BIOSYSTEMATISTS Meeting, March 9th, 2004, at UCB

Matt Haber, Philosphy, UC Davis

Title: On Probability and Systematics

ABSTRACT: Within the field of phylogenetic systematics, an ongoing debate has revolved around the appropriate choice of methodology for cladistic analysis, i.e., the construction of phylogenetic trees and inference of ancestral states. A recent paper (Siddall, M. E., and A. G. Kluge. 1997. "Probabilism and phylogenetic inference." Cladistics 13:313-336.) highlights this controversy in theoretical systematics. Siddall & Kluge's paper has generated a lot of reaction, and characterizes the kinds of arguments which have strongly influenced biologists' choice of methodology. Siddall & Kluge advocate a privileged status for a phylogenetic technique called parsimony, to the exclusion of other, statistically based, phylogenetic methods. The present paper will address some of the debates of their argument. Rather than try to address every point made by Siddall & Kluge, I instead use their paper to draw out two general lines of argument that are representative of a particular school of thought in phylogenetics. (This school of thought is typically referred to as the Cladistic school.) The two lines of argument that I identify are what I term (i) the argument from falsificationism, and (ii) the argument from probability. The first of these has been addressed elsewhere both by philosophers and biologists, and will merely be commented upon below. The argument from probability, though, is the primary focus of this paper. I show that the argument from probability, as invoked by Siddall & Kluge, is ambiguous, e.g., between metaphysical and epistemic possibility. Upon disambiguation, the argument from probability is either invalid (or unsound), or simply misses the intended target. In working through this disambiguation, I precisely identify and clarify Siddall & Kluge's concerns, and show that statistical phylogenetic techniques ought not be considered problematic.