Origin and Philosophy of the Groundplan-divergence Method of Cladistics

The Groundplan-divergence Method of constructing phylogenetic trees was created during the 1950's for illustrating systematic principles, but it was taken up widely by researchers carrying out monographic research. Based upon a study of certain Hawaiian ferns, the method attempts to deduce pathways of genetic change on the basis of phenetic evidence. It tries to estimate the amounts (grades), directions (clades), and sequences (steps) of phylogenetic divergence, using the con- cept of generalized or groundplan characteristics as the basis of judging primitive- ness. Groundplan-divergence analysis is concerned with pathways of actual biological changes rather than with their chronology (when the changes took place) or their geography (where they took place). Obvious parallelisms, reticulations, and co-ex- istences of ancestors and derivatives are accepted and embodied in the cladistics. The method involves the interplay of phenetic classification, the detection of taxa of hybrid origin, analysis of character trends, synthesis of generalized character states, estimation of divergence levels, grouping according to divergence formulas, and the connecting of lines with hybrid reticulations. The method is not able to overcome problems that are caused by such factors as major gaps in the phylogenetic record, important missing characters, excessive hybridization, and evolutionary pat- terns that involve massive randomness or overwhelming amounts of parallelism. Examples of various uses of this method are cited. In view of recent resurgence in cladistic theory and methodology, it seems appropriate to review the history and philosophy of the Ground- plan-divergence Method, one of the pioneering attempts to place the construction of phylogenetic trees on an objective basis. The method remains as useful today as it was when it was introduced over a quarter of a century ago for purposes of teaching principles of systematic botany. As will be seen, the approach is a conservative one, utilizing time-hon- ored ideas of phylogenetic induction and bringing them together in a particular way. The method is based upon traditional concepts of the relationships of classification to phylogeny. Although both are involved, it makes no claims for either perfect parsimony or complete compatibil- ity. It allows for a certain amount of parallelism as well as reticulation. The last, the origin of taxa by hybridization, has not previously been discussed in connection with Groundplan-divergence, nor for that matter has the entire method itself been reviewed in any detail, although various authors to be cited below have discussed various aspects of it. Until the 1950's, phylogenetic trees were based on subjective analysis and drawn without definite reference to evolutionary theories. There was no effort to quantify relationships and each author drew his tree intuitively. Little or no explanation went along with it. There was no 173