que el tamaño, la forma y las tasas evolutivas del pico no están correlacionadas con la tasa de especiación. Sin embargo, encontramos evidencia de que cada eje de variación en la morfología del pico se diversifica en eventos de especiación intercalados con períodos de evolución gradual, en consonancia con un modelo de equilibrio puntuado de cambio de caracteres. Para determinar las correlaciones, incorporamos una filogenia molecular calibrada en el tiempo y escaneos de superficie de alta resolución en 3D de la estructura del pico a partir de pieles de estudio de museos. En general, nuestros hallazgos sugieren que el tamaño y la forma del pico se diversifican en asociación con el proceso de especiación, pero no a través de ningún efecto en la tasa de especiación. Estudios previos han demostrado que otros rasgos, como el canto, el nicho ecológico y el plumaje, sí influyen en la especiación y, en última instancia, en la riqueza de especies en los tangaras.
