A chain is no stronger than its weakest link: Double decay analysis of phylogenetic hypotheses

In decay analyses the support for a particular split in most-parsimonious t rees is its decay index, that is, the extra steps required of the shortest trees that do not include the split. By focusing solely on the support for splits, traditional decay analysis may provide an incomplete and potentiall y misleading summary of the support for phylogenetic relationships common t o the mast-parsimonious tree or trees. Here, we introduce double decay anal ysis, a new approach to assessing support for phylogenetic relationships. D ouble decay analysis is the determination of the decay indices of all n-tax on statements/partitions common to the most-parsimonious tree. The results of double decay analyses are presented in a partition table, but various ap proaches to graphical representation of the results, including the use of r educed consensus support trees, are also discussed. Double decay analysis p rovides a more comprehensive summary and facilitates a better understanding of the strengths and weaknesses of complex phylogenetic hypotheses than do es traditional decay analysis. The limitations of traditional decay analyse s and the utility of double decay analyses are illustrated with both contri ved data and real data for sauropod dinosaurs.