Phylum

A phylum (plural: phyla)"Phylum" is adopted from the Greek polytonic|φυλαί "phylai", the clan-based voting groups in Greek city-states.] is a taxonomic rank between Kingdom and above Class. "Phylum" is equivalent to the botanical term division. [cite book
title=The American Heritage New Dictionary of Cultural Literacy
url=http://dictionary.reference.com/browse/phylum
accessdate=2008-10-04
edition=third edition
year=2005
publisher=Houghton Mifflin Company
chapter=Life sciences
quote=Phyla in the plant kingdom are frequently called divisions.]

Although a phylum is often spoken of as if it were a hard and fast entity, no satisfactory definition of a phylum exists. Consequently the number of phyla varies from author to author. The relationship of phyla is increasingly well known, and larger clades can be erected to contain many of the phyla.

Informally, phyla can be thought of as grouping animals based on general body plan, [cite book
last=Valentine
first=James W.
year=2004
title=On the Origin of Phyla
publisher=University Of Chicago Press
location=Chicago
id=0226845486
pages=7"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."] developmental or internal organizations. [cite book
last=Parker
first=Andrew
year=2003
title=In the blink of an eye: How vision kick-started the big bang of evolution
publisher=Free Press
location=Sydney
id=0743257332
pages=1–4"The job of an evolutionary biologist is to 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."] 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. 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 oncyophorans were marine.citation
author=Davidson, E. H.; Erwin, D. H.
year=2006
journal=Science
volume=311
issue=5762
pages=796–800
url=http://www.sciencemag.org/cgi/content/abstract/311/5762/796
publisher=American Association for the Advancement of Science]

The origin of phyla has traditionally been interpreted as a sudden and rapid event early in the Cambrian period, known as the Cambrian explosion. [Wonderful life] However, this interpretation stemmed from an incomplete knowledge of the fossil record, and a circular definition of a phylum; organisms of the time were mainly similar to, but not strictly members of, modern phyla.cite journal
author=Budd, G.E.
coauthors=Jensen, S.
year=2000
title=A critical reappraisal of the fossil record of the bilaterian phyla
journal=Biological Reviews
volume=75
issue=02
pages=253–295
doi=10.1017/S000632310000548X
url=http://www.journals.cambridge.org/abstract_S000632310000548X
accessdate=2007-05-26] The significance of this event depends on (1) for how long modern phyla had existed prior to the Cambrian – fossil embryos and new interpretations of the Ediacara biota suggest that there may be a fair Precambrian root to the phyla; and (2) how soon phyla appeared in the Cambrian, a factor that depends on both the definition of a phylum and our interpretation of early fossils which may not display vital characteristics – for instance, non-mineralised parts of organisms rarely preserve.citation
author=Briggs, D. E. G.; Fortey, R. A.
year=2005
title=Wonderful strife: systematics, stem groups, and the phylogenetic signal of the Cambrian radiation
journal=Paleobiology
volume=31
issue=2 (Suppl)
pages=94–112
doi=10.1666/0094-8373(2005)031 [0094:WSSSGA] 2.0.CO;2]

The magnitude of the event was also overestimated as early authors felt it necessary to erect a new phylum for any organism that could not be accommodated in modern phyla. This approach is misleading and unhelpful; by one definition, such organisms do not fall into any phylum, but are classified as "aunts" of a phylum.

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 relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to 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.

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? Implicitly, the general definition is that they should be closely enough related that it is clear that they are more closely related to one another than to any other group. However, this too is problematic, as the definition is a function of our current knowledge about relationships. As more data becomes available, particularly from molecular studies, we are better able to judge the relationships between groups; therefore the phyla will eventually be merged as it becomes apparent that they are related to one another; for instance, the onychophora and tardigrada are now accepted as stem group arthropods; by the general definition, these three phyla should be combined.

This has led to calls for the concept of a phylum to be abandoned in favour of cladistics, a method in groups are placed on a "family tree" without any formal ranking of group size. 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 palaeontolgists Graham Budd an Sören Jensen. The definition was posited by palaeontologists 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 aquired.

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. 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.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.citation
author = Budd, G.E.
year = 1998
journal = Lethaia
volume = 31
issue = 3
pages = 197–210
url = http://www.blackwell-synergy.com/doi/abs/10.1111/j.1502-3931.1998.tb00508.x
publisher = Blackwell Synergy

]

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.

Lists

Animal phyla

Fungal divisions

ee also

*Cladistics
*Phylogenetics
*Systematics
*Taxonomy

References

External links

* [http://waynesword.palomar.edu/trnov01.htm Major Phyla Of Animals]
* [http://www.pandasthumb.org/archives/2005/04/down_with_phyla_1.html Are phyla "real"? Is there really a well-defined "number of animal phyla" extant and in the fossil record?]

Etymology:
* [http://www.bartleby.com/61/54/P0275400.html American Heritage Dictionary] : New Latin phylum, from Greek phūlon, class.
* [http://www.etymonline.com/index.php?l=p&p=15 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"