Phylum

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The hierarchy of biological classification's eight major taxonomic ranks, which is an example of definition by genus and differentia. A kingdom contains one or more phyla. Intermediate minor rankings are not shown.

In biology, a phylum (plural: phyla)^{[note 1]} is a taxonomic rank below Kingdom and above Class. "Phylum" is equivalent to the botanical term division.^[1] Phylum is one of the major biological divisions called Taxa. Although "phylum" is often used as if it were a clearly defined term, no satisfactory definition of it exists. In fact, "phylum" may be a misnomer indicative of ignorance.^[2] Consequently the number of phyla varies from one author to the next. The relationships among phyla are becoming increasingly well known, and larger clades can be found to contain many of the phyla.

General description and familiar examples

Informally, phyla can be thought of as grouping organisms based on general body plan,^[3] as well as developmental or internal organizations.^[4] For example, though seemingly divergent, spiders and crabs both belong to Arthropoda, whereas earthworms and tapeworms, similar in shape, are from Annelida and Platyhelminthes, respectively. Although the International Code of Botanical Nomenclature allows the use of the term "phylum" in reference to plants, the term "Division" is almost always used by botanists.

The best known animal phyla are the Mollusca, Porifera, Cnidaria, Platyhelminthes, Nematoda, Annelida, Arthropoda, Echinodermata, and Chordata, the phylum to which humans belong, along with all other vertebrate species, as well as some invertebrates such as the lamprey. Although there are approximately 35 phyla, these nine include over 96% of animal species. Many phyla are exclusively marine, and only one phylum, the Onychophora (velvet worms) is entirely absent from the world's oceans–although ancestral onycophorans were marine.^[5]

Defining a phylum

At the most basic level, 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).^[6] Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is an unsatisfactory approach, but the phenetic definition is more useful when addressing questions of a morphological nature – such as how successful different body plans were.

Definition based on genetic relation

The largest 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 related closely enough for them to be clearly more closely related to one another than to any other group.^[6] However, even this is problematic, as the requirement depends on our current knowledge about organisms' relationships: As more data becomes 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, since the onychophora and the tardigrada have now been accepted as stem groups of the arthropods, these three phyla should be combined.

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.^[6] So as to provide a handle on the size and significance of groups, a "body-plan" based definition of a phylum has been proposed by paleontologists Graham Budd and Sören Jensen. The definition was posited by paleontologists because it is extinct organisms that are typically hardest to classify, because they can be extinct off-shoots that diverged from a phylum's history before the characters that define the modern phylum were all acquired.

Definition based on body plan

By Budd and Jensen's definition, phyla are defined by a set of characters shared by all their living representatives. This has a couple of small problems – for instance, characters common to most members of a phylum may be secondarily lost by some members. It is also defined 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 more major problem is that it relies on an objective decision of which group of organisms should be considered a phylum.

Its utility is that 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.^[6] 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.^[6] Further, organisms in the stem group to a phylum can bear all the aspects of the "body plan" of the phylum without all the characters necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan.^[7]

Based upon this definition, which some say is unreasonably affected by the chance survival of rare groups, which vastly increase the size of phyla, representatives of many modern phyla did not appear until long after the Cambrian – as late as the Carboniferous in the case of the Priapulids.^[8]

Lists

Animal phyla

Groups formerly ranked as phyla

Plant divisions

Fungal divisions

See also

• Cladistics
• Phylogenetics
• Systematics
• Taxonomy

Notes

1. ↑ "Phylum" is adopted from the Greek φυλαί phylai, the clan-based voting groups in Greek city-states.

References

1. ↑ "Life sciences". The American Heritage New Dictionary of Cultural Literacy (third ed.). Houghton Mifflin Company. 2005. http://dictionary.reference.com/browse/phylum. Retrieved 2008-10-04. "Phyla in the plant kingdom are frequently called divisions." 
2. ↑ Bengtson,, S. (1986). "Introduction: The problem of the problematica,". In Hoffman, A.; Nitecki, M. H.. Problematic Fossil Taxa. Oxford,: Oxford University Press,. pp. 3–11. 
3. ↑ Valentine, James W. (2004). On the Origin of Phyla. Chicago: University Of Chicago Press. pp. 7. ISBN 0226845486. "Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their what? 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."
4. ↑ Parker, Andrew (2003). In the blink of an eye: How vision kick-started the big bang of evolution. Sydney: Free Press. pp. 1–4. ISBN 0743257332. "The job of an evolutionary biologist is to create dinosaurs for the devil to consume, make sense of the conflicting diversity of form – there is not always a relationship between internal and external parts. Early in the history of the subject, it became obvious that internal organisations were generally more important to the higher classification of animals than are external shapes. The internal organisation puts general restrictions on how an animal can exchange gases, obtain nutrients and reproduce."
5. ↑ Davidson, E. H; Erwin, D. H (2006). "Gene Regulatory Networks and the Evolution of Animal Body Plans". Science (American Association for the Advancement of Science) 311 (5762): 796–800. doi:10.1126/science.1113832. PMID 16469913. http://www.sciencemag.org/cgi/content/abstract/311/5762/796. 
6. ↑ ^{6.0 6.1 6.2 6.3 6.4} Budd, G.E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews 75 (02): 253–295. doi:10.1017/S000632310000548X. http://www.journals.cambridge.org/abstract_S000632310000548X. Retrieved 2007-05-26. 
7. ↑ Budd, G.E. (1998). Lethaia (Blackwell Synergy) 31 (3): 197–210. doi:10.1111/j.1502-3931.1998.tb00508.x. http://www.blackwell-synergy.com/doi/abs/10.1111/j.1502-3931.1998.tb00508.x. 
8. ↑ 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. 
9. ↑ Feldkamp, S. (2002) Modern Biology. Holt, Rinehart, and Winston, USA. (pp. 725)
10. ↑ Species Register. "Flatworms — Phylum Platyhelminthes". Marine Discovery Centres. http://www.woodbridge.tased.edu.au/mdc/Species%20Register/phylum_platyhelminthes.htm. Retrieved 2007-04-09. 
11. ↑ ""Kingdom Plantae Tree of Life"". http://www.fossilmuseum.net/Tree_of_Life/KingdomPlantae.htm. 

External links

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

Etymology:

• American Heritage Dictionary: New Latin phylum, from Greek phūlon, class.
• Online Etymological Dictionary: from Gk. phylon "race, stock," related to phyle "tribe, clan," and phylein "bring forth" of physikos "pertaining to nature," from physis "nature"