in biology, a treelike representation of phylogenetic relationships within any natural group of organisms or within the organic world as a whole. The concept of a genealogical tree and the method for depicting it are borrowed from human genealogy.

The idea of a chain of beings preceded the concept of a genealogical tree. In 1766 the Russian scientist P. S. Pallas was the first to prove that the linear arrangement of organisms does not reflect the true relationship between animal classes, and consequently he proposed representing organic systems in the form of genealogical tables. This idea was particularly developed after the publication of C. Darwin’s The Origin of the Species (1859), in which the phenomenon of divergence was illustrated by means of genealogical trees. In the 19th century the genealogical tables of the animal and plant kingdoms were depicted by E. Heckel and others in the form of actual trees (hence the name) with group names arranged on the branches.

Genealogical trees are drawn only on the basis of a mono-phyletic development of living nature and cannot be used to illustrate a polyphyletic origin of the organic world. A genealogical tree is constructed to show the phylogenetic relationships and the level of organization of equally ranking groups, including phyla, classes, and orders. The relative level of organization of each group corresponds with its position on a genealogical tree. The origin of each group and its relation to other groups is designated by the point at which a particular branch departs from the common trunk or from a larger branch. If a genealogical tree is supposed to illustrate both phylogenetic ties and subordinate relationships in a group, the large unifying taxons (phyla, classes, orders, and families) are arranged closer to the base of the trunk (or the main taxon is at the trunk’s base) or they are arranged according to their rank on the primary or secondary lateral branches.

The end-shoots of a genealogical tree designate the lowest ranking categories, which are genera and sometimes species. Genealogical trees based on only modern forms reflect purely systematic relationships. If paleontological material is also used, genealogical trees are plotted on diagrams representing the subdivisions of geologic time. The point of divergence of a particular branch indicates the time that the corresponding taxon emerged.

True phylogenetic relationships among different groups of organisms and within individual groups are very complex and can only be approximated on a plane. Therefore, the two-dimensional flat representation of a genealogical tree is frequently replaced by a three-dimensional representation.