Figure 2.

Comparing measures of phylogenetic diversity (PD). Faith's phylogenetic diversity (PD_Faith) is the sum of the branch lengths on the minimum spanning tree linking a set of terminal taxa to the root. This inclusion of an unnamed root taxon results in an implicit complementarity aspect to PD_Faith. Not all definitions of PD include the root, which avoids this complementarity issue, but has created a source of confusion in the literature. Values indicate branch lengths – note here that the tips are all not the same distance from the root; species are identified with letters shown at tips. PD_Faith for sets (A,B) and (C,D) would be 7. A PD measure that sums just the branch lengths on the minimum spanning tree would lead to a value of 1 + 1 = 2 for the (A,B) subset and 2 + 2 = 4 for the (C,D) subset.