A Measure of the Information Content of Phylogenetic Trees, and Its Use as an Optimality Criterion

The D measure is presented as a quantification of historical constraints in phylo-genetic data. It is derived from information-theoretic considerations, and offers increased resolution of the criteria used in parsimony analyses. Thus, it can be used as an optimality measure in phylogenetic studies. The D measure is more sensitive to the evolutionary nature (symple-siomorphy, synapomorphy, autapomorphy) of characters on trees than two commonly used phylogenetic optimality measures: the consistency index; and the F-ratio. The consistency index does not distinguish between shared and unique characters on a tree. The F-ratio is affected by factors relevant to undirected, rather than directed, trees. Neither the D measure nor the F-ratio are biased a priori towards choosing postulates of reversal or parallelism in equal-length trees. Neither the D measure nor the F-ratio always indicate the shortest tree when used alone. The D measure can be used to choose among equal-length shortest trees that have been produced by standard parsimony techniques.