TAXONOMY

in a broad sense, the science of classification, but more strictly the classification of living and extinct organismsi.e., biological classification. The term is derived from the Greek taxis (arrangement) and nomos (law). Taxonomy is, therefore, the methodology and principles of systematic botany and zoology and sets up arrangements of the kinds of plants and animals in hierarchies of superior and subordinate groups. Popularly, classifications of living organisms arise according to need and are often superficial. Anglo-Saxon terms such as worm and fish have been used to refer, respectively, to any creeping thingsnake, earthworm, intestinal parasite, or dragonand to any swimming or aquatic thing. Although the term fish is common to the names shellfish, crayfish, and starfish, there are more anatomical differences between a shellfish and a starfish than there are between a bony fish and a man. Vernacular names vary widely. The American robin (Turdus migratorius), for example, is not the English robin (Erithacus rubecula), and the mountain ash (Sorbus) has only a superficial resemblance to a true ash. Biologists, however, have attempted to view all living organisms with equal thoroughness and thus have devised a formal classification. A formal classification provides the basis for a relatively uniform and internationally understood nomenclature, thereby simplifying cross-referencing and retrieval of information. The usage of the terms taxonomy and systematics with regard to biological classification varies greatly. American evolutionist Ernst Mayr has stated that taxonomy is the theory and practice of classifying organisms and systematics is the science of the diversity of organisms; the latter in such a sense, therefore, has considerable interrelations with evolution, ecology, genetics, behaviour, and comparative physiology that taxonomy need not have. in a broad sense, the science of classification, but more strictly the classification of living and extinct organisms, i.e., biological classification. The term is derived from the Greek taxis (arrangement) and nomos (law). The taxonomist creates from a varied array of organisms a hierarchy of groupings, or taxa (singular: taxon), that have an orderly relationship to each other. The basic unit in taxonomy is the species. Most modern taxonomists attempt to group organisms so as to reflect evolutionary (phyletic) relationships. The basic method of taxonomy is to compare the characteristics of the structure of living and extinct organisms by the methods of comparative anatomy and to interpret their differences and similarities in the light of comparative genetics, biochemistry, physiology, embryology, behaviour, ecology, and geography. Authorities once divided organisms into two simple categories, or taxathe kingdoms Plantae and Animalia. A third kingdom, Protista, was later proposed for those organisms that are not clearly plant or animal, such as the protozoa, bacteria, and algae. Improvements in biochemical and genetic research and advances in electron microscopy enabled further classification. Microorganisms without a distinct nucleus, called prokaryotes, are placed in the kingdom Monera; bacteria are the principal organisms in this kingdom. The classification of eukaryotes (microorganisms having a distinct nucleus) remains controversial. At present most authorities classify all living and extinct organisms into either a five-kingdom system or a four-kingdom system. The five-kingdom system recognizes Animalia, Plantae, Monera, Fungi, and Protista (basically all eukaryotic microorganismsi.e., algae, protozoans, and slime molds), and the four-kingdom system recognizes only the kingdoms Animalia, Plantae, Monera, and Virus; i.e., while recognizing the separation of the eukaryotes and prokaryotes, the four-kingdom system places the protozoans with the animals and the fungi and algae with the plants. The main subordinate taxa are successively the phylum (or division for plants), class, order, family, genus, and species. The basic units of the classificatory system are readily recognizable kinds, or species. The first step in the process of classification is the systematic grouping together of organisms with related characteristics; for example, single-hoofed animals related to the horse (horse, ass, and zebra) are placed in the horse genus, Equus, and similarly the grouping of acorn-bearing trees into the oak genus, Quercus. When the genus name is combined with that of the species, relationships are indicated, and by this means additional species may be included as they are discovered. This scheme of generic and specific names in Latin was established by Carolus Linnaeus in the mid-18th century. Additional reading Willi Hennig, Phylogenetic Systematics (1966, reissued 1979), is a statement of phylogenetic principles. Mark Ridley, Evolution and Classification: The Reformation of Cladism (1986), offers a very clear and readable account of the different philosophical approaches used in taxonomy and systematics. General texts include George Gaylord Simpson, Principles of Animal Taxonomy (1961, reissued 1990); Lynn Margulis and Karlene V. Schwartz, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 2nd ed. (1988), a survey of the lives and relationships of many groups of organisms; Godfrey M. Hewitt, Andrew W.B. Johnston, and J. Peter W. Young (eds.), Molecular Techniques in Taxonomy (1991), covering various topics and methods in molecular systematics; V.V. Sivarajan, Introduction to the Principles of Plant Taxonomy, ed. by N.K.P. Robson, 2nd ed. (1991), an overview of the established concepts of plant systematics for beginning students; Peter L. Forey et al., Cladistics: A Practical Course in Systematics (1992), an introduction to the theory and practice of cladistics; and Ernst Mayr and Peter D. Ashlock, Principles of Systematic Zoology, 2nd ed. (1991). Also of interest are S.P. Lapage et al. (eds.), International Code of Nomenclature of Bacteria, 1990 revision (1992); W. Greuter et al. (eds.), International Code of Botanical Nomenclature (Tokyo Code) (1994); and W.D.L. Ride et al. (eds.), International Code of Zoological Nomenclature, 3rd ed. (1985). A.J. Cain The Editors of the Encyclopdia Britannica