Chapter 3: Systematics
John G. Lundberg and Lucinda A. Mcdade
The waters of Earth support an enormous diversity of fishes. Recent compilations include 17,600 to 21,000 living species of fishes (Nelson 1984), or about half of all vertebrate species, and newly discovered species are described and named each year. Based on the fossil record, the number of extinct fishes is vast and our knowledge of them is expanding. Further, even superficial examination indicates that recognizable groups (taxa) of fishes exist above the species level. It is the task of systematics to discover taxonomic patterns of diversity and to relate these patterns to underlying evolutionary processes. Systematic ichthyology seeks to provide the ordered reference system of fish life and, therefore, is fundamental to both applied and pure aspects of fish biology. All disciplines that require knowledge of fish identification, relationships, and spatial and temporal distributions rely directly upon systematics. Its major goals are listed in Box 3.1 and are described further in the sections that follow.
There are three schools of modern systematic theory and practice: phylogenetic systematics, evolutionary systematics, and phenetics. Phylogenetic and evolutionary systematists share the premise that we can objectively infer the evolutionary history of life and that classification should be based on this history. These two schools differ in that phylogenetic systematists insist that all taxa be genealogical entities whereas evolutionary systematists argue that other aspects (e.g., morphological or ecological distinctiveness) should be considered in delimiting taxa (see Section 3.6.2). Many pheneticists argue that it is not possible to uncover evolutionary history with sufficient accuracy to provide the basis for classification (see Section 3.5). They maintain that it is best to classify organisms based on objective measures of their overall phenotypic similarity. In this chapter, we emphasize phylogenetic systematics or cladistics. This approach to systematics has won wide acceptance among ichthyologists (Nelson 1972). We will point to differences of opinion among systematists, but there is not space to exhaustively compare the schools. Additional journals and books on the theory and practice of systematics follow.
General. Journals: Systematic Zoology, Systematic Botany, Evolution, Cladistics, Taxon; books: Blackwelder (1967), Crowson (1970), Ross (1974).
Phylogenetic Systematics. Hennig (1966), Eldredge and Cracraft (1980), Nelson and Platnick (1981), Wiley (1981).
Evolutionary Systematics. Simpson (1961), Mayr (1969).