Talk by René Zaragüeta: “Why, Willi, why? Phylogenetics and the dichotomy of trees”

Date and time: Monday, April 13th, 11.30 am.

Location: Carlos Santamaría Building, Room B14
Speaker: René Zaragüeta i Bagils (Sorbonne, Paris)
Title: “Why, Willi, why? Phylogenetics and the dichotomy of trees”

Abstract: A cladogram is usually considered as resolved when all its branching points are bifurcations. The question I ask is: Why? Why all phylogenetic methods search dichotomous trees? Is evolution, or speciation, dichotomous? I suggest that Hennig’s principle of dichotomy is theoretically grounded. Cladograms—taxa and their relationships—are the result of a Cartesian analysis, which consists of the decomposition of taxa into homologies, i.e. hypotheses of degree of identity. Now, degree of identity is best represented by a ternary relationship, where two features are more identical to each other than any is to a third one. The foundation of taxa and their relationships upon homologies thus results in an intrinsically dichotomous pattern.

I speculate that the theoretical principle of dichotomy was present in Hennig’s theory. However, somehow, Hennig “forgot” his own arguments and the justification for this part of his theory.

Finally, I draw consequences of the theoretical foundation of dichotomy: if the evolutionary process needs not to be dichotomous, in which way are phylogenetic trees phylogenetic?