The fair proportion metric calculates the evolutionary distinctness of tips
in a tree through summing the total amount of branch length each tip represents, where
each branch in the tree is evenly divided between all descendants. Parallelizable.
References
Isaac, N.J.B., Turvey, S.T., Collen, B., Waterman, C. and Baillie, J.E.M. (2007).
Mammals on the EDGE: conservation priorities based on threat and phylogeny. PLoS ONE, 2, e296.