Kingdom (biology)

Kingdom (biology)

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In biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, kingdom (Latin: regnum, pl. regna) is a taxonomic rank
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...

, which is either the highest rank or in the more recent three-domain system
Three-domain system
The three-domain system is a biological classification introduced by Carl Woese in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria and Archaebacteria...

, the rank below domain
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...

. Kingdoms are divided into smaller groups called phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 (in zoology) or divisions in botany. The complete sequence of ranks is life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

, domain
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...

, kingdom, phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

, class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

, order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

, family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

, genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

, and species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

.

Currently, textbooks from the United States use a system of six kingdoms (Animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

ia, Plantae, Fungi, Protista, Archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

) while British, Australian and Latin American textbooks may describe five kingdoms (Animalia, Plantae, Fungi, Protista, and Prokaryota or Monera
Monera
Monera is a superseded kingdom that contains unicellular organisms without a nucleus , such as bacteria....

).

Historically, the number of kingdoms in widely accepted classifications has grown from two to six. However, phylogenetic research from about 2000 onwards does not support any of the traditional systems.

Two kingdoms


The classification of living things into animals and plants is an ancient one. Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 (384–322 BC) classified animal species in his work the History of Animals
History of Animals
History of Animals is a zoological natural history text by Aristotle.-Arabic translation:The Arabic translation of Historia Animalium comprises treatises 1-10 of the Kitāb al-Hayawān .-See also:...

, and his pupil Theophrastus
Theophrastus
Theophrastus , a Greek native of Eresos in Lesbos, was the successor to Aristotle in the Peripatetic school. He came to Athens at a young age, and initially studied in Plato's school. After Plato's death he attached himself to Aristotle. Aristotle bequeathed to Theophrastus his writings, and...

 (c. 371–c. 287 BC) wrote a parallel work on plants (the History of Plants
Historia Plantarum
Historia Plantarum is Latin and literally means History of Plants, although in reality it means something closer to "on plants" or "treatise on plants". There has been more than one book by this title....

).

Carolus Linnaeus
Carolus Linnaeus
Carl Linnaeus , also known after his ennoblement as , was a Swedish botanist, physician, and zoologist, who laid the foundations for the modern scheme of binomial nomenclature. He is known as the father of modern taxonomy, and is also considered one of the fathers of modern ecology...

 (1707–1778) laid the foundations for modern biological nomenclature, now regulated by the Nomenclature Codes
Nomenclature Codes
Nomenclature codes or codes of nomenclature are the various rulebooks that govern biological taxonomic nomenclature, each in their own broad field of organisms...

. He distinguished two kingdoms of living things: Regnum Animale ('animal kingdom') for animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s and Regnum Vegetabile ('vegetable kingdom') for plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s. (Linnaeus also included mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s, placing them in a third kingdom, Regnum Lapideum.) Linnaeus divided each kingdom into classes, later grouped into phyla
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 for animals and divisions for plants.

Three kingdoms


In 1674, Antonie van Leeuwenhoek, often called the "father of microscopy", sent the Royal Society
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...

 of London a copy of his first observations of microscopic single-celled organisms. Until then the existence of such microscopic organisms was entirely unknown. At first these organisms were divided into animals and plants and placed in the appropriate Kingdom. However, by the mid-19th century it had become clear that "the existing dichotomy of the plant and animal kingdoms [had become] rapidly blurred at its boundaries and outmoded". In 1866, following earlier proposals by Richard Owen
Richard Owen
Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...

 and John Hogg
John Hogg (biologist)
John Hogg was a British naturalist who wrote about amphibians, birds, plants, and protist. In 1839 he became a member of the Royal Society....

, Ernst Haeckel
Ernst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...

 proposed a third kingdom of life. Haeckel revised the content of this kingdom a number of times before settling on a division based on whether organisms were unicellular (Protista) or multicellular (animals and plants).

Four kingdoms


The development of microscopy, and the electron microscope
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

 in particular, revealed an important distinction between those unicellular organisms whose cells do not have a distinct nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

, prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

s, and those unicellular and multicellular organisms whose cells do have a distinct nucleus, eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s. In 1938, Herbert F. Copeland
Herbert Copeland
Herbert Faulkner Copeland was an American biologist who contributed to the theory of biological kingdoms. His father was Edwin Copeland. He was responsible for the fourth kingdom, Monera.- Bibliography :...

 proposed a four-kingdom classification, moving the two prokaryotic groups, bacteria and "blue-green algae", into a separate Kingdom Monera.
It gradually became apparent how important the prokaryote/eukaryote distinction is, and in the 1960s Stanier and van Niel popularized Édouard Chatton's much earlier proposal to recognize this division in a formal classification. This required the creation, for the first time, of a rank above kingdom, a superkingdom or empire, also called a domain
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...

.

Five kingdoms



The differences between fungi and other organisms regarded as plants had long been recognized. For example, at one point Haeckel moved the fungi out of Plantae into Protista, before changing his mind. Robert Whittaker
Robert Whittaker
Robert Harding Whittaker was a distinguished American plant ecologist, active in the 1950s to the 1970s.Born in Wichita, Kansas, he obtained a B.A. at Washburn Municipal College in Topeka, Kansas, and, following military service, his Ph.D...

 recognized an additional kingdom for the Fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

. The resulting five-kingdom system, proposed in 1969 by Whittaker, has become a popular standard and with some refinement is still used in many works and forms the basis for new multi-kingdom systems. It is based mainly on differences in nutrition
Nutrition
Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....

; his Plantae were mostly multicellular autotroph
Autotroph
An autotroph, or producer, is an organism that produces complex organic compounds from simple inorganic molecules using energy from light or inorganic chemical reactions . They are the producers in a food chain, such as plants on land or algae in water...

s, his Animalia multicellular heterotroph
Heterotroph
A heterotroph is an organism that cannot fix carbon and uses organic carbon for growth. This contrasts with autotrophs, such as plants and algae, which can use energy from sunlight or inorganic compounds to produce organic compounds such as carbohydrates, fats, and proteins from inorganic carbon...

s, and his Fungi multicellular saprotrophs. The remaining two kingdoms, Protista and Monera, included unicellular and simple cellular colonies. The five kingdom system may be combined with the two empire system.

Six kingdoms


From around the mid-1970s onwards, there was an increasing emphasis on molecular level comparisons of genes (initially ribosomal
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....

 RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 genes) as the primary factor in classification; genetic similarity was stressed over outward appearances and behavior. Taxonomic ranks, including kingdoms, were to be groups of organisms with a common ancestor, whether monophyletic (all descendants of a common ancestor) or paraphyletic (only some descendants of a common ancestor). Based on such RNA studies, Carl Woese
Carl Woese
Carl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...

 divided the prokaryotes (Kingdom Monera) into two groups, called Eubacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and Archaebacteria
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, stressing that there was as much genetic difference between these two groups as between either of them and all eukaryotes. Eukaryote groups, such as plants, fungi and animals may look different, but are more similar to each other in their genetic makeup at the molecular level than they are to either the Eubacteria or Archaebacteria. (It was also found that the eukaryotes are more closely related, genetically, to the Archaebacteria than they are to the Eubacteria.) Although the primacy of the eubacteria-archaebacteria divide has been questioned, it has also been upheld by subsequent research.

Woese attempted to establish a "three primary kingdom" or "urkingdom" system. In 1990, the name "domain" was proposed for the highest rank. The six-kingdom system shown below represents a blending of the classic five-kingdom system and Woese's three-domain system
Three-domain system
The three-domain system is a biological classification introduced by Carl Woese in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria and Archaebacteria...

. Such six-kingdom systems have become standard in many works.
Woese also recognized that the kingdom Protista was not a monophyletic group and might be further divided at the level of kingdom.

Cavalier-Smith's six kingdoms



Thomas Cavalier-Smith
Thomas Cavalier-Smith
Professor Thomas Cavalier-Smith , FRS, FRSC, NERC Professorial Fellow, is a Professor of Evolutionary Biology in the Department of Zoology, at the University of Oxford...

 has published extensively on the evolution and classification of life, particularly protists. His views have been influential but controversial, and not always widely accepted. In 1998, he published a six-kingdom model, which has been revised in subsequent papers. The version published in 2009 is shown below. (Compared to the version he published in 2004, the alveolates and the rhizaria
Rhizaria
The Rhizaria are a species-rich supergroup of unicellular eukaryotes. This supergroup was proposed by Cavalier-Smith in 2002. They vary considerably in form, but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods...

ns have been moved from Kingdom Protozoa to Kingdom Chromista.) Cavalier-Smith does not accept the importance of the fundamental eubacteria–archaebacteria divide put forward by Woese and others and supported by recent research. His Kingdom Bacteria includes the Archaebacteria as part of a subkingdom along with a group of eubacteria (Posibacteria). Nor does he accept the requirement for groups to be monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

. His Kingdom Protozoa includes the ancestors of Animalia and Fungi. Thus the diagram below does not represent an evolutionary tree.

International Society of Protistologists Classification 2005


The "classic" six-kingdom system is still recognizably a modification of the original two-kingdom system: Animalia remains; the original category of plants has been split into Plantae and Fungi; and single-celled organisms have been introduced and split into Bacteria, Archaea and Protista.

Research published in the 21st century has produced a rather different picture. In 2004, a review article by Simpson and Roger noted that the Protista were "a grab-bag for all eukaryotes that are not animals, plants or fungi". They argued that only monophyletic groups–an ancestor and all of its descendents — should be accepted as formal ranks in a classification. On this basis, the diagram opposite (redrawn from their article) showed the real 'kingdoms' (their quotation marks) of the eukaryotes. A classification which followed this approach was produced in 2005 for the International Society of Protistologists, by a committee which "worked in collaboration with specialists from many societies". It divided the eukaryotes into the same six "supergroups". Although the published classification deliberately did not use formal taxonomic ranks, other sources have treated each of the six as a separate kingdom.
In this system, the traditional kingdoms have vanished. For example, research shows that the multicellular animals (Metazoa) are descended from the same ancestor as the unicellular choanoflagellate
Choanoflagellate
The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals...

s and the fungi. A classification system which places these three groups into different kingdoms (with multicellular animals forming Animalia, choanoflagellates part of Protista and Fungi a separate kingdom) is not monophyletic. The monophyletic group is the Opisthokonta, made up of all those organisms believed to have descended from a common ancestor, some of which are unicellular (choanoflagellates), some of which are multicellular but not closely related to animals (some fungi), and others of which are traditional multicellular animals.

However, in the same year as the International Society of Protistologists' classification was published (2005), doubts were being expressed as to whether some of these supergroups were monophyletic, particularly the Chromalveolata, and a review in 2006 noted the lack of evidence for several of the supposed six supergroups.

, there is widespread agreement that the Rhizaria belong with the Stramenopiles and the Alveolata, in a clade dubbed the SAR supergroup
SAR supergroup
The SAR supergroup is a clade that includes Stramenopiles , Alveolates, and Rhizaria.The first letter of each group provides the "SAR" in the name.It includes most of the organisms in the Chromalveolata grouping, but not Hacrobia....

, so that Rhizara is not one of the main eukaryote groups. Beyond this, there does not appear to be a consensus.
Rogozin et al. in 2009 noted that "The deep phylogeny of eukaryotes is an extremely difficult and controversial problem." , there appears to be a consensus that the 2005 six supergroup model does not reflect the true phylogeny of the eukaryotes and hence how they should be classified, although there is no agreement as to the model which should replace it.

Summary


The sequence from the two-kingdom system up to Cavalier-Smith's six-kingdom system can be summarized in the table below.

Note that the equivalences in this table are not perfect. For example, Haeckel placed the red algae
Red algae
The red algae are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species  of mostly multicellular, marine algae, including many notable seaweeds...

 (his Florideae, modern Florideophyceae
Florideophyceae
Florideophyceae, demoted to subclass Floridae in some classification systems, is a class of red algae. It is sometimes merged with the Bangiaceae into the Rhodophyceae, leaving the subclass Florideae in use....

) and blue-green algae (his Archephyta, modern Cyanobacteria) in his Plantae.

One or other of the kingdom-level classifications of life is still widely employed as a useful way of grouping organisms, notwithstanding the problems with this approach:
  • Kingdoms such as Bacteria represent grade
    Evolutionary grade
    In alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.-Definition:...

    s rather than clade
    Clade
    A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

    s, and so are rejected by phylogenetic classification systems.
  • Research in the 21st century does not support the classification of the eukaryotes into any of the standard systems. , the situation appears to be that there is no set of kingdoms sufficiently supported by current research to gain widespread acceptance; as Roger & Simpson say: "with the current pace of change in our understanding of the eukaryote tree of life, we should proceed with caution."

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