A human construct for grouping organisms into hierarchical categories. The most inclusive categories of any classification scheme are called kingdoms, which are delimited so that organisms within a single kingdom are more related to each other than to organisms grouped in the other kingdoms. Classification (grouping) is a part of biological systematics or taxonomy, science that involves naming and sorting organisms into groups.
Historically, organisms have been arranged into kingdoms based on practical characteristics, such as motility, medicinal properties, and economic value for food or fiber. In the nineteenth and twentieth centuries, advances such as electron and light microscopy, biochemistry, genetics, ethology, and greater knowledge of the fossil record provided new evidence upon which to construct more sophisticated classification schemes. Proposals were made to assign organisms into four, five, and even thirteen kingdoms. Earlier two-kingdom and three-kingdom systems were devised without awareness of the profound distinction between prokaryotes and eukaryotes, which Edouard Chatton (1937) recognized as a fundamental evolutionary discontinuity. Prokaryotic cells do not have either a nucleus or any other internal membrane-bounded structures, the so-called organelles. By contrast, eukaryotic cells have both a nucleus and organelles. The deoxyribonucleic acid (DNA) of eukaryotes is combined with protein to form chromosomes. All bacteria are prokaryotes. Plants, fungi, animals, and protoctists are eukaryotes. See Eukaryotae, Prokaryotae
By the late 1990s, biologists widely accepted a system that classifies all organisms into five kingdoms based on key characteristics of function and structure. These are Superkingdom Prokarya [Kingdom Bacteria (Monera) with two subkingdoms, Archaea and Eubacteria] and Superkingdom Eukarya with four kingdoms, Protoctista (or Protozoa in some classification schemes), Fungi, Animalia, and Plantae. Key charactistics used to classify organisms into these categories include the mode by which the organism obtains its nutrients; presence or absence of an embryo; and whether and how the organism achieves motility.
Cladistics may be defined as an approach to grouping organisms that classifies them according to the time at which branch points occur along a phylogenetic tree. Such a phylogenetic tree is represented by a diagram called a cladogram. In this approach, classification is based on a sequence of phylogenetic branching. In a cladogram, the phylogenetic tree branches dichotomously and repeatedly, reflecting cladogenesis (production of biological diversity by evolution of new species from parental species).
Modern classification is based on evidence derived from developmental pattern, biochemistry, molecular biology, genetics, and detailed morphology of extant organisms and their fossils. Because information is drawn from such diverse sources, and because 10–30 million (possibly 100 million) species are probably alive today, informed judgments must be made to integrate the information into classification hierarchies. Only about 1.7 million species have been formally classified in the taxonomic literature of biology to date. Thus, as new evidence about the evolutionary relationships of organisms is weighed, it must be anticipated that biological classification schemes will continue to be revised. See Animal systematics, Bacterial taxonomy, Plant taxonomy, Virus classification