Kingdom

Kingdom is the highest category in the hierarchical classification of organisms created by Carolus Linnaeus around 1750. Linnaeus recognized two kingdoms, plants and animals, a scheme that worked reasonably well for large multicellular organisms but failed as microscopes revealed diverse unicellular organisms. In 1959 Robert Whittaker devised a five-kingdom system that maintained kingdoms Plantae and Animalia but added kingdoms Monera, Protista, and Fungi (see Table).

Whittaker placed bacteria in their own kingdom, Monera, because of fundamental organizational differences between prokaryotic bacterial cells, which lack membrane-enclosed nuclei and organelles , and the eukaryotic cells of other organisms that possess internal membranes. Plantae, Fungi, and Animalia consist of complex, multicellular eukaryotic organisms that differ from each other in details of cell structure and in how they secure and process energy. Protista is a collection of single-celled eukaryotic organisms and simple multicellular forms, some animal-like, some plantlike.

Molecular evidence, particularly from ribosomal ribonucleic acid (RNA), suggests that the five-kingdom scheme is also too simple. Some biologists believe that Protista should be partitioned into three or more kingdoms. Similarly, kingdom Monera contains two very biochemically distinct groups of prokaryotes: archaebacteria, and eubacteria. A proposed system acknowledges this ancient evolutionary split by creating a higher level of classification, domain, above kingdom. This system distinguishes three domains: Archaea, Eubacteria, and Eukarya (containing protists, plants, fungi, and animals).

see also Animalia; Archaea; Eubacteria; Fungi; Linnaeus, Carolus; Plant; Protisa

Cynthia A. Paszkowski

Bibliography

Margulis, Lynn, and Karlene V. Schwartz. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth. New York: W. H. Freeman and Company, 1998.

king·dom / ˈkingdəm/ • n. 1. a country, state, or territory ruled by a king or queen: ∎  a realm associated with or regarded as being under the control of a particular person or thing: the kingdom of dreams. 2. the spiritual reign or authority of God. ∎  the rule of God or Christ in a future age. ∎  heaven as the abode of God and of the faithful after death. 3. each of the three traditional divisions (animal, vegetable, and mineral) in which natural objects have conventionally been classified. ∎  Biol. the highest category in taxonomic classification.

kingdom In traditional classification systems, the highest category into which organisms are classified. The original two kingdoms, Plantae (see plant) and Animalia (see animal), have been supplemented by others, and most modern classification systems recognize five kingdoms: Bacteria (or Prokaryotae; see bacteria), Protoctista (including protozoa and algae), Fungi (see fungi), Plantae, and Animalia. However, the discovery of the archaebacteria (see Archaea) has led taxonomists to suggest a superordinate category in the taxonomic hierarchy – the domain.

kingdom Topmost level (taxon) of the most widely adopted taxonomy for living organisms, the Five Kingdoms system. The Five Kingdoms are Animalia (animal), Plantae (plant), Fungi (fungus), Prokaryote, and Protoctist. Two subkingdoms are often recognized within Prokaryotae, Archaebacteria and Eubacteria, but the bacteria are so diverse that many taxonomists think they comprise more than one kingdom. Some believe that they merit the status of a new, even higher category, domains. See also eukaryote; plant classification

kingdom, in taxonomy: see classification.

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