Cladistic Methodology: A Discussion of the Theoretical Basis for the Induction of Evolutionary History

Statement of the problem.-How can we estimate from whatever might be known about the similarities and differences among the members of a collection of "units of evolution," the phylogeny or evolutionary history for that collection? This is the basic problem to which cladistic methods address themselves. It is a very old problem. Biologists have been thinking about it continuously since the time of the formal publication of the theory of evolution by natural selection (Darwin 18, Tyler 95, Bessey 3, Lam 62, Huxley 53, Hennig 50, Simpson 85, 86). It is also an extiemely difficult problem for it involves the induction, or guessing, of events which in many cases took place over millions of years, but for which little or no direct evidence remains other than the diversity of living forms which are seen today as a consequence of these millions of years of evolution. Although the intellectual appeal of wondering about the true historical relationships among various living forms is great, it remains today that relatively little is known with confidence about the phylogeny of most groups. As an upper bound, the evolutionary history of horses (Stirton 94, Simpson 84) is considered among the best known. In poolly preserved groups, such as the angiosperms, estimates of phylogenetic relationship (Cronquist 16, Davis & Heywood 19) in some cases border on speculation. Nonetheless, that the study of a group rarely permits incontrovertible statements about its evolutionary history is not sufficient to discourage motivation by the challenge of estimating history, especially as it serves to stimulate work in comparative morphology, biogeography, population genetics, comparative serology, paleontology, comparative molecular biology, and other related fields (Nelson 76, 77, Smith 87, Kimura 59, Goodman 41, Beck 2, Margoliash et al 70, Wagner 99). In particular, the problem of estimating evolutionary history is comprised of several concomitant parts, each, in and of itself, a difficult and largely unresolved problem. Some of these parts are as follows: 1. What are the units whose evolutionary history is to be estimated? To what extent do a priori considerations of the form of evolutionary history affect the definition and choice of units? To what extent does the definition and choice of units affect the determination of subsequent methodology?