THE USE OF HIERARCHIES AS ORGANIZATIONAL MODELS IN SYSTEMATICS

A hierarchy is an abstract organizational model of inter-level relatio nships among entities. When isomorphic with nature, hierarchies are us eful for organizing and manipulating our knowledge. Hierarchies have b een used in biological systematics to represent several distinct, but interrelated, facets of the evolution of life with different organizat ional properties, and these distinctions have been confused by the rub ric the hierarchy of life'. Evolution, as descent with modification, i s inherently dualistic. The organizational structure of a hierarchy ca n be used to represent dualistic properties as inter-level relationshi ps. Cladistics is monistic: with a singular focus on patterns of desce nt. Descent has conceptual priority over modification, but the organiz ational relationship is not exclusive. 'Cladistic classification' is a n oxymoron because cladistics lacks the class concepts needed to const ruct a classification, a point recognized by those who suggest abandon ing Linnaean classification in favour of a newly devised monophyletic systematization. Cladistic analysis of descent can be supplemented wit h an analysis of modification that provides the class concepts needed to construct an evolutionary/phylogenetic classification. When a stron g monophyletic pattern of modification is detected (in addition to its monophyletic pattern of descent), the criterion of subsequent modific ation provides the basis for formally recognizing a certain monophylet ic group at a given rank, as opposed to a group that is one node more inclusive or one node less. The criterion of subsequent modification a lso permits detection of strong paraphyletic patterns of modification, when they exist. By setting standards of evidence needed to recognize paraphyletic groups, one concomitantly strengthens the basis for form ally recognizing selective monophyletic groups. (C) 1998 The Linnean S ociety of London.