Biological classification, or scientific classification in biology, is a method by which biologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy.
Classification has been defined by Mayr as "The arrangement of entities in a hierarchical series of nested classes, in which similar or related classes at one hierarchical level are combined comprehensively into more inclusive classes at the next higher level." A class is defined as "a collection of similar entities", where the similarity consists of the entities having attributes or traits in common.
A classification, as defined above, is necessarily hierarchical. In a biological classification, rank is the level (the relative position) in a hierarchy. (Rarely, the term "taxonomic category" is used instead of "rank".) There are seven main ranks defined by the international nomenclature codes: kingdom, phylum/division, class, order, family, genus, species. "Domain" (proposed by Carl Woese), a level above kingdom, has become popular in recent years, but has not been accepted into the codes.
A taxon is usually assigned a rank when it is given its formal name. The most basic rank is that of species. The next most important rank is that of genus: when an organism is given a species name it is assigned to a genus, and the genus name is part of the species name. The third-most important rank, although it was not used by Linnaeus, is that of family.
The species name is sometimes called a binomial, that is, a two-term name. For example, the zoological name for the human species is Homo sapiens: this is usually italicized in print (and underlined when italics are not available). In this case, Homo is the generic name and refers to the genus; it is capitalized; sapiens indicates the species: it is not capitalized.
There are slightly different ranks for zoology and for botany, including subdivisions such as tribe.
From well before Linnaeus, plants and animals were considered separate Kingdoms. Linnaeus used this as the top rank, dividing the physical world into the plant, animal and mineral kingdoms. As advances in microscopy made classification of microorganisms possible, the number of kingdoms increased, five and six-kingdom systems being the most common.
Domains are a relatively new grouping. The three-domain system was first invented in 1990, but not generally accepted until later. One main characteristic of the three-domain method is the separation of Archaea and Bacteria, previously grouped into the single kingdom Bacteria (a kingdom also sometimes called Monera). Consequently, the three domains of life are conceptualized as Archaea, Bacteria, and Eukaryota (comprising the nuclei-bearing eukaryotes). A small minority of scientists add Archaea as a sixth kingdom, but do not accept the domain method.
Thomas Cavalier-Smith, who has published extensively on the classification of protists, has recently proposed that the Neomura, the clade that groups together the Archaea and Eukarya, would have evolved from Bacteria, more precisely from Actinobacteria. His classification of 2004 treats the archaebacteria as part of a subkingdom of the Kingdom Bacteria, i.e. he rejects the three-domain system entirely.