Taxonomic Hierarchy of Life

The current system for classifying life places all organisms in one of three domains: Archaea, Bacteria, or Eukarya. Domain is the largest taxonomic grouping. Kingdom is the second largest taxonomic grouping, under domain. The classification system has changed since the 1700s, when naturalist Carolus Linnaeus first introduced the conventions of modern classification, or taxonomy. Among his ideas for simplifying and standardizing organism names, Linnaeus proposed a two-kingdom system in which all organisms were considered either plants or animals. That classification does not reflect the current understanding and is no longer practical. However, one aspect of Linnaeus's work that still applies today is the use of a system for identifying organisms, called binomial nomenclature. Binomial nomenclature is a two-word naming convention for each type of organism that includes its genus and species name. By tradition, the scientific name of an organism includes two Latin words that are used by all scientists, regardless of their spoken language. The genus and species names are the two most specific categories of the hierarchy of life. These names are italicized with only the genus name capitalized, as in Panthera tigris for tigers.

There are eight major categories in the hierarchy of life: domain, kingdom, phylum, class, order, family, genus, and species. There are also some categories that fall in between, such as subphylum. The categories are nested such that each domain includes at least one kingdom, each kingdom includes at least one phylum, and so on, all the way down the list. Each genus includes at least one species. The groups high on the list are more general than those low on the list. For example, the kingdom Animalia includes all animals, but the family Felidae includes the less diverse group of cats. One of the three domains, Eukarya, includes all organisms with a eukaryotic cell type. Eukaryotic cells are complex, with a nucleus and other organelles surrounded by one or two membranes. The other two domains, Archaea and Bacteria, include organisms that seem superficially similar. They are small, single-celled prokaryotes. Prokaryotic cells are smaller and simpler than eukaryotic cells, containing no nucleus or other membrane-bound organelles. However, there are differences in the cell structure and general lifestyles of Archaea and Bacteria that provide clues about their evolutionary distance from one another. Once their genomes were sequenced, it became clear that Archaea are not closely related to Bacteria at all. Furthermore, DNA evidence suggests that Archaea are more closely related to organisms within Eukarya than they are to Bacteria. However, there is some debate over the determination of relative relatedness among the three domains because horizontal gene transfer (the direct transfer of DNA between individuals, without reproduction) throughout evolutionary history has complicated the issue. The direct transfer of genes between organisms (through plasmids, viruses, or other vectors) can cause genetic similarity that is not due to common ancestry. The domains of Bacteria and Archaea each contain a single kingdom, however the kingdom name in Archaea (Archaebacteria) is rarely used anymore and is replaced with just the domain name. The domain Eukarya contains four kingdoms: Plantae, Fungi, Animalia, and Protista. The organisms of these kingdoms are sorted primarily by how they obtain their nutrition.