Classification

I

Introduction

Classification, in biology, the identification, naming, and grouping of organisms into a formal system. The vast numbers of living forms must be named and arranged in an orderly manner so that biologists all over the world can be sure they know the exact organism that is being examined and discussed. Groups of organisms must be defined by the selection of important characteristics, or shared traits, that make the members of each group similar to one another and unlike members of other groups. Modern classification schemes also attempt to place groups into categories that will reflect an understanding of the evolutionary processes underlying the similarities and differences among organisms. Such categories form a kind of pyramid, or hierarchy, in which the different levels should represent the different degrees of evolutionary relationship. The hierarchy extends upwards from several million species, each made up of individual organisms that are closely related, to a few kingdoms, each containing large assemblages of organisms, many of which are only distantly related.

To construct classification schemes that correspond as closely as possible to the natural world, biologists examine and compare the anatomy, functions, genetic systems, behaviour, ecology, and fossil histories of as many organisms as possible. More than 1.5 million different groups have been identified and at least partly described, and many more remain to be studied. All branches of biology contribute to such studies; the specialities that are immediately concerned with the problems of classification are taxonomy and systematics. Although the two disciplines overlap considerably, taxonomy is more involved with nomenclature (naming) and with constructing hierarchical systems, and systematics with uncovering evolutionary relationships.

II

Hierarchical Levels

Biologists classify individual organisms at the basic level of the species, which is the only category that can be regarded as occurring in nature. The higher categories are abstract groupings of species. A species is composed of organisms that resemble one another in many important characteristics. Moreover, in organisms that have sexual reproduction, a species is made up of interbreeding populations that, ideally, cannot produce fertile offspring with members of any other species.

Species that do not interbreed with each other but are clearly related by important shared traits are grouped into a genus (plural, genera), and the separate species are given a two-word name (binomial nomenclature). The first word is the genus name and the second word is an adjective, usually descriptive or geographic. This means of naming was established in 1758 by the Swedish naturalist Carolus Linnaeus, the founder of modern taxonomy. He used Latin names because the scholars of his day communicated in that language. Linnaeus gave humans the genus name Homo (man) and the species name Homo sapiens (wise man). To construct the hierarchy of classification, one or more genera are grouped into a family, families are grouped into orders, orders into classes, classes into phyla, and phyla into kingdoms. The groups of organisms within these seven major categories, at every level of the hierarchy, are termed taxa, and each taxon has a definition that encompasses the important traits shared by all its constituent taxa.

To allow further subdivision, the prefixes sub- and super- may be added to any category. In addition, special intermediate categories—such as branch (between kingdom and phylum), cohort (between class and order), and tribe (between family and genus)—may be used in complex classifications.

On any level, a taxon should indicate a common evolutionary background, that is, all its members should have evolved from a common ancestor. The taxon is then said to be monophyletic. Where an established taxon includes two or more members that have converged so that they have traits in common but have evolved from different ancestral lines, the taxon is said to be polyphyletic. An attempt is then usually made to divide and redefine the taxon so that monophyletic taxa result.

III

Five-Kingdom System

Two kingdoms of living forms, Plantae and Animalia, have been recognized since Aristotle, in the 4th century bc, established the first taxonomy. In their way of life and evolutionary path, rooted plants are so distinct from mobile, food-ingesting animals that the concept of the two kingdoms remained intact until recently. Only in the 19th century, long after it was revealed that one-celled organisms could not fit comfortably into either of the two categories, was it proposed that unicellular forms be placed in a third kingdom, the Protoctista. Furthermore, long after photosynthesis was discovered to be the basic nutritional mode of plants, the fungi, which feed by absorption, continued to be classified as plants because of their apparently rooted manner of growth.

Recently, as techniques for examining the cell have improved dramatically, it has become clear that the major division in the living world is not that between plants and animals but between organisms whose cells have no enclosed nucleus and organisms whose cells have nuclei bound by membranes. The former are called prokaryotes (“before kernels”) and the latter eukaryotes (“true kernels”). Prokaryotic cells are also lacking in organelles—mitochondria, chloroplasts, advanced flagella, and other special cell structures—at least some of which occur in all eukaryotic cells. The bacteria and blue-green algae are prokaryotic cells, and they have been recognized in modern taxonomies as a fourth kingdom, Prokaryota, formerly known as the kingdom Monera.

Eukaryotic cells arose much later and may have evolved as symbiotic associations of prokaryotes. The kingdom Protoctista is composed of diverse one-celled organisms, either free-living or colony-forming. Each of the multicellular kingdoms is believed to have arisen more than once from protoctist ancestors. The kingdom Animalia comprises organisms that are multicellular, have their cells organized into different tissues, are mobile or partly mobile by means of contractile tissues, and digest food internally. The kingdom Plantae is made up of multicellular organisms that usually have walled cells and that contain chloroplasts in which they produce their own food by photosynthesis. However, in 1999 preliminary results of a major cladistics study indicated that the plant kingdom should actually be reclassified as three kingdoms for green, red, and brown plants, as the study proposed that these three groups evolved from three different varieties of one-celled, plant-like organisms. The fifth kingdom, Fungi, comprises multicellular or multinucleate organisms that digest food externally and absorb it through the surfaces of protoplasmic tubes called hyphae (of which their bodies are composed).

This five-kingdom classification of the living world (see chart) is thus based on three levels of organization: the primitive prokaryotic; the relatively simple and primarily unicellular eukaryotic; and the complex, multicellular eukaryotic. Within the last of these levels, the three major directions of evolution are each based on a different kind of nutrition and are expressed in the different kinds of tissue organization that are characteristic of animals, plants, and fungi.

See also Botany; Zoology.