Sometimes one might be interested in calculating the rate of evolution of a particular character like body size in a certain clade, say, mammals. You have a phylogenetic tree with branch lengths that are proportional to time, and data on the phenotypes of species on the tips of that tree. It is usually a good idea to log-transform your data if they involve a measurement from a living thing (see Box 4.1, below). If we assume that the character has been evolving under a Brownian motion model, we have two parameters to estimate: $$\bar{z}(0)$$, the starting value for the Brownian motion model – equivalent to the ancestral state of the character at the root of the tree – and σ2, the diffusion rate of the character. It is this latter parameter that is commonly considered as the rate of evolution for comparative approaches1.