In biology, a phylum (/ˈfaɪləm/; plural: phyla)[note 1] is a taxonomic rank below kingdom and above class. Traditionally, in botany the term division was used instead of "phylum", although from 1993 the International Code of Nomenclature for algae, fungi, and plants accepted the designation "phylum". The kingdom Animalia contains approximately 35 phyla, Plantae contains 12, and Fungi contains 7. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.
General description and familiar examples
The definitions of zoological phyla have changed importantly from their origins in the six Linnaean classes and the four "embranchements" of Georges Cuvier. Haeckel introduced the term phylum, based on the Greek word phylon ('tribe' or 'stock'). In plant taxonomy, Eichler (1883) classified plants into five groups, named divisions.
Informally, phyla can be thought of as grouping organisms based on general specialization of body plan. 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). 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"—how unrelated do organisms need to be 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 judge 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). 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.
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. Representatives of many modern phyla did not appear until long after the Cambrian.
|Others (Radiata or Parazoa)|
|Phylum||Meaning||Common name||Distinguishing characteristic||Species described|
|Acanthocephala||Thorny headed worms||Thorny-headed worms||Reversible spiny proboscis that bears many rows of hooked spines||approx. 1,100|
|Acoelomorpha||Without gut||Acoels||No mouth or alimentary canal (alimentary canal = digestive tract in digestive system)||approx. 350|
|Annelida||Little ring||Annelids||Multiple circular segment||17,000+ extant|
|Arthropoda||Jointed foot||Arthropods||Segmented bodies and jointed limbs, with Chitin exoskeleton||1,134,000+|
|Brachiopoda||Arm foot||Lamp shells||Lophophore and pedicle||300-500 extant|
|Bryozoa||Moss animals||Moss animals, sea mats||Lophophore, no pedicle, ciliated tentacles, anus outside ring of cilia||5,000 extant|
|Chaetognatha||Longhair jaw||Arrow worms||Chitinous spines either side of head, fins||approx. 100 extant|
|Chordata||With a cord||Chordates||Hollow dorsal nerve cord, notochord, pharyngeal slits, endostyle, post-anal tail||approx. 100,000+|
|Cnidaria||Stinging nettle||Anemones / Jellyfish||Nematocysts (stinging cells)||approx. 11,000|
|Ctenophora||Comb bearer||Comb jellies||Eight "comb rows" of fused cilia||approx. 100 extant|
|Cycliophora||Wheel carrying||Symbion||Circular mouth surrounded by small cilia, sac-like bodies||3+|
|Echinodermata||Spiny skin||Echinoderms||Fivefold radial symmetry in living forms, mesodermal calcified spines||approx. 7,000 extant; approx. 13,000 extinct|
|Entoprocta||Inside anus||Goblet worm||Anus inside ring of cilia||approx. 150|
|Gastrotricha||Hair stomach||Hairybacks||Two terminal adhesive tubes||approx. 690|
|Gnathostomulida||Jaw orifice||Jaw worms||approx. 100|
|Hemichordata||Half cord||Acorn worms, pterobranchs||Stomochord in collar, pharyngeal slits||approx. 100 extant|
|Kinorhyncha||Motion snout||Mud dragons||Eleven segments, each with a dorsal plate||approx. 150|
|Loricifera||Corset bearer||Brush heads||Umbrella-like scales at each end||approx. 122|
|Micrognathozoa||Tiny jaw animals||—||Accordion-like extensible thorax||1|
|Mollusca||Soft||Mollusks / molluscs||Muscular foot and mantle round shell||112,000|
|Nematoda||Thread like||Round worms||Round cross section, keratin cuticle||25,000–1,000,000|
|Nematomorpha||Thread form||Horsehair worms||approx. 320|
|Nemertea||A sea nymph||Ribbon worms||approx. 1,200|
|Onychophora||Claw bearer||Velvet worms||Legs tipped by chitinous claws||approx. 200 extant|
|Orthonectida||Straight swim||Single layer of ciliated cells surrounding a mass of sex cells||approx. 20|
|Phoronida||Zeus's mistress||Horseshoe worms||U-shaped gut||11|
|Placozoa||Plate animals||Differentiated top and bottom surfaces, two ciliated cell layers, amoeboid fiber cells in between||1|
|Platyhelminthes||Flat worm||Flatworms||approx. 25,000|
|Porifera*||Pore bearer||Sponges||Perforated interior wall||5,000+ extant|
|Priapulida||Little Priapus||Penis worms||approx. 16|
|Rhombozoa||Lozenge animal||—||Single anteroposterior axial cell surrounded by ciliated cells||75|
|Rotifera||Wheel bearer||Rotifers||Anterior crown of cilia||approx. 2,000|
|Sipuncula||Small tube||Peanut worms||Mouth surrounded by invertible tentacles||144–320|
|Tardigrada||Slow step||Water bears||Four segmented body and head||1,000+|
|Xenacoelomorpha||Strange flatworm||—||Ciliated deuterostome||2|
Land plant phyla (divisions)
The ten Divisions into which the living embryophytes (land plants) are often placed are shown in the table below. To these may be added two algal Divisions, Chlorophyta and Charophyta, which are included with land plants in the clade Viridiplantae (see also current definitions of Plantae). The definition and classification of plants at this level varies from source to source and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Pteridophyta, while others place them both in Pteridophyta, 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 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.
|Division||Meaning||Common name||Distinguishing characteristics|
|Anthocerotophyta||Anthoceros-like plants||hornworts||horn-shaped sporophytes, no vascular system|
|Bryophyta||Bryum-like plants, moss plants||mosses||persistent unbranched sporophytes, no vascular system|
|liverworts||ephemeral unbranched sporophytes, no vascular system|
|clubmosses & spikemosses||microphyll leaves, vascular system|
|Pteridophyta||Pteris-like plants, fern plants||ferns & horsetails||prothallus gametophytes, vascular system|
|conifers||cones containing seeds and wood composed of tracheids|
|Cycadophyta||Cycas-like plants, palm-like plants||cycads||seeds, crown of compound leaves|
|Ginkgophyta||Ginkgo-like plants||ginkgo, Maidenhair tree||seeds not protected by fruit (single living species)|
|Gnetophyta||Gnetum-like plants||gnetophytes||seeds and woody vascular system with vessels|
|Magnoliophyta||Magnolia-like plants||flowering plants, angiosperms||flowers and fruit, vascular system with vessels|
Protista phyla (Divisions)
|Group Description||Phylum||Meaning||Common name||Distinguishing characteristics||Example|
|Heterotrophs (no locomotor apparatus)||Rhizopoda||root-foot||Amoeba||Amoeboids have the ability to change their shape.||Amoeba|
|Actinopoda||ray-foot||-------||long, thin axopodia||Radiolarians|
|Foraminifera||hole bearers||Forams||Complex shells with one or more chambers||Forams|
|Photosynthetic protists||Dinoflagellata||whirling scourge||Dinoflagellates||Unicellular, have two dissimilar flagella||Red tides|
|Euglenophyta||Good-eyed plant||Euglenids||Have a pellicle, which gives shape to the cell.||Euglena|
|Chrysophyta||golden plant||golden algae||Diatoma|
|Rhodophyta||rose plant||red algae||Cells do not have flagella or centrioles; use phycobiliproteins which gives red tint||Coralline Algae|
|Phaeophyta||gray plant||brown algae||chloroplasts surrounded by four membranes - form differentiated tissues||Kelp|
|Heterotrophs (flagella)||Sarcomastigophora||Trypanosoma cruzi|
|Heterotrophs (restricted mobility)||Oomycota||Water Molds|
Currently there are 29 phyla accepted by List of Prokaryotic names with Standing in Nomenclature (LPSN)
- Acidobacteria, phenotipically diverse and mostly uncultured
- Actinobacteria, High-G+C Gram positive species
- Aquificae, only 14 thermophilic genera, deep branching
- Caldiserica, formerly candidate division OP5, Caldisericum exile is the sole representative
- Chlamydiae, only 6 genera
- Chlorobi, only 7 genera, green sulphur bacteria
- Chloroflexi, green non-sulphur bacteria
- Chrysiogenetes, only 3 genera (Chrysiogenes arsenatis, Desulfurispira natronophila, Desulfurispirillum alkaliphilum)
- Cyanobacteria, also known as the blue-green algae
- Deinococcus-Thermus, Deinococcus radiodurans and Thermus aquaticus are "commonly known" species of this phyla
- Elusimicrobia, formerly candidate division Thermite Group 1
- Firmicutes, Low-G+C Gram positive species, such as the spore-formers Bacilli (aerobic) and Clostridia (anaerobic)
- Lentisphaerae, formerly clade VadinBE97
- Proteobacteria, the most known phyla, containing species such as Escherichia coli or Pseudomonas aeruginosa
- Spirochaetes, species include Borrelia burgdorferi, which causes Lyme disease
- Tenericutes, alternatively class Mollicutes in phylum Firmicutes (notable genus: Mycoplasma)
- Thermotogae, deep branching
- Crenarchaeota, Second most common archaeal phylum
- Euryarchaeota, most common archaeal phylum
- Nanoarchaeota, ultra-small symbiotes, single known species
- ↑ The term was coined by Haeckel from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan."
- ↑ "Life sciences". The American Heritage New Dictionary of Cultural Literacy (third ed.). Houghton Mifflin Company. 2005. Retrieved 2008-10-04.
Phyla in the plant kingdom are frequently called divisions.
- ↑ Berg, Linda R. (2 March 2007). Introductory Botany: Plants, People, and the Environment (2 ed.). Cengage Learning. p. 15. ISBN 9780534466695. Retrieved 2012-07-23.
- ↑ Collins AG, Valentine JW (2001). "Defining phyla: evolutionary pathways to metazoan body plans." Evol. Dev. 3: 432-442.
- ↑ Valentine 2004, p. 8.
- ↑ Naik, V.N. (1984). Taxonomy of Angiosperms. Tata McGraw-Hill. p. 27. ISBN 9780074517888.
- ↑ Valentine, James W. (2004). On the Origin of Phyla. Chicago: University Of Chicago Press. p. 7. ISBN 0-226-84548-6.
Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy.
- 1 2 3 4 5 Budd, G.E.; Jensen, S. (May 2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews. 75 (2): 253–295. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID 10881389. Retrieved 2007-05-26.
- ↑ Rouse G.W. (2001). "A cladistic analysis of Siboglinidae Caullery, 1914 (Polychaeta, Annelida): formerly the phyla Pogonophora and Vestimentifera". Zoological Journal of the Linnean Society. 132 (1): 55–80. doi:10.1006/zjls.2000.0263.
- ↑ Pawlowski J, Montoya-Burgos JI, Fahrni JF, Wüest J, Zaninetti L (October 1996). "Origin of the Mesozoa inferred from 18S rRNA gene sequences". Mol. Biol. Evol. 13 (8): 1128–32. doi:10.1093/oxfordjournals.molbev.a025675. PMID 8865666.
- ↑ Budd, G. E. (1998). "Arthropod body-plan evolution in the Cambrian with an example from anomalocaridid muscle". Lethaia. Blackwell Synergy. 31 (3): 197–210. doi:10.1111/j.1502-3931.1998.tb00508.x.
- ↑ Briggs, D. E. G.; Fortey, R. A. (2005). "Wonderful strife: systematics, stem groups, and the phylogenetic signal of the Cambrian radiation". Paleobiology. 31 (2 (Suppl)): 94–112. doi:10.1666/0094-8373(2005)031[0094:WSSSGA]2.0.CO;2.
- ↑ Feldkamp, S. (2002) Modern Biology. Holt, Rinehart, and Winston, USA. (pp. 725)
- ↑ Hodda, M (2011). "Phylum Nematoda Cobb, 1932. In: Zhang, Z.-Q. (Ed.) Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness". Zootaxa. 3148: 63–95.
- ↑ Zhang, Z (2013). "Animal biodiversity: An update of classification and diversity in 2013. In: Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013)". Zootaxa. 3703 (1): 5–11. doi:10.11646/zootaxa.3703.1.3.
- ↑ Species Register. "Flatworms — Phylum Platyhelminthes". Marine Discovery Centres. Retrieved 2007-04-09.
- ↑ Mauseth 2012, pp. 514, 517.
- 1 2 Cronquist, A.; A. Takhtajan; W. Zimmermann (1966). "On the higher taxa of Embryobionta". Taxon. International Association for Plant Taxonomy (IAPT). 15 (15): 129–134. doi:10.2307/1217531. JSTOR 1217531.
- ↑ Chase, Mark W. & Reveal, James L. (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
- ↑ Mauseth, James D. (2012). Botany : An Introduction to Plant Biology (5th ed.). Sudbury, MA: Jones and Bartlett Learning. ISBN 978-1-4496-6580-7. p. 489
- 1 2 Mauseth 2012, 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. ISBN 0-521-66097-1.
- ↑ Mauseth 2012, p. 509.
- ↑ Mauseth 2012, p. 535.
- ↑ Mauseth 2012, p. 540.
- ↑ Mauseth 2012, p. 542.
- ↑ Mauseth 2012, p. 543.
- ↑ http://www.nicholls.edu/biol-ds/Biol156/Lectures/Protista.pdf
- ↑ http://www.memidex.com/rhizopoda
- ↑ J.P. Euzéby. "List of Prokaryotic names with Standing in Nomenclature: Phyla". Retrieved 2010-12-30.
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