ABSTRACT
Introduction
Systematics is commonly, but not universally, understood in terms of phylogeny reconstruction, with results presented as phylogenetic trees. Cladistics is a powerful approach to systematics. It seeks to solve the problem of the structure of the natural world, through the discovery of characters and taxa, with results presented as cladograms, through which 'the data of biology integrate at the level of classification' (Nelson and Platnick, 1984: 154). Cladograms are not phylogenetic trees, but are related to them in a complex fashion (Nelson and Platnick, 1981: 14, 135–151, 171–172). A cladogram may summarize a suite of several, or many, phylogenetic trees and thus may be considered to be more general than a phylogenetic tree. Cladistics may then be considered to be more general than phylogeny reconstruction. Cladistics seeks to solve the problem of the structure of the natural world by addressing it in the following way: 'What are the interrelationships among a suite of taxa?' The solution to this question is conceptually simple. For any cladistic problem the number of terminal taxa is specified at the outset and there is a finite number of possible solutions when the solutions are presented as hierarchical branching diagrams (Felsenstein, 1978). For instance, for four taxa the total number of possible solutions is 26, with 15 fully resolved solutions (all inter-relationships solved), 10 partially resolved solutions (some inter-relationships solved), and one completely unresolved solution (no inter-relationships solved). Resolved solutions suggest that among the taxa being considered, some are more closely related to each other than they are to the remainder; taxa at higher levels are thus discovered. Choosing among solutions may be done on the basis of how well available data fit each one, with data originating in observations of specimens (in the case of morphology, cell structure, etc.) or of gels (in the case of molecular sequences).
