 Feature diversity raises measurement challenges. Not only do we not know, in general, the future value of different features, but also we cannot even list the features for most species. Phylogenetic pattern provides one way to estimate and quantify variation at the feature level. The predicted total feature diversity of a set of species is referred to as its “phylogenetic diversity” (PD). The amount of PD, and the estimated relative feature diversity, of a set of species is calculated as the minimum total length of all the phylogenetic branches required to connect all the species in that set on the phylogenetic tree (Faith 1992). This definition follows from an evolutionary model in which branch lengths reflect evolutionary changes (new features), and shared ancestry accounts for shared features among species. Note that a set of three species may have lower PD (lower feature diversity) than a set of two species (see figure below).