On Systematists’ Single Objective Tree of Ancestors and Descendants

Abstract

It is often said that there is just one “objective” tree of life: a single accurate branching hierarchy of species reflecting order of descent. For any two species there is a single correct answer as to whether one is a “daughter” of the other, whether the two are “sister species” by virtue of their descent from a common parental species, whether they belong to a family line that excludes any given third species, and so on. This position is not right. We may whittle a tree of life, paring troublesome branches, in order to portray an ordering that admits of no legitimate dissent. But the history of life can sustain many legitimate arrangements of the same branches of species. The same can be said about other taxonomically relevant groups besides species, such as “Least Inclusive Taxonomic Units” (LITUs), so the basic claim survives even if we abandon traditional species. Similarly, the claim survives even if we distinguish between synchronic and diachronic groups, even if we consider polytomies, even if we distinguish between models and the world modeled, and even if we recognize an objective world. Nor is the claim merely an epistemic one.