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).
|A COMPARISON OF THE FIVE KINGDOMS|
|Internal cell membranes||Absent Present (Prokaryotes)||Present (Eukaryotes)||Present (Eukaryotes)||Present (Eukaryotes)||Present (Eukaryotes)|
|Cell wall||Present||Present or Absent||Present||Present||Absent|
|Organization||Unicellular||Unicellular or Multicellular||Multicellular||Mainly Multicellular||multicellular|
|Mode of nutrition||Autotrophs or Heterotrophs||Autotrophs or Heterotrophs||Autotrophs||Heterotrophs||Heterotrophs|
|Representative groups||Archaea, eubacteria||Protozoa, algae, slime molds||Mosses, ferns, seed plants||Molds, yeasts, mushrooms||Animals with and without backbones|
|Note: An autotroph is an organism that uses solar energy or energy from inorganic chemicals to make organic molecules. A heterotroph obtains organic molecules by consuming other organisms or their products.|
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
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.
The term kingdom is one of the seven major classification groups that biologists use to identify and categorize living things. These seven groups are hierarchical or range in order of size. The kingdom group is the first and largest group. The classification scheme for all living things is: kingdom, phylum, class, order, family, genus, and species.
As the broadest of all classification groups, kingdom is made up of phyla (singular, phylum). From the time of Greek philosopher and scientist Aristotle (384–322 b.c.) to the mid-twentieth century, only two kingdoms were recognized—Animalia (animal) and Plantae (plant). However, with increasingly modern and sophisticated techniques, biologists eventually came to recognize a five kingdom approach. These additional kingdoms were necessary in order to include other forms of life that did not belong in either the plant or animal kingdom. Today, the five kingdoms are monerans, protists, fungi, plants, and animals.
Monera, the smallest kingdom (with only about 4,000 species), includes the prokaryotic bacteria (single cells that do not have a nucleus) and certain types of algae. Bacteria play an important role as decomposers, and some monerans can make their own food through photosynthesis. Organisms in the kingdom Protista are eukaryotic (their cells contain a nucleus) but are both plantlike and animal-like. Some algae and protozoans are protists. The kingdom Fungi consists of molds, yeasts, and mushrooms. These are all multicelled organisms that live by absorbing food. Although these organisms look like plants, they do not make their own food. Members of the kingdom Plantae make their own food and often grow flowers and form seeds. The kingdom Animalia includes multicelled organisms that live by taking in food. Animalia is made up of vertebrates (animals with a backbone) and invertebrates (animals without a backbone). This is the largest kingdom, containing more than 2,000,000 species.
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.