A biological classification is a hierarchical arrangement of species, subspecific units and higher taxa, with the corresponding scientific nomenclature; classification is also the part of systematic biology concerned with generating such an arrangement. Scientific classifications have ancient roots in folk taxonomies. Between the classical Antiquity and the Renaissance, major conceptual advancement was due to Aristotle and Cesalpino, but modern classifications owe mainly to John Ray and eventually to Linnaeus, who introduced binomial nomenclature. Modern classifications are increasingly aiming to mirror phylogenetic relationships, an effort that may eventually require abandoning the traditional Linnaean ranks such as the genus, the family, the order and the class. Nomenclature is disciplined by international codes – these provide rules for introducing new names and for selecting the names to be used in the case of conflict between synonymous or homonymous names.

Key Concepts

  • Main steps on the way from naif folk taxonomies to modern scientific classifications initiated by Linnaeus were provided by Aristotle, Andrea Cesalpino and John Ray.
  • Linnaeus introduced the binomial nomenclature still in use in zoology and botany for the scientific names of species.
  • Georges–Louis Buffon defined the species as a reproductive community, members of which can freely interbreed, thus generating fertile offspring, whereas members of different species, even if similar, cannot breed with them.
  • For a classification, the equation natural = evolution based was clearly reinforced by Charles Darwin's (1809–1882) work, especially by his On the Origin of Species (1859). However, Darwin also introduced into biological systematics a potentially disruptive thought: that the species, the hitherto undisputed units of biological classifications, are subject to steady change. In Darwin's own words, species differ from varieties only by matter of degree.
  • With Charles Darwin, evolutionary biology provided the foundation for a natural classification mirroring phylogenetic relationships, a research programme actually launched by Willi Hennig one century later.
  • The last few decades of the twentieth century witnessed heated debates on the theoretical foundations and methodological aspects of biological systematics among the phenetic, cladistics and evolutionary schools.
  • The traditional Linnaean classification of living beings has the structure of a hierarchy, with a series of ranks or categories (species, genus, family, order, class, phylum and kingdom).
  • The Linnaean hierarchy has been criticised as its use takes for granted a branched topology and requires acknowledging absolute ranks, rather than simple relations of inclusive nesting.
  • Biological nomenclature is governed by international codes whose rules are intended to provide unique and universally accepted names for any recognised taxon (species, infraspecific entity or supraspecific group).
  • Conflicts between synonyms (different names for the same taxon) or homonyms (same name for different taxa) are basically resolved by application of a principle of priority.

Keywords: taxon; Linnaean hierarchy; biological nomenclature; taxonomy; cladistics; type specimen; taxonomic rank

Figure 1. Schematics of the Linnaean hierarchical system of classification. On the left are the main ranks of the system. As an example, the position of the wolf in the classification is traced down from the kingdom to the species.
Figure 2. (a) A traditional classification of the Tetrapoda. The structure is hierarchical and formally acknowledges two ranks, the Superclass and the Class. (b) A phylogenetic tree of the Tetrapoda with names for each internal node (diamonds). The taxon Reptilia of the traditional classification has disappeared because it results paraphyletic, as it contains all the Sauropsida to the exclusion of Aves. (c) A phylogeny‐based classification. Only monophyletic taxa are admitted, not all nodes need to be represented, taxon ranking is still in use. (d) A rank‐free phylogeny‐based classification. Indentation represents clade nesting.
Figure 3. The pink terrestrial iguana of the Galápagos Islands Conolophus marthae was described in 2009 by selecting a living specimen as the species' holotype. Photo courtesy of Gabriele Gentile.


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Further Reading

Kitching IJ , Humphries CJ , Williams DM and Forey PL (1998) Cladistics, 2nd edn. Clarendon Press: Oxford.

Mayr E (1982) The Growth of Biological Thought. The Belknap Press of Harvard University Press: Cambridge, MA.

Minelli A (1993) Biological Systematics. Chapman & Hall: London.

Minelli A (2019) The galaxy of the non‐Linnaean nomenclature. History and Philosophy of the Life Sciences 41: 31.

Schuh RT (1999) Biological Systematics: Principles and Applications. Cornell University Press: Ithaca, NY.

Wheeler WC (2012) Systematics: A Course of Lectures. Wiley‐Blackwell: Malden, MA.

Winston JE (1999) Describing Species. Columbia University Press: New York.

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Minelli, Alessandro, and Fusco, Giuseppe(Jul 2020) Classification. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001519.pub4]