**History**:
– Term ‘taxon’ first used in 1926 by Adolf Meyer-Abich for animal groups.
– Word ‘taxonomy’ coined a century before from Greek components.
– Proposed for plants by Herman Johannes Lam in 1948 and adopted in 1950.
– The glossary of the International Code of Zoological Nomenclature (1999) defines taxon as a taxonomic unit.
**Definition**:
– Taxon is a taxonomic unit, named or not, inferred to be phylogenetically related.
– It has common distinguishing characteristics and includes all included taxa of lower rank.
– Defined in the International Code of Zoological Nomenclature (1999).
**Ranks**:
– Hierarchy of biological classifications has eight major taxonomic ranks.
– Taxon can be assigned a taxonomic rank when given a formal name.
– Prefixes indicate ranking importance: super-, sub-, infra-.
– Rank is relative and restricted to a particular systematic schema.
**Cladistics and Phylogenetics**:
– Modern systematists use cladistic methods requiring taxa to be monophyletic.
– Clade is the basic unit, equivalent to a taxon.
– Some long-established taxa are not clades, like the class Reptilia.
– Phylogenetic taxonomy challenges traditional Linnean classification.
– Ongoing development of the PhyloCode as an alternative to Linnean classification.
**References**:
– Cantino & de Queiroz (2000) on the International Code of Phylogenetic Nomenclature.
– Magnol (1689) on the classification of plant families.
– Quammen (2007) on taxonomy.
– de Queiroz & Gauthier (1990) on phylogeny in taxonomy.
– Naik (1984) on the taxonomy of angiosperms.
**Classification**:
– Taxa are arranged hierarchically in a classification system.
– The Linnaean system is a widely used method of classification.
– Taxa are grouped based on shared characteristics and evolutionary relationships.
– Taxonomic ranks help in placing taxa in the hierarchy.
– Taxa can be reclassified based on new scientific discoveries.
**Nomenclature**:
– Taxonomic nomenclature assigns names to taxa.
– The binomial nomenclature system gives each species a two-part Latin name.
– Taxonomic names are standardized to avoid confusion.
– The International Code of Zoological Nomenclature governs naming rules.
– Nomenclature ensures clarity and consistency in scientific communication.
**Evolutionary Relationships**:
– Taxa reflect the evolutionary history of organisms.
– Phylogenetic trees show the relationships between taxa.
– Taxa closer on the tree share a more recent common ancestor.
– Evolutionary relationships help in understanding biodiversity.
– Taxa can be grouped into clades based on shared ancestry.
**Importance**:
– Taxa are essential for organizing the diversity of life.
– They aid in species identification and classification.
– Taxa help in studying evolutionary patterns and processes.
– Conservation efforts rely on understanding taxa distributions.
– Taxa provide a framework for biological research and education.
In biology, a taxon (back-formation from taxonomy; pl.: taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking, especially if and when it is accepted or becomes established. It is very common, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion, especially in the context of rank-based ("Linnaean") nomenclature (much less so under phylogenetic nomenclature). If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.
Initial attempts at classifying and ordering organisms (plants and animals) were presumably set forth long ago by hunter-gatherers, as suggested by the fairly sophisticated folk taxonomies. Much later, Aristotle, and later still, European scientists, like Magnol, Tournefort and Carl Linnaeus's system in Systema Naturae, 10th edition (1758),, as well as an unpublished work by Bernard and Antoine Laurent de Jussieu, contributed to this field. The idea of a unit-based system of biological classification was first made widely available in 1805 in the introduction of Jean-Baptiste Lamarck's Flore françoise, and Augustin Pyramus de Candolle's Principes élémentaires de botanique. Lamarck set out a system for the "natural classification" of plants. Since then, systematists continue to construct accurate classifications encompassing the diversity of life; today, a "good" or "useful" taxon is commonly taken to be one that reflects evolutionary relationships.
Many modern systematists, such as advocates of phylogenetic nomenclature, use cladistic methods that require taxa to be monophyletic (all descendants of some ancestor). Their basic unit, therefore, the clade is equivalent to the taxon, assuming that taxa should reflect evolutionary relationships. Similarly, among those contemporary taxonomists working with the traditional Linnean (binomial) nomenclature, few propose taxa they know to be paraphyletic. An example of a long-established taxon that is not also a clade is the class Reptilia, the reptiles; birds and mammals are the descendants of animals traditionally classed as reptiles, but neither is included in the Reptilia (birds are traditionally placed in the class Aves, and mammals in the class Mammalia).