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In biology, a phylum (;
plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full version of the word or phr ...

plural
: phyla) is a level of classification or
taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, ...
below
kingdom Kingdom may refer to: Monarchy * A type of monarchy * A realm ruled by: **A king, during the reign of a male monarch **A queen regnant, during the reign of a female monarch Taxonomy * Kingdom (biology), a category in biological taxonomy Arts an ...
and above
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
. Traditionally, in
botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek wo ...

botany
the term
division Division or divider may refer to: Mathematics *Division (mathematics), the inverse of multiplication *Division algorithm, a method for computing the result of mathematical division Military *Division (military), a formation typically consisting o ...
has been used instead of phylum, although the
International Code of Nomenclature for algae, fungi, and plants The ''International Code of Nomenclature for algae, fungi, and plants'' (ICN) is the set of rules and recommendations dealing with the formal botanical name A botanical name is a formal scientific name Science (from the Latin Latin (, ...
accepts the terms as equivalent. Depending on definitions, the animal kingdom
Animalia Animals (also called Metazoa) are multicellular Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the L ...

Animalia
contains approximately 31 phyla; the plant kingdom
Plantae Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel t ...
contains about 14, and the fungus kingdom
Fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

Fungi
contains about 8 phyla. Current research in
phylogenetics In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

phylogenetics
is uncovering the relationships between phyla, which are contained in larger
clades A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual cont ...
, like
Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the ...

Ecdysozoa
and
Embryophyta The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that w ...
.


General description

The term phylum was coined in 1866 by
Ernst Haeckel Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology that stu ...

Ernst Haeckel
from the Greek (, "race, stock"), related to (, "tribe, clan"). 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"). ''"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)."'' which translates as: However, 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 ''Stamm'' .e., race(''Phylon''). In
plant taxonomy Plant taxonomy is the science that finds, identifies, describes, classifies, and names plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all livin ...
, 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. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four ' of
Georges Cuvier Jean Léopold Nicolas Frédéric, Baron Cuvier (; 23 August 1769 – 13 May 1832), known as Georges Cuvier, was a French naturalist Natural history is a domain of inquiry involving organisms, including animals, fungus, fungi, and plants, in ...

Georges Cuvier
. Informally, phyla can be thought of as groupings of organisms based on general specialization of
body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
. 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
definition), or a group of organisms with a certain degree of evolutionary relatedness (the
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

phylogenetic
definition). 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. 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 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
annelid The annelids (Annelida , from Latin ', "little ring"), also known as the ringed worms or segmented worms, are a large phylum, with over 22,000 extant taxon, extant species including ragworms, earthworms, and leeches. The species exist in and ha ...
s, so the phyla were merged (the bearded worms are now an annelid
family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society. Ideally, families would off ...
). On the other hand, the highly parasitic phylum
Mesozoa The Mesozoa are minuscule, worm Worms are many different distantly related animals that typically have a long cylindrical tube-like body, no limbs, and no eyes. Worms vary in size from microscopic to over in length for marine polychaete ...
was divided into two phyla (
Orthonectida Orthonectida () is a small phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The pl ...
and
Rhombozoa Dicyemida, also known as Rhombozoa, is a phylum of tiny parasite Parasitism is a symbiotic relationship between species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an ...
) when it was discovered the Orthonectida are probably
deuterostome Deuterostomia (; in Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...
s and the Rhombozoa
protostome Protostomia () is the clade A clade (), also known as a monophyletic group or natural group, is a group of organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ...
s. This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of cladistics, a method in which groups are placed on a "family tree" without any formal ranking of group size.


Definition based on body plan

A definition of a phylum based on body plan has been proposed by paleontologists Graham 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 characters 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. 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. 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. 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.


Known phyla


Animals

Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, 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.


Plants

The kingdom Plantae is defined in various ways by different biologists (see Plant#Current definitions of Plantae, Current definitions of Plantae). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta, to form the clade Viridiplantae. The table below follows the influential (though contentious) Cavalier-Smith's system of classification, Cavalier-Smith system in equating "Plantae" with Archaeplastida, a group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions. 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, 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), 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 order (biology), 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. subclass (biology), subclasses.


Fungi

Phylum Microsporidia is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see #Protista phyla (divisions), Protista, below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), 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.


Protista

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. Protista is a polyphyletic taxon, 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's system of classification, Cavalier-Smith system. Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. The phyla listed here are used for Chromista and Protozoa by the Catalogue of Life, adapted from the system used by the International Society of Protistologists. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista.


Bacteria

Currently there are 29 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN) # Acidobacteria, phenotypically diverse and mostly uncultured # Actinobacteria, High-G+C Gram positive species # Aquificae, only 14 thermophilic genera, deep branching # Armatimonadetes # Bacteroidetes # Caldiserica, formerly candidate division OP5, ''Caldisericum exile'' is the sole representative # Chlamydiae, only 6 genera # Chlorobi, only 7 genera, green sulphur bacteria # Chloroflexi (phylum), Chloroflexi, green non-sulphur bacteria # Chrysiogenetes, only 3 genera (''Chrysiogenes arsenatis'', ''Desulfurispira natronophila'', ''Desulfurispirillum alkaliphilum'') # Cyanobacteria, also known as the blue-green algae # Deferribacteres # Deinococcus-Thermus, ''Deinococcus radiodurans'' and ''Thermus aquaticus'' are "commonly known" species of this phyla # Dictyoglomi # Elusimicrobia, formerly candidate division Thermite Group 1 # Fibrobacteres # Firmicutes, Low-G+C Gram positive species, such as the spore-formers Bacilli (aerobic) and Clostridia (anaerobic) # Fusobacteria # Gemmatimonadetes # Lentisphaerae, formerly clade VadinBE97 # Nitrospira # Planctomycetes # Proteobacteria, the most known phyla, containing species such as ''Escherichia coli'' or ''Pseudomonas aeruginosa'' # Spirochaetes, species include ''Borrelia burgdorferi'', which causes Lyme disease # Synergistetes # Tenericutes, alternatively class Mollicutes in phylum Firmicutes (notable genus: ''Mycoplasma'') # Thermodesulfobacteria # Thermotogae, deep branching # Verrucomicrobia


Archaea

Currently there are five phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN). # Crenarchaeota, second most common archaeal phylum # Euryarchaeota, most common archaeal phylum # Korarchaeota # Nanoarchaeota, ultra-small symbiotes, single known species # Thaumarchaeota


See also

* Cladistics * Phylogenetics * Systematics * Taxonomy (biology), Taxonomy


Notes


References


External links


Are phyla "real"? Is there really a well-defined "number of animal phyla" extant and in the fossil record?


{{Authority control Phyla, Plant divisions,