Taxonomy

Taxonomy

A phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains bacteria, archaea, and eukaryotes as described initially by Carl Woese. Trees constructed with other genes are generally similar, although they may place some early-branching groups very differently, presumably owing to rapid rRNA evolution. The exact relationships of the three domains are still being debated.
Main article: Taxonomy
Classification is the province of the disciplines ofsystematics and taxonomy. Taxonomy places organisms in groups called taxa, while systematics seeks to define their relationships with each other.[55]
This classification technique has evolved to reflect advances in cladistics and genetics, shifting the focus from physical similarities and shared characteristics to phylogenetics.
Traditionally, living things have been divided into five kingdoms:[56]
Monera
Protista
Fungi
Plantae
Animalia
However, many scientists now consider this five-kingdom system outdated. Modern alternative classification systems generally begin with the three-domain system:[57]
Archaea (originally Archaebacteria)
Bacteria (originally Eubacteria)
Eukarya (including protists, fungi, plants, and animals)
These domains reflect whether the cells have nuclei or not, as well as differences in the cell exteriors.[57]
Further, each kingdom is broken down recursively until each species is separately classified. The order is:
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
There is also a series of intracellular parasites that are "on the edge of life"[58] in terms of metabolic activity, meaning that many scientists do not actually classify these structures as alive, due to their lack of at least one or more of the fundamental functions by which life is defined. They are classified as:
Viruses
Viroids
Prions
The scientific name of an organism is obtained from its genus and species. For example, humans would be listed as Homo sapiens. Homowould be the genus and sapiens is the species. Whenever writing the scientific name of an organism, it is proper to capitalize the first letter in the genus and put all of the species in lowercase. Additionally, the entire term would be italicized or underlined.[59][60]
The dominant classification system is called Linnaean taxonomy, which includes ranks and binomial nomenclature. How organisms are named is governed by international agreements such as the International Code of Botanical Nomenclature (ICBN), the International Code of Zoological Nomenclature (ICZN), and the International Code of Nomenclature of Bacteria (ICNB).
A merging draft, BioCode, was published in 1997 in an attempt to standardize nomenclature in these three areas, but it has yet to be formally adopted.[61] The BioCode draft has received little attention since 1997; its originally planned implementation date of January 1, 2000, has passed unnoticed. However, a 2004 paper concerning the cyanobacteria does advocate a future adoption of a BioCode and interim steps consisting of reducing the differences between the codes.[62] The International Code of Virus Classification and Nomenclature (ICVCN) remains outside the BioCode.