Classification of organisms


May 19, 2022

Classification of living things (seibutsu no bunrui) explains how to classify living things in a unified manner. See also Taxonomy, Scientific Names, Category: Taxonomy, Wikispecies.


Each known species of organism is given a scientific name (genus name + species minor name or genus name + species adjective). The first half of the scientific name is the genus name, which is a collection of species that are closely related to the genus. These are classified and the classification group is also given a scientific name. Furthermore, by systematizing this classification hierarchically (minor classification> middle classification> major classification, etc.), we try to clarify the relationships between various biological groups and, by extension, the genealogy of evolution. In each era, taxonomy has continued to seek a system of classification that is as convincing as possible based on the information available up to that time. In Linnaeus's era, there was a need for a system that was as convincing as possible, focusing on morphology, using information obtained from microscopes when they came to be used, and using dyes when biochemistry developed. .. Therefore, it is considered that the classification system is gradually approaching the correct figure while changing with the times. At the end of the 20th century, molecular genetics methods that refer to the genes themselves were adopted, and many taxa are under pressure for major revisions. Therefore, such a system should continue to be forced to change. However, at each point in time, sentences cannot be created unless one of the systems is adopted. Therefore, Wikipedia adopts a specific system for each group. Therefore, it should be noted that the description may differ from other books, etc., and it may be difficult to discuss which is correct.

About modern phylogenetic nomenclature

As scientific knowledge about living organisms accumulated, the classification of living organisms was revised many times, but the results of molecular phylogenetic analysis at the end of the 20th century made major revisions. This section describes a modern phylogenetic nomenclature based on the results of this molecular phylogeny.


Eocyte hypothesis

The theory that the entire organism is divided into bacterial strains and archaeal strains, of which eukaryotes have evolved from archaea, is becoming more predominant (eocite theory in a broad sense, bifurcation theory). Molecular phylogenetic analysis suggests that it evolved from Asgard archaea among archaea. Asgard archaea also have many similarities to eukaryotes in terms of acquired features such as the membrane transport system.

Relationship with conventional classification

The relationship between the framework of classification of organisms proposed in the past and the three-domain theory is simplified and shown below. Even in the eocyte hypothesis and the bifurcation theory, the taxon of eukaryotes is not dismantled. The word "kingdom" that appears in "animal kingdom", "plant kingdom", etc. in the above table is one of the taxonomic ranks of living things, and was once positioned as the highest taxonomic rank. On the other hand, in the three-domain theory, the whole organism is first divided into three domains, and the "kingdom" is treated as a classification class lower than these domains. In Japan's primary education, the classification of living things is explained based on the two-world theory (until 2011) or the five-world theory (after 2012) before the three-domain theory.

Large taxa of eukaryotes

Also for the major classification within the eukaryotic domain A classification system based on the results of molecular phylogenetic analysis has been proposed and announced. Below is a classification by the International Society of Protistology.