The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon (φῦλον, 'race, stock'), related to phyle (φυλή, 'tribe, clan').[4][5] Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as a group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock / tribe] (Phylon)."[a] In plant taxonomy, August W. Eichler (1883) classified plants into five groups named divisions, a term that remains in use today for groups of plants, algae and fungi.[1][6]
The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier.[7]
At its most basic, a phylum can be defined in two ways: as a group of organisms with a certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition).[8] Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were.
Definition based on genetic relation
The most important objective measure in the above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group.[8] Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine the relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids, so the phyla were merged (the bearded worms are now an annelid family).[9] On the other hand, the highly parasitic phylum Mesozoa was divided into two phyla (Orthonectida and Rhombozoa) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes.[10]
This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size.[8]
Definition based on body plan
A definition of a phylum based on body plan has been proposed by paleontologistsGraham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characteristics that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by a set of characters shared by all its living representatives.
This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on a subjective decision about which groups of organisms should be considered as phyla.
The approach is useful because it makes it easy to classify extinct organisms as "stem groups" to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities.[8] However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display a character unique to a sub-set of the crown group.[8] Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.[11]
A classification using this definition may be strongly affected by the chance survival of rare groups, which can make a phylum much more diverse than it would be otherwise.[12]
Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species,[13] and some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.[14]
Monociliated epidermis (also in some gastrotrichs, e.g. Chordodasys, Xenodasys[24]), unique jaws (absent in Agnathiella[25]) without manubria (also absent in some rotifers, e.g. Acanthocephala) or homologous elements[26] and a 5-part unpaired "basal plate"[27] (absent in some genera) without a "manus",[26] and occipitalia (possibly absent in Problognathia minima and an undescribed species and genus[28])
Single layer of cells (monoblastic) and through-gut (i.e. anus and mouth), meaning that the cells contact both the internal digestive cavity and external ambient.
Sperm with a subplasmalemmal manchette, nine accessory microtubules surrounding the axoneme, an annulus inside the neck region and an extracellular helical band of secretion surrounding the head[42]
Embryonic development undergoes inside a plasmodium,[43]:13 adults made up only of a simple nervous and muscular system,[43]:13 internal reproductive cells,[43]:13 which are gonochoric (with the exception of Stoecharthrum,[43]:11 and, possibly, Pelmatosphaera[44]), and external cells alternating in rings, of 1–3 cell rows,[43]:13 of ciliated and unciliated cells[43]:13 (except in Stoecharthrum, where cells after ring 22 are all ciliated,[45] and Pelmatosphaera, where all cells are ciliated[44][46])
Body composed of an anterior epistome (prosome), middle lophophore (mesosome), and a posterior trunk (metasome) that has a single pair of metanephridia, tipped with an end bulb (ampulla), actinotroch larva (except in Phoronis ovalis)[39]:638–644
Microscopic, ellipsoidal body shape with pronounced bilateral symmetry expressed by paired spiny sclerites. Single, presumably oral opening on assumed ventral side (no anus).[65]
The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta,[71] while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes),[72] or for conifers alone as below.
Since the first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders, many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses.[73]
Most are saprobes and reproduce sexually and asexually.[90]
approx. 1060
Total: 8
Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain,[92] and it is considered a protozoan by the International Society of Protistologists[93] (see Protista, below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor),[94] which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish the Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina, Kickxellomycotina, Mucoromycotina, and Zoopagomycotina.[92]
The kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi.[75]:120 Protista is a paraphyletic taxon,[95] which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system.[96]
Protist taxonomy has long been unstable,[97] with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by the Catalogue of Life,[98] and correspond to the Protozoa-Chromista scheme,[93] with updates from the latest (2022) publication by Cavalier-Smith.[99] Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP.[100]
Mostly parasitic, at least one stage of the life cycle with flattened subpellicular vesicles and a complete apical complex, non-photosynthetic apicoplast.[100]
Biflagellates with a transverse ribbon-like flagellum with multiple waves beating to the cell's left and a longitudinal flagellum beating posteriorly with only one or few waves.[100]
Ellipsoid or vermiform phagotrophs, two slightly spiraling rows of around 12 cilia each, thecal plates below the membrane supported by microtubules and rotationally symmetrical, tubular and saccular cristae.[100][108]
Complex life cycle containing amoebae, flagellates and cysts.[100]Amoeboflagellates with an amoeba, a flagellate, and a cyst stage in their life cycles. Amoebae usually cylindrical, with a monopodial locomotive form, relatively fast-moving via eruptive lobopodia. Flagellates usually with two or four flagella that arise at the anterior end of a feeding groove. Golgi apparatus lacking a classic stacked form. Mitochondria with discoidal cristae, some species with acristate, hydrogen-producing mitochondrion-related organelles.[111]
Aerobic flagellates (none, 1, 2 or 4 flagella) with dorsal semi-rigid pellicle of one or two submembrane dense layers, ventral feeding groove, branching ventral pseudopodia, typically filose.[107]
Phagotrophic pyriform biflagellates with a unique complex cytoskeleton, tubular cristae, tripartite mastigonemes, cortical alveoli.[118][119]
7
Total: 26, but see below.
The number of protist phyla varies greatly from one classification to the next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae,[98] but other systems consider these phyla part of Protista.[120] In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista, and all alveolates except ciliates in one phylum Myzozoa, later lowered in rank and included in a paraphyletic phylum Miozoa.[99] Even within a phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta. These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete.[100]
Currently there are 5 phyla that have been validly published according to the Bacteriological Code.[121] There were many changes to the phyla and the names used for them in 2023.[122]
Methanobacteriota, a phylum including a range of archaeans, including methanogens and halophilic archaea (halobacteria), according to LPSN. Previously called Euryarchaeota [122]
Promethearchaeota (phylum within the kingdom Promethearchaeati, which was previously known as Asgard Archaea), thought to include the ancestor of Eukaryotes.[124]
Thermoproteota, second most common archaeal phylum (previously known as Crenarchaeota)[125]
↑"Wohl aber ist eine solche reale und vollkommen abgeschlossene Einheit die Summe aller Species, welche aus einer und derselben gemeinschaftlichen Stammform allmählig sich entwickelt haben, wie z. B. alle Wirbelthiere. Diese Summe nennen wir Stamm (Phylon)."
↑"Life sciences". The American Heritage New Dictionary of Cultural Literacy (thirded.). Houghton Mifflin Company. 2005. Retrieved 4 October 2008. Phyla in the plant kingdom are frequently called divisions.
↑Haeckel, Ernst (1866). Generelle Morphologie der Organismen[The General Morphology of Organisms] (in German). Vol.1. Berlin, (Germany): G. Reimer. pp.28–29.
↑Chase, Mark W. & Reveal, James L. (October 2009), "A phylogenetic classification of the land plants to accompany APG III", Botanical Journal of the Linnean Society, 161 (2): 122–127, doi:10.1111/j.1095-8339.2009.01002.x
123Mauseth, James D. (2012). Botany: An Introduction to Plant Biology (5thed.). Sudbury, MA: Jones and Bartlett Learning. ISBN978-1-4496-6580-7. p. 489
↑Crandall-Stotler, Barbara; Stotler, Raymond E. (2000). "Morphology and classification of the Marchantiophyta". In A. Jonathan Shaw; Bernard Goffinet (eds.). Bryophyte Biology. Cambridge: Cambridge University Press. p.21. ISBN978-0-521-66097-6.
↑Wyatt, T.; Wösten, H.; Dijksterhuis, J. (2013). "Advances in Applied Microbiology Chapter 2 - Fungal Spores for Dispersion in Space and Time". Advances in Applied Microbiology. 85: 43–91. doi:10.1016/B978-0-12-407672-3.00002-2. PMID23942148.
↑Holt, Jack R.; Iudica, Carlos A. (1 October 2016). "Blastocladiomycota". Diversity of Life. Susquehanna University. Retrieved 29 December 2016.
↑Holt, Jack R.; Iudica, Carlos A. (9 January 2014). "Chytridiomycota". Diversity of Life. Susquehanna University. Retrieved 29 December 2016.
↑Davis, R. A. (19 March 2012). "Kingdom PROTISTA". College of Mount St. Joseph. Archived from the original on 4 August 2022. Retrieved 28 December 2016.
12"Taxonomic tree". Catalogue of Life. 23 December 2016. Archived from the original on 1 August 2021. Retrieved 28 December 2016.
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123Denis V. Tikhonenkov, Kirill V. Mikhailov, Ryan M. R. Gawryluk, Artem O. Belyaev, Varsha Mathur, Sergey A. Karpov, Dmitry G. Zagumyonnyi, Anastasia S. Borodina, Kristina I. Prokina, Alexander P. Mylnikov, Vladimir V. Aleoshin & Patrick J. Keeling (2022). "Microbial predators form a new supergroup of eukaryotes". Nature. 612: 714–719. doi:10.1038/s41586-022-05511-5. PMID36477531. WikidataQ115933632.{{cite journal}}: CS1 maint: multiple names: authors list (link){{Cite Q}}: Missing first/last names in Wikidata for Denis V. Tikhonenkov, Kirill V. Mikhailov, Ryan M. R. Gawryluk, Artem O. Belyaev, Varsha Mathur, Sergey A. Karpov, Dmitry G. Zagumyonnyi, Anastasia S. Borodina, Kristina I. Prokina, Alexander P. Mylnikov, Vladimir V. Aleoshin & Patrick J. Keeling (help)
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