Strain (biology)

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Hierarchy of taxonomic levels (without intermediate levels)

In biological systematics, the stem is a hierarchical rank of the taxonomy . A trunk in this sense is also called a phylum (plural phyla ). In eukaryotes (living beings whose cells have a nucleus ) each tribe is subordinate to a kingdom . In contrast, prokaryotes ( bacteria and archaea ) are not divided into realms, but directly into tribes.

Among the prokaryotes, “stem” is also used in a different, non-taxonomic meaning, since different lineages within a species are also referred to as tribes.

Stem as a taxonomic rank in eukaryotes

In the domain of the eukaryotes (Eukaryota), the tribe is a hierarchical level between kingdom and class . Every living being described should be assigned to a tribe, so the trunk belongs to the categorical ranks . It is sometimes further divided into sub-tribes ( Subphyla , singular: Subphylum ). Example: The vertebrates are a sub-tribe in the tribe of the chordates . Several strains are in some cases to over strain or Superphylum summarized.


In botany and mycology , the synonym department ( English division , Latin divisio ) is permitted in addition to the stem . The name ending is standardized in botany and mycology, the names end in botany on -phyta, in mycology on -mycota. Traditionally, a distinction is made between nine strains of algae, eight of the higher plants ( Embryophyta ) and four of fungi. According to recent findings, this classification is largely obsolete, but is still widely used.


The founder of modern taxonomy, Carl von Linné , divided the kingdom (regnum) of animals (Animalia) hierarchically into classes (classis), orders (ordo), genus (genus) and species (species). The French anatomist and naturalist Georges Cuvier envisaged four highest-ranking departments in the animal kingdom, which he called embranchements and which roughly correspond to Linné's classes: Vertébrés ( vertebrates ), Mollusques ( molluscs ), Articulés ( articulated animals ) and Radiaires (" radiant animals "); Only then do the Cuvier classes follow .

The term stem ( Latin phylum ) as the highest rank and its name was first introduced by the German zoologist Ernst Haeckel . Haeckel names five strains: coelenterates, echinoderms, articulates, molluscs, vertebrates. The sponges (Spongiae) were recognized by him, but not attributed to the animal kingdom, but to the protists . For Haeckel, tribes were fundamentally different types of organization, each of which can be traced back to a common tribal species.

As a result of the advances in knowledge in zoology, about thirty tribes are currently differentiated (see systematics of multicellular animals ), but the number varies somewhat depending on the scientific understanding. It is difficult to classify the protists . Traditionally conceived as a single tribe, they are currently divided into a plurality of tribes. For example, Thomas Cavalier-Smith distinguished 18 tribes of protists and summarized them as an empire under the name Protozoa .

In science, two classification schemes are currently in use side by side. In addition to the classic systematics, which can be traced back to Linné , the phylogenetic systematics or cladistics founded by Willi Hennig has appeared . In the phylogenetic system, all units should be based on community of descent and include all descendants of a common parent species ( holophyletic units). The successive splits result in a tree structure. Groups that also go back to a common parent species ( i.e. are monophyletic ), but do not include all of their descendants, are called paraphyletic by him . Paraphyletic units are allowed in the classical system, but not in the phylogenetic system. As a result of this reform, the aspect of common form and organization, on which the classic concept of the phylum was based, took a back seat. Most of the morphologically based strains turned out to be holophyletic. But: According to this system, the common building plan is no longer fundamental for the systematic classification, appropriately defined units must be abandoned if they are not holophyletic, even if the taxa summarized in this way could be easily recognized as belonging together through their common building plan.

In general, the classic animal phyla are mostly used as organizational units in the phylogenetic system, but they no longer correspond to a certain, defined rank as in the classic system, so they do not necessarily have to be of equal rank. Some systematists draw the conclusion from this that tribes, like all other ranks of the classical system, are purely arbitrary units that should be abandoned in favor of a pure sister group relationship. Most systematists, however, continue to use stems, like the other ranks of the classical system. This does not result in a contradiction to the phylogenetic system, as long as the (unnecessary) condition is dropped that the sister groups must be of equal rank. The further use has the advantage that far more information can be passed on with an abbreviated notation than with the completely non-illustrative, purely sister-group-based system with its endless sequence of intermediate groups. In addition, comparisons, for example the number of species, are made easier.

This use of the term is only intended to facilitate communication about the organisms. If it can be shown that a group of species that was previously considered a tribe forms a subgroup of another taxon considered a tribe, it is no longer called a tribe in order to avoid misunderstandings. A more recent example is the siboglinidae : In the past they were considered regular Pogonophora now than the family Siboglinidae the root of annelids (Annelida).

As a trunk or phylum, a group of organisms with a similar shape or a similar " blueprint " is usually summarized at present, if this is so dissimilar to that of the members of other tribes that they cannot both be combined on a common basic pattern. With a few exceptions, the current tribes can be traced back to the Cambrian on the basis of fossil finds .

Taxa in the rank of a tribe (and all other taxa above the superfamily ) are not subject to the rules of the ICZN . This means that the rules laid down in the code do not have to be applied to the names of tribes. This applies in particular to the rules for the validity (availability) of names and the principle of priority, according to which the first proposed / oldest name should be used if several names are available for the same group. The names of tribes are therefore more or less fixed by the consensus of the experts involved, who use a certain name - or not. In practice, most scientists also give priority to the tribes and use the traditional names even when they redefine the taxa involved, e.g. B. cut taxa out of them or add others. Deviations mainly occur with followers of the PhyloCode . However, most scientists ignore this newly proposed set of rules.

Tribe as a taxonomic rank among prokaryotes

For the domains bacteria (Bacteria) and Archaea (Archaea), the rank level trunk (phylum) is directly below that of the domain, so these two domains are divided into phyla. The number of tribes is controversial in the river and within science; their number has increased noticeably in recent years. It is currently assumed that there are at least 35 bacterial strains and 18 archaea strains.

Strains within a prokaryotic species and in viruses

No taxonomic rank are as tribes ( english strains ) designated from coming ungslinien of prokaryotes and viruses . These culture strains ( pure culture ) are used, among other things, as a nomenclatory type ( type strain ) to define species (species) and subspecies (subspecies). A strain in this sense consists of a clone , because it is a by asexual reproduction generated population is.

An example of a strain in this sense is the bacterial strain Rhodococcus jostii RHA1 within the bacterial species Rhodococcus jostii . This species is classified as follows in the bacteria system:

That is, within the species Rhodococcus jostii there is the “strain” Rhodococcus jostii RHA1; the species Rhodococcus jostii in turn belongs to the "tribe" (alias division or phylum) of the actinobacteria. The example illustrates the ambiguity of the term strain or bacterial strain . To denote the rank directly below the domain, one can use several synonyms for bacteria : Latin phylum or division (Latin divisio , English division ).

The term stem is also used in this sense for some fungi ( yeasts , molds ). Examples here .

In virology , in accordance with the rules of the International Committee on Taxonomy of Viruses (ICTV) , the term phylum is generally used instead of department or strain (in the sense of a taxonomic rank). The name endings are -viricota for Phyla and -viricotina for Subphyla, Superphyla are currently (as of April 2020) not permitted. In virology, a strain is a non-taxonomic name and is always subordinate to a species.

Individual evidence

  1. Ernst Haeckel: General Morphology of Organisms: general principles of the organic science of forms, mechanically justified by the descent theory reformed by Charles Darwin . G. Reimer Verlag, Berlin 1866, p. 408: The tribes of the animal kingdom .
  2. For example Zhi-Qiang Zhang (2011): Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness. Zootaxa 3148, pp. 9-11.
  3. ^ T. Cavalier-Smith: Kingdom Protozoa and Its 18 Phyla . In: Microbiological Reviews . Vol. 57, No. 4, 1993, pp. 953-994.
  4. Peter Ax: The Phylogenetic System. Systematization of living nature on the basis of its phylogenetic lake. Gustav Fischer Verlag, 1984. ISBN 978-3-437-30450-7 .
  5. Markus Lambertz, Steven F. Perry (2015): Chordate phylogeny and the meaning of categorial ranks in modern evolutionary biology. Proceedings of the Royal Society B 282, 1807. doi: 10.1098 / rspb.2014.2327
  6. Gonzalo Giribet, Gustavo Hormiga, Gregory D. Edgecombe (2016): The meaning of categorical ranks in evolutionary biology. Organisms Diversity & Evolution 16 (3): pp. 613-639. doi: 10.1007 / s13127-016-0263-9
  7. James W. Valentine: On the Origin of Phyla . University of Chicago Press, Chicago 2004. ISBN 978-0-226-84549-4
  8. André Adoutte, Guillaume Balavoine, Nicolas Lartillot, Renaud de Rosa: evolution Animal: the end of the intermediate taxa? . In: Trends in Genetics . Volume 15, Issue 3, 1999, pp. 104-108.
  9. Philip Hugenholtz (2002): Exploring prokaryotic diversity in the genomic era . Genome Biology Vol. 3, No. 2.
  10. ICTV Master Species List 2018b.v2 (MSL # 34v)