Escaping the matrix: a new algorithm for phylogenetic comparative studies of co-evolution.

An algorithm for generating host cladograms from parasite-host cladograms derived from parasite phylogenies, Phylogenetic Analysis for Comparing Trees (PACT), is described. PACT satisfies Assumption 0, that all the information in each parasite-host cladogram must be used in a co-evolutionary analysis, and that the host relationships depicted in the final host cladogram must be logically consistent with the phylogenetic relationships depicted in every part of every parasite-host cladogram used to construct the host cladogram. It accounts for cases of speciation by host switching and expansion of host range, and reticulated host relationships, in addition to co-speciation, sympatric speciation, and extinction in all input parasite-host cladograms, and does so without a priori weighting schemes and without a posteriori manipulation of the data.