“Phylogenetic ANCOVA: Estimating changes in evolutionary rates as well as relationships between traits”

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Jesualdo A. Fuentes-G., Elizabeth A. Housworth, Ashley Weber, and Emília P. Martins

PANCOVA estimates different rates of phenotypic evolution as well as shifts in directional trends and trait covariation

Crested oropendola (Psarocolius decumanus) building nest in the Amazon (Amazonas, Brazil).
(Credit: Jorge I. Atehortúa, http://jatehortua.wixsite.com/atehophoto)

Is social behavior associated with changes in bird encephalization? This question about bird sociality and relative brain size can be addressed in two basic ways. The first way deals with the direction of change. For example, evolutionary shifts in sociality could be associated with bigger relative brain sizes. The second way deals with phenotypic diversification. For example, evolutionary shifts in sociality could lead to a larger range of relative brain sizes. There are powerful phylogenetic comparative methods to analyze the question introduced above, but they tend to focus on only one of the two ways of addressing the question. In a paper appearing in The American Naturalist, a team of biologists and mathematicians from Indiana University and Brown University presents a new comparative method that allows both ways to be explored simultaneously.

A female southern lapwing (Vanellus chilensis) takes a walk with her chicks in the Aburrá Valley (Antioquia, Colombia).
(Credit: Jorge I. Atehortúa, http://jatehortua.wixsite.com/atehophoto)

Their new method corresponds to a phylogenetic version of the analysis of covariance (ANCOVA) that estimates unequal evolutionary rates under maximum likelihood. As an example, the authors revisit the aforementioned question about bird brain evolution, focusing on relative telencephalon mass and parental investment as informative of social cohesion. They confirm that the telencephalon of social birds tends to be larger and more variable in size. This is a result that, traditionally, would have required the application of several comparative methods. The new method could clarify not only bird brain sizes, but whether novel environments promote sexual size dimorphism in lizards, how circulating testosterone differs for primates under particular mating systems, and the effect of diet on carnivoran skull robustness, among other questions that associate phenotypic changes with major historical shifts such as key innovations, the colonization of new habitats, or substantial environmental change. Read the Article