PHYLOGENETIC MODELS AND CLASSIFICATION

Cracraft, J. (Dept. Anatomy, University of Illinois at the Medical Center, Chicago, Ill. 60680). 1974. Phylogenetic models and classification. Syst. Zool., 23:71-90.-Several statements are suggested as necessary for the formulation of any theory of phylogeny reconstruction; these include statements about kinship relationships, the origination and diversification of taxonomic units, and clustering. From these basic statements an attempt is made to construct phylogenetic models corresponding to the theory and methods of the two principle schools of systematic thought. The models are compared, and it is concluded that the phylogenetic systematic model is a more powerful tool in reconstructing phylogeny than is the evolutionary systematic model. The ways in which the phylogenetic models influence the form and content of classifications are discussed. Only information about phylogenetic (cladistic) relationships can be stored in and retrieved from Linnaean hierarchical classifications, and thus classifications should be based on a phylogenetic systematic model because such a classification will reproduce precisely the information (phylogeny) used to construct it. [Phylogeny; Classification; Phylogenetic models.] The study of phylogeny concerns the evolutionary history of lineages of organisms, and for many biologists an important question has been and remains: How do we reconstruct that history? Many different opinions have been expressed with regard to phylogeny reconstruction, and the theory and methodology are complex and confusing. The basic premises that seem to be common to all theories of phylogeny have not been enumerated, and much that has been written appears to circumvent discussions designed to clarify these basic issues. The purpose of this paper is to discuss what are perhaps the basic conditions or statements necessary for any theory of phylogeny reconstruction. It will be suggested that these statements can be used in constructing several different "models" of phylogeny reconstruction, each of which roughly corresponds to a different "school" of systematic theory in existence today. It is probably fair to look upon these different research strategies as "models," because each is a synthetic method that seeks to order natural phenomena, i.e., observational data. It is hoped that by conceptualizing these alternative viewpoints, future discussions can be directed to substantiative issues, namely, the defining statements of each model. When passing judgment on alternative systematic theories, what some evolutionary biologists often do not fully appreciate is that preference for a particular model should be made on the basis of how its logical structure explains or orders observational data in a consistent manner. Thus, it will be another purpose of this paper to suggest that one way in which we might exercise a choice for a specific phylogenetic model would be to consider the following question: How do different models of phylogeny reconstruction influence the form and content of classifications? Because the form and content seem to be conceptually related to specific notions of phylogeny (see below), it follows that differences in the logical structure and the ability of these different classification theories to order data might themselves provide insight into the worthiness of the various phylogenetic models. PHYLOGENETIC MODELS: BASIC COMPONENTS Any attempt to reconstruct phylogeny necessarily mirrors an investigator's prior conception as to how the phylogenetic process actually took place. It would seem, however, that all models of phylogeny will be constrained to include certain basic components. These might be: A. A statement about the nature of kinship relationships to be accepted among taxonomic units.