In biology, a kingdom is the second highest taxonomic rank, just below domain. Kingdoms are divided into smaller groups called phyla (singular phylum).
Traditionally, textbooks from the United States and some of Canada have used a system of six kingdoms (Animalia, Plantae, Fungi, Protista, Archaea or Archaebacteria, and Bacteria or Eubacteria), while textbooks in other parts of the world, such as Bangladesh, Brazil, Greece, India, Pakistan, Spain, and the United Kingdom have used five kingdoms (Animalia, Plantae, Fungi, Protista and Monera).
Some recent classifications based on modern cladistics have explicitly abandoned the term kingdom, noting that some traditional kingdoms are not monophyletic, meaning that they do not consist of all the descendants of a common ancestor. The terms flora (for plants), fauna (for animals), and, in the 21st century, funga (for fungi) are also used for life present in a particular region or time.
Definition and associated terms
When Carl Linnaeus introduced the rank-based system of nomenclature into biology in 1735, the highest rank was given the name "kingdom" and was followed by four other main or principal ranks: class, order, genus and species. Later two further main ranks were introduced, making the sequence kingdom, phylum or division, class, order, family, genus and species. In 1990, the rank of domain was introduced above kingdom.
History
<span class="anchor" id="Three kingdoms: animal, vegetable, and mineral"></span> Two kingdoms of life
The classification of living things into animals and plants is an ancient one. Aristotle (384–322 BC) classified animal species in his History of Animals, while his pupil Theophrastus ( – ) wrote a parallel work, the Historia Plantarum, on plants.
Carl Linnaeus (1707–1778) laid the foundations for modern biological nomenclature, now regulated by the Nomenclature Codes, in 1735. He distinguished two kingdoms of living things: Regnum Animale ('animal kingdom') and Regnum Vegetabile ('vegetable kingdom', for plants). Linnaeus also included minerals in his classification system, placing them in a third kingdom, Regnum Lapideum.
<span class="anchor" id="Three kingdoms: plants, protists, and animals"></span> Three kingdoms of life
thumb|upright=1.5|right|Haeckel's original (1866) conception of the three kingdoms of life, including the new kingdom Protista. Notice the inclusion of the cyanobacterium [[Nostoc with plants.]]
In 1674, Antonie van Leeuwenhoek, often called the "father of microscopy", sent the Royal Society of London a copy of his first observations of microscopic single-celled organisms. Until then, the existence of such microscopic organisms was entirely unknown. Despite this, Linnaeus did not include any microscopic creatures in his original taxonomy.
At first, microscopic organisms were classified within the animal and plant kingdoms. However, by the mid–19th century, it had become clear to many that "the existing dichotomy of the plant and animal kingdoms [had become] rapidly blurred at its boundaries and outmoded".
In 1860 John Hogg proposed the Protoctista, a third kingdom of life composed of "all the lower creatures, or the primary organic beings"; he retained Regnum Lapideum as a fourth kingdom of minerals.
In the 1960s, Roger Stanier and C. B. van Niel promoted and popularized Édouard Chatton's earlier work, particularly in their paper of 1962, "The Concept of a Bacterium"; this created, for the first time, a rank above kingdom—a superkingdom or empire—with the two-empire system of prokaryotes and eukaryotes.
<span class="anchor" id="Five kingdoms"></span> Five kingdoms
The differences between fungi and other organisms regarded as plants had long been recognised by some; Haeckel had moved the fungi out of Plantae into Protista after his original classification, 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 upon differences in nutrition; his Plantae were mostly multicellular autotrophs, his Animalia multicellular heterotrophs, and his Fungi multicellular saprotrophs.
The remaining two kingdoms, Protista and Monera, included unicellular and simple cellular colonies.
Following publication of Whittaker's system, the five-kingdom model began to be commonly used in high school biology textbooks.
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