1:59 PM
Goohara
Rubus Berry Plants
Phylum
Picture Of
Rubus Berry Plants
Rubus Berry Plants
Phylum
In biology, a phylum (plural: phyla) is a taxonomic rank below Life, Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species and above Class. "Phylum" is equivalent to the botanical term division. Phylum is one of the major biological divisions called Taxa. The Kingdom Animalia contains approximately forty phyla. The relationships among phyla are becoming increasingly well known, and larger clades can be found to contain many of the phyla.
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 36 animal 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.
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. 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.
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.
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.
General description and familiar examples
Informally, phyla can be thought of as grouping organisms based on general body plan, as well as developmental or internal organizations. 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 36 animal 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.
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). 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. 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. 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. 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.
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.
Related : Phylum From Wikipedia, the free encyclopedia, Rubus Berry Plants
0 ความคิดเห็น:
Post a Comment