A simple parsimony-based approach to assess ancestor-descendant relationships

Abstract

One of the main goals of systematics is to reconstruct the tree of life. Half a century ago, the breakthrough of cladistics was a major step towards this objective because it allowed us to assess relatedness patterns among species, an abstract kind of relationship. Unfortunately, the philosophy of cladism forbade to go further and to seek more realistic relationships, like the ancestor-descendant relationship, which is the expected fundamental kind of relationship of the tree of life according to Darwinian evolution. Here, I describe a simple parsimony-based procedure which can be used to transform a classical cladogram into a genuine phylogenetic tree, i.e. a caulogram. It consists in deleting as many unobserved and unnamed nodes as possible and replacing them with observed and named species. A new Bayesian non-stochastic weighting scheme is used to assess character reliability for both this procedure and classical cladogram construction. I illustrate the whole process by assessing the relationships between the species of the moss genus Didymodon sensu lato (Pottiaceae) and discuss the resulting caulogram by confronting it with the previous methodology from the evolutionary literature. I finally argue that strictly adhering to cladist epistemology is untenable and that we must seek new formal methods to find ancestral species as well as ancestral higher taxa.