Inferring evolutionary process from phylogenetic tree shape

Inferences about macroevolutionary process have traditionally depended solely on the fossil record, but such inferences can be strengthened by also considering the shapes of the phylogenetic trees that link extant taxa. The realization that phylogenies reflect macroevolutionary processes has fed to a growing literature of theoretical and comparative studies of tree shape. Two aspects of tree shape are particularly important: tree balance and the distribution of branch lengths. We examine and evaluate recent developments in and connections between these two aspects, and suggest directions for future research. Studies of tree shape promise useful and powerful tests of macroevolutionary hypotheses. With appropriate further research, tree shape may help us detect mass extinctions and adaptive radiations, measure continuous variation in speciation and extinction rates, and associate changes in these rates with ecological or biogeographical causes. The usefulness of tree shape extends well beyond the study of macroevolution. We discuss applications to other areas of biology, including coevolution, phylogenetic inference, population biology, and developmental biology.