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 were due to Aristotle and Cesalpino, but modern classifications owe mainly to John Ray and eventually to Linnaeus, who introduced binominal 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 ruled 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 the modern scientific classification 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 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 genealogical 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

Figure 1.

(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 needs to be represented, taxon ranking is still in use. (d) A rank‐free phylogeny‐based classification. Indentation represents clade nesting.

Figure 2.

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.)



Donegan TM (2009) Type specimens, samples of live individuals and the Galapagos Pink Land Iguana. Zootaxa 2201: 12–20.

Dubois A (2009) Endangered species and endangered knowledge. Zootaxa 2201: 26–29.

Gentile G and Snell HL (2009) Conolophus marthae sp.nov. (Squamata, Iguanidae), a new species of land iguana from the Galápagos archipelago. Zootaxa 2201: 1–10.

Greuter W, Garrity G, Hawksworth DL et al. (2011) Draft BioCode (2011): Principles and rules regulating the naming of organisms. Taxon 60(1): 201–212.

Minelli A (2003) The status of taxonomic literature. Trends in Ecology and Evolution 18(2): 75–76.

Nemésio A (2009) On the live holotype of the Galápagos pink land Iguana, Conolophus marthae Gentile & Snell, 2009 (Squamata: Iguanidae): is it an acceptable exception? Zootaxa 2201: 21–25.

Nguembock B, Fjeldså J, Couloux A and Pasquet E (2008) Phylogeny of Laniarius: molecular data reveal L. liberatus synonymous with L. erlangeri and ‘plumage coloration’ as unreliable morphological characters for defining species and species groups. Molecular Phylogenetics and Evolution 48(2): 396–407.

Polaszek A, Agosti D, Alonso‐Zarazaga M et al. (2005) A universal register for animal names. Nature 437(7058): 477.

Smith EFG, Arctander P, Fjeldsa J and Amir OG (1991) A new species of shrike (Laniidae: Laniarius) from Somalia, verified by DNA sequence data from the only known individual. Ibis 133(3): 227–235.

Further Reading

Berlin B (1992) Ethnobiological Classification. Princeton, NJ: Princeton University Press.

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

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

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

Quicke DLJ (1993) Principles and Techniques of Contemporary Taxonomy. London: Blackie.

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

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

Wiley EO (1981) Phylogenetics: The Theory and Practice of Phylogenetic Systematics. New York: Wiley.

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

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Minelli, Alessandro, and Fusco, Giuseppe(Sep 2012) Classification. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001519.pub3]