The genealogical sorting index: software and web site for quantifying the exclusivity of lineages

Background: We introduce the software that calculates a new statistic that we call the genealogical sorting index, or gsi. This statistic quantifies the common ancestry of groups of individuals on a phylogenetic tree. The significance of an observed genealogical sorting index value is measured with permutation testing. Results: The algorithm for calculating the genealogical sorting index was written in R, a statistical programming environment. We intend to make the code available as an open source package for R. In addition, we have developed a web service (http://www.molecularevolution.org/) for users interested in quickly calculating the genealogical sorting index and performing permutation tests on phylogenetic trees. Conclusions: The genealogical sorting index appropriately quantifies the extent to which commonly labeled individuals are united in a tree. It is intuitive, simple, and easily calculated. The software that calculates the genealogical sorting index and measures its significance is made freely available.