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Hierarchy of taxonomic levels (without intermediate levels)

Taxon ( listen ? / I , that, pl . : taxa; from ancient Greek τάξις táxis , (order) order, rank ) denotes a unit in the systematics of biology to which a group of living beings is assigned according to certain criteria . This system is usually expressed through a separate name for this group. Audio file / audio sample

In principle, a distinction can be made between form taxa and real taxa .

Form taxa have similar characteristics, but do not have to be directly related to one another in phylogeny ( e.g. invertebrates , deciduous trees or single-celled organisms ). The corresponding characteristics emerged independently of one another in different places ( analogous development ) .

Real taxa, on the other hand, ideally form closed communities of descent ("natural groups"), so that the corresponding systematics provides an image of the evolutionary relationships . The relationships show a hierarchical , multi-branched tree structure ("family tree"). The biological system is also structured in a correspondingly hierarchical manner: Most taxa are subtaxa of a higher-level taxon and contain subtaxa themselves. In the traditional systematics, the hierarchy levels, which are also referred to as ranks or ranks, are named. The highest level is the domain (contains the taxa bacteria , archaea and eukaryotes ). The next lower rank is the phylum (within the bacteria and archaea) or the kingdom (within the eukaryotes), which in turn are divided into lower ranks. In the case of the eukaryotes, these further ranks are traditionally referred to in descending order as tribes , classes , orders , families , genera and species . These ranks are also often divided into superordinate and / or subordinate grades (e.g. superordinate or subgeneral). The actual taxa are the individually named units within a rank. With the growing importance of cladistics in biology, the methods of which to determine kinship relationships result in a multiple of potential hierarchy levels compared to traditional systematics, the use of traditional ranks is increasingly being dispensed with.

Since viruses are not considered living beings, the virus taxonomy is outside the hierarchy discussed here.

The relationship of the taxa grouped together to form a superordinate taxon is expressed by at least one common characteristic that distinguishes these taxa from other taxa of the same rank. In the cladistic system, these characteristics each have a common evolutionary origin by definition , that is, they are homologous to one another and were inherited from an ancestral species . While in cladistics the taxon, which is defined by a certain parent species, always includes all of its descendants (i.e. all subtaxas of all ranks), there are also taxa in traditional systematics that do not contain all descendants of the parent species. For example, the class of reptiles does not include birds (they form a separate class of terrestrial vertebrates ), although it is widely believed that birds are the closest living relatives of crocodiles (a traditional order of reptiles).

The rules for assigning scientific names to taxa are part of the relevant nomenclature rules ( ICZN , ICBN , ICNB ). The classification according to the degree of kinship in the corresponding hierarchical system is made by the academic discipline of the taxonomy based on the biological systematics . Different methodological approaches and / or feature interpretations can lead to alternative results in the determination of the relationships and therefore to taxon concepts that differ from one another and often to differently named taxa. Traditionally, names derived from Latin or Greek are often given. Such a named taxon is called a nominal or formal taxon.

Form taxa

Form taxa are groups of living beings that have been recognized by the system as not belonging to one another in terms of phylogeny, but are nevertheless retained with their own name for practical reasons. Examples of such form taxa are:

  • the invertebrates (Invertebrata), which are characterized by the fact that they have no bones . This group, named by Jean-Baptiste de Lamarck , includes the vast majority of all animal species. Invertebrates such as sponges (Porifera), insects (Insecta) or starfish (Asteroidea) are only very distantly related to one another and do not form a natural family group.
  • the lichens (Mycophycophyta), which as symbiotic communities of fungi and algae are not independent living beings, but consist of two fundamentally different partners. Nevertheless, some of them are still listed as a separate taxon with associated sub-groups such as genera and species .
  • the Fungi imperfecti (Deuteromycota), which represent a collective group for all those fungi that either have no sexual reproductive structures or whose reproductive pathways are not yet understood. Sometimes even asexual stages are recorded by mushrooms, which are otherwise quite capable of sexual reproduction. In tribal history, many organisms in this group belong to other groups of fungi such as the tube mushrooms (Ascomycota) or the stand mushrooms (Basidiomycota). Here, anamorphs and teleomorphs are not infrequently listed under different taxa.
  • the protists (Protista), which are defined as the totality of all organisms with a real cell nucleus (Eukaryota), without being animals (Animalia), fungi (Fungi) or plants (Plantae). They are often called single-celled organisms , although this group includes numerous multicellular organisms that, such as the brown algae (Phaeophyta), are among the largest creatures on our planet.

While today it is largely undisputed that the above examples are form taxa, the question of what is to be regarded as a real taxon is problematic and depends on fundamental considerations with regard to the system: what is a valid taxon from the point of view of some can be be merely a formal group from another's perspective .


The easiest way to resolve are discrepancies that only arise due to synonymous names. For example, two biologists who encounter the same species in different regions can initially assign different names to it, or two species originally regarded as separate can be recognized as subpopulations of a single one. In this way, for example, the brown hare ( Lepus europaeus ) has received forty-three different scientific names (synonyms) over time. Another possibility is that, based on new scientific knowledge, a grouping that was believed to be out of date will be regarded as a real taxon again, but will initially be given a new name in ignorance of a name that may have been centuries back. In both cases, the conflict is resolved by giving the oldest available name as a "valid" name over the "younger synonyms" according to the priority rule.

Only in rare cases are there exceptions, e.g. B. Equus ferus Boddaert, 1785 before Equus caballus Linnaeus 1758, because according to a decision of the ICZN commission in 2003, the younger name was given priority over the older one. Another exception is Tyrannosaurus rex Osborn, 1905, exceptionally with priority over Manospondylus gigas Cope, 1892.

The reverse case, in which two different taxa are assigned the same name , can also occur. Here, too, the name can only be used for the taxon named first (if at all); this name cannot be used for the other taxon.

Of greater theoretical importance for the validity of a taxon, however, are deeper systematic considerations on the question of which criteria are used to determine which group of living beings forms a real taxon and which does not. The most important systems at the beginning of the 21st century are the classical and cladistic taxonomy. Your following comparison is idealized to work out the differences; In biological practice, both systems are often used side by side or in combination.

Taxa in classical taxonomy

The classic taxonomy goes back to a classification system introduced by the Swedish naturalist Carl von Linné (whose name was Carl Linnæus at the time). According to this, not only are all living beings grouped in a hierarchy of nested taxa, but each group is also assigned a category level. The basic category, the basis for the entire classification, forms the species , which can be defined according to the most varied of aspects, but in sexually reproducing organisms is often determined by the criterion of interfertility, which means that members of a species share under natural conditions be able to produce fertile offspring. This definition of the term species, like its alternatives, is not without problems; a more detailed explanation can be found in the article on species .

Naming rules for classical taxa

According to a convention that goes back to Linnaeus, each species is given a two-part (binary) name, which is composed of the name of the next higher category, the genus , and the so-called specific epithet (in zoology: the species name) and therefore also as binomials referred to as. These two parts of the species binomial are usually written in italics according to the conventions today: the generic name is capitalized and the specific epithet / name is small. According to these rules, all taxa of higher rank have a name consisting of a single word that starts with a capital letter and is not written in italics. Each genus name can only be given once within the organism kingdom (the same genus name can, however, be given in parallel in zoology and botany). The second part of the type binomial can be assigned and used several times in different genera.

In addition to the genre, Linnaeus also introduced the ranks of order , class and empire . Later this system was the location between genus and order family and located between class and poor tribe (in zoology respectively) the department (in the Botany extended). Recently, additional ranks such as domain , row , cohort, legion or tribe have been introduced. All categories can be further subdivided by the prefixes “over-” or “under-”. In the case of some taxa, it is already possible to determine which category of the classic ranking systems they belong to by their names: For example, the ending “-aceae” denotes a family of plants, while a family of animals has the ending “-idae”. The exact naming rules are laid down in the respective and mutually independent sets of rules of the biological nomenclature .

Three -part ( trinomial ) names are used to name subspecies . They denote taxa within a species that can be well characterized and differentiated from one another, but whose individuals are still fertile for crossbreeding with those of other taxa within the same species, which are also well defined. In the case of the trinomial of subspecies, the subspecies name, always in lower case, is placed after the species binomial, and just like the species binomial, the subspecies trinomial is written completely in italics.

Differentiation of classical taxa

Linné's distinction between taxa was still based on purely formal criteria, the so-called essential characteristics. For example, he divided the flowering plants according to the number and arrangement of the stamens and carpels of the flower, i.e. according to their reproductive structures. His contemporary, the French naturalist Georges Louis Leclerc Graf von Buffon , however, pointed out the arbitrariness of this criterion.

Since the pioneering work of the British geologist and naturalist Charles Darwin , the idea has instead prevailed that the classification of living things into taxa should take into account their natural relationships. But this view and procedure was not new either. Much earlier researchers, including Linnaeus, had always adjusted the taxonomy as soon as new knowledge about the relationships had become known and accordingly no longer assigned the bats to the birds and the whales to the fish. Definitely arbitrary taxa based on purely formal criteria, such as flying animals, have not been used since 1700 at the latest and no longer flowed into Linnaeus' modern scientific naming.

However, classical systematics, such as the so-called evolutionary systematics formulated by the German-American evolutionary biologist Ernst Mayr and the American paleontologist George Gaylord Simpson , try not only to incorporate the phylogenetic branches into the definition of taxa, but also to use other criteria for classification in order to to depict essential innovations in evolutionary history.

  • For example, in his classification presented in 1990, Ernst Mayr contrasts the unicellular protists (Protista) in a separate subdomain with the multicellular cells (Metabionta), to which he counts animals (Animalia), plants (Plantae) and fungi (Fungi), although the criterion is the multicellular structure very likely emerged several times independently of one another in the course of evolutionary history. According to Mayr, the decisive factor here is not the degree of relationship, which can only be determined by molecular genetics, but also the external appearance ( morphology ).
  • The system of the important evolutionary biologist Lynn Margulis (1988, 1996) deliberately combines the two groups of archaebacteria (then Archaebacteria) and real bacteria (then Eubacteria) into the taxon of the prokaryotes (Procaryota), contrary to the (probable) relationships , because they are organisms whose cells do not have a nucleus have a different organizational form than the eukaryotes (eukaryota), in whose cells one is located. The structural difference that went hand in hand with the “invention” of the cell nucleus is therefore considered to be more important than the most accurate possible representation of the evolutionary family tree.
  • Another example can be seen in the two groups of birds (Aves) and reptiles (Reptilia). They are listed as equal classes in classical taxonomy, although today it is undisputed that the former evolved from the latter, which is also reflected in the fact that a group of reptiles, the crocodiles (Crocodylia), is more closely related to birds than to others Creeping such as the snakes (Serpentes) - see also Archosaurs (Archosauria). The separation of birds as a separate class, however, is suggested by traditional taxonomists such as Mayr or Simpson not only with reference to the significant differences in anatomy and physiology that separate birds from reptiles today, but also with regard to the significant ecological role change that came with the Conquest of the airspace was connected, viewed as justified.
  • The seals (Pinnipedia), which are descended from land-dwelling predators (Carnivora), but are often listed as an independent order of the same rank as the predators due to the extreme change in their habitat , are a comparable case.
Classical classification of terrestrial vertebrates

In any case, it is characteristic that not only the criterion of ancestral relationship, but also significant morphological differences (discontinuities), ecological niche changes, the complexity of the anatomical blueprint or the species diversity of a group are used to delimit a taxon. From this it follows that members of a taxon can, through their further evolutionary development, pass into another taxon of the same rank, as has happened, for example, with the birds, which, as already mentioned, form an equal group next to the reptiles according to the classical view. Conversely, groups formed according to purely phylogenetic criteria that contain sub-taxa that differ morphologically from one another are often rejected from a classical perspective; for example, the summary of the sister groups of birds and crocodiles to form the archosaurs (Archosauria) is rejected as absurd.

While today's supporters of the classical systematics mostly reject polyphyletic taxa, i.e. groups that do not even include their last common ancestor, there is broad agreement that paraphyletic taxa, groups that do not contain all descendants of their last common ancestor, not only allow, but because of the - despite newly introduced levels such as cohort or legion - the limited number of categories are almost inevitable.

An advantage of the classic taxon definition is its comparative stability: Since scientific views about the exact relationships between living things can change much more quickly than views about the external appearance, such a classification is particularly useful for non-biological areas of application such as in agriculture. and forestry of greater practical relevance. However, the disadvantage today is the relative arbitrariness in the delimitation of the taxa: In a given case, without precise information from the taxonomist, outsiders often cannot see which criterion - diversity, ecological niche change or genealogical relationship - was used to delimit the taxon . Critics of the classical taxonomy have therefore ironically demanded, following the English biologist Thomas Henry Huxley, the introduction of a separate realm "Psychozoa" for humans, which should be on an equal footing with the realms of animals, plants and fungi in order to address the significant ecological niche change of humans when leaving the African savannah to bring to bear taxonomically.

Taxa in cladistic taxonomy

The so-called cladistic taxonomy, which goes back to the German systematist and entomologist Willi Hennig and is based on the results of the cladistic systematics, takes a completely different approach to delimiting taxa . Its basic principle is that only natural groups of living things should be named. As such, only reproductive and descent communities are accepted, i.e. groups of living beings that form a common gene pool (species) or contain all descendants of a common ancestor. These monophyletic taxa are also called clades (plural, from singular the clade ) or monophyla. Criteria such as morphological discontinuities, species diversity, etc. are deliberately not used to delimit a taxon; the classification should therefore only faithfully reproduce the natural relationships between living beings. Among cladist-oriented taxonomists, the term taxon is sometimes used as a synonym for clade, since other taxa are not accepted as valid.

The rejection of paraphyletic groups goes hand in hand with the rejection of the categories that were seen as remnants of the typologically oriented classification of the 18th and 19th centuries. According to the cladistic taxonomy, with the exception of the somewhat differently defined species, they are considered to be biologically meaningless, because in nature there is sometimes a vertical but no horizontal order of taxa: It makes sense to speak of the carnivores (Carnivora ) are a subgroup of mammals (Mammalia), which in turn form a subgroup of vertebrates (Vertebrata). On the other hand, the statement that the ducks (Anatidae) have the same rank as the composites (Asteraceae) is considered scientifically empty, so the comparability of taxa of equal rank from a classical perspective is disputed. In order not to jeopardize the transmission of biological information, the old taxon names are almost always retained - without taking into account the ranks contained in these names.

In a cladistic system, the meaning of the traditional ranks is transferred to the so-called sister groups (Adelphotaxa), under which one understands two taxa that once emerged from a parent species through the process of speciation, which after the splitting then became more strictly cladistic Perception as extinct, i.e. no longer valid. Through further splitting processes (cladogenesis), two groups of living beings, which are now numerous species, can develop from the two original sister species over time, but which are in the same relationship - as Adelphotaxa - to one another at all times. Phenomena such as horizontal gene transfer or, especially in plants, hybrid formation can complicate this picture, but these processes are not of more fundamental importance in the long term, at least for organisms with a real cell nucleus (eukaryota).

Examples of sister taxa are:

  • the crocodiles (Crocodylia) and birds (Aves), which are grouped together as archosaurs (Archosauria). Only modern living beings are taken into account here, so the term sister taxon changes depending on whether fossil species are also included or not.
  • the ray fins (Actinopterygii) and muscle fins (Sarcopterygii), but only if the latter group also includes the terrestrial vertebrates (Tetrapoda).
Terrestrial vertebrate cladogram; Reptiles dissolved

The term sister taxa must not be confused with the original subspecies from which it emerged. While two sister taxa can ideally be derived from the characteristic analysis of living beings that exist today, their parent species or the two sister species into which it is divided can hardly be reconstructed today, even with the best fossil records. According to the strict opinion, the only exceptions are scientifically observed speciation - an extremely rare phenomenon. For this reason, ancestors or descendants of a taxon are never identified in cladistic taxonomies: All existing (monophyletic) taxa of a given group, which can also include fossil groups, are classified in a system of sister taxa, each of which is a logical summary or subset , but can never be regarded as the historical predecessor or successor of other taxa. This system is represented diagrammatically in a so-called cladogram , a tree diagram in which each branch ideally splits into two sub-branches. At the end of the finest ramifications sit the (fossil or modern) species; the nodes, on the other hand, are not named, they stand for the virtual, that is, never fixed, root species.

When naming taxa according to the cladistic system, certain difficulties arise: As already mentioned, classic taxa often include their rank in the name, and the binary nomenclature for species stipulates that a species name contains information about its genus. Firstly, it is problematic to name taxa located between genus and species, and secondly, a change in the systematic position of a species always entails a renaming. That is why there are efforts today to overcome the previous nomenclature in favor of new naming rules tailored to the cladistic taxonomy. From July 6th to 9th, 2004, a conference took place in Paris at which the International Society for Phylogenetic Nomenclature (ISPN) was founded, which monitors the rules of the so-called PhyloCode developed in international cooperation which, according to its authors, should replace all previous nomenclature regulations such as the botanical (ICBN) or zoological nomenclature code (ICZN) in the long term . The first publication date of the conference reports is given as the start date of the PhyloCode.

Its proponents consider the taxonomy, established according to cladistic criteria, to be the only really scientific method for describing, classifying and naming taxa, which, in contrast to the traditional classification, which is viewed as a purely pigeonhole system, reflects real biological information. Critics such as the aforementioned Ernst Mayr or Gaylord Simpson, on the other hand, consider the cladistic taxonomy to be too unstable and therefore not practical.

As already mentioned at the beginning, the above comparison of the two taxonomies must be viewed as idealized. While it is becoming increasingly common in classical taxonomy to create monophyletic taxa whenever possible and only switch to paraphyletic groups in exceptional cases, the classical binary names are often still in use in cladistic taxonomists, at least at the species and genus level. At the beginning of the 21st century it remains to be seen whether the nomenclature called PhyloCode will become established. Even with higher taxa, paraphyletic groups are still used, at least informally.

Operational Taxonomic Units (MOTU)

With the help of molecular biological methods, taxonomic affiliations can be recognized. After DNA analysis and comparison in gene libraries , molecular operational taxonomic units (MOTUs) can be defined, which can make it possible to define species. This means that 18 species of Saprolegnia have been confirmed and a further 11 potential species have been identified.


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  • Willi Hennig : Basic features of a theory of phylogenetic systematics. Deutscher Zentralverlag, Berlin 1950, Koeltz, Königstein 1980 (repr.), ISBN 3-87429-188-X .
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  • Lynn Margulis , Karlene V. Schwartz: The five realms of the organisms. Spectrum d. Wiss., Heidelberg 1989. ISBN 3-89330-694-3 .
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  • Carl Woese: Default taxonomy, Ernst Mayr's view of the microbial world. in: Proceedings of the National Academy of Sciences (PNAS). NAS, Washington DC 95.1998, p. 11043. ISSN  0027-8424

Web links

Wiktionary: Taxon  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Karin Mölling: Superpower of Life. Travel to the amazing world of viruses. 1st edition, Beck, Munich 2015, ISBN 978-3-406-66969-9 .
  2. ^ Erwin J. Hentschel, Günther H. Wagner: Zoological dictionary . 6th edition. Gustav Fischer Verlag , Jena 1996, p. 427 .
  3. International Commission on Zoological Nomenclature: Usage of 17 specific names based on wild species which are pre-dated by or contemporary with those based on domestic animals (Lepidoptera, Osteichthyes, Mammalia): conserved. Opinion 2027 (Case 3010) . In: Bulletin of Zoological Nomenclature . 60, No. 1, 2003, pp. 81-84.
  4. (English)
  5. Joseph Boxhorn: Observed Instances of Speciation (English)
  6. Mark Blaxter, Jenna Mann, Tom Chapman, Fran Thomas, Claire Whitton, Robin Floyd, Eyualem Abebe: Defining operational taxonomic units using DNA barcode data. In: Philos Trans R Soc Lond B Biol Sci. 360, No. 1462, October 29, 2005, pp. 1935–1943, doi : 10.1098 / rstb.2005.1725 ( PDF ).
  7. Jose Vladimir Sandoval-Sierra, Maria P. Martín, Javier Diéguez-Uribeondo: Species identification in the genus Saprolegnia (Oomycetes): defining DNA-based molecular operational taxonomic units. In: Fungal Biol. , 118, No. 7, July 2014, pp. 559-578, doi : 10.1016 / j.funbio.2013.10.005 .
This article was added to the list of excellent articles on April 22, 2004 in this version .