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Phylum

Phylum, in biology, major category, or taxon, of organisms with a common design or organization. This design is shared by all members of the phylum, even though structural details may differ greatly because of evolution. The assumption is made by biologists that all members of a phylum have a common ancestry.

A phylum is part of the hierarchy of classification of organisms. It is an arbitrary grouping; that is, it is developed from a combination of scientific observation, theorizing, and guesswork in an attempt to find order in the complexity of living and extinct life forms. The same is true of all classification levels above and below it except for species, which consist of organisms known to be capable, at least potentially, of interbreeding (see Species and Speciation).

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Classification Methods

Grouping organisms according to shared characteristics is not a simple task, and scientists often disagree about the best way to classify organisms. Some think that organisms should be grouped according to differences or similarities in the way they look or act. Other scientists argue that classification should be based on characteristics derived from a shared evolution. Conflicting philosophies about classification have resulted in a variety of classification methods, each with their own set of assumptions, techniques, and results.

How Species Are Grouped

Scientists classify organisms using a series of hierarchical categories called taxa (taxon, singular). This hierarchical system moves upward from a base containing a large number of organisms with very specific characteristics. This base taxon is part of a larger taxon, which in turn becomes part of an even larger taxon. Each successive taxon is distinguished by a broader set of characteristics.

The base level in the taxonomic hierarchy is the species. Broadly speaking, a species is a group of closely related organisms that are able to interbreed and produce fertile offspring. On the next tier of the hierarchy, similar species are grouped into a broader taxon called a genus (genera, plural). The remaining tiers within the hierarchy are formed by grouping genera into families, then families into orders, and orders into classes. In the classification of animals, bacteria, protists (unicellular organisms, such as amoebas, with characteristics of both plants and animals), and fungi, classes are grouped into phyla (see Phylum), while plant classes are grouped into divisions. Both phyla and divisions are grouped into kingdoms. Some scientists go on to group kingdoms into domains.

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Taxonomy

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Taxonomy, science of classifying organisms. Probably the first scientific study of plants was the attempt to classify them. At first, because of the limited knowledge of plant structures, artificial classifications, beginning with the most ancient one into herbs, shrubs, and trees, were necessary. These simple categories merely cataloged, in a tentative way, the rapidly accumulating material, in preparation for a classification based on natural relationships. Modern taxonomic classification, based on the natural concepts and system of the Swedish botanist Carolus Linnaeus, has progressed steadily since the 18th century, modified by advances in knowledge of morphology, evolution, and genetics.

For more information about Taxonomy, read the full article at wikipedia.org.

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Classification of Organisms

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The classification of living organisms has been controversial throughout time, and these schemes are among those in use today. Firstly, Aristotle’s system distinguished only between plants and animals on the basis of movement, feeding mechanism, and growth patterns. This system groups prokaryotes, algae, and fungi with the plants, and moving, feeding protozoa with the animals. Then, the increasing sophistication of laboratory methods and equipment, however, revealed the differences between prokaryotic and eukaryotic cells, prompting a classification system that reflects them; then most recently, five kingdoms have emerged to take both cellular organization and mode of nutrition into account.

Greek philosopher Aristotle (384-322 bc) grouped life forms as either plant or animal. Microscopic organisms were unknown.

Plants - Plants and Fungi
Animals - Animals

In 1735 Swedish naturalist Carolus Linnaeus formalized the use of two Latin names to identify each organism, a system called binomial nomenclature. He grouped closely related organisms and introduced the modern classification groups: kingdom, phylum, class, order, family, genus, and species. Single-celled organisms were observed but not classified.

Kingdom Plantae - includes Plants and Fungi organisms
Kingdom Animalia - Animals

In 1866 German biologist Ernst Haeckel proposed a third kingdom, Protista, to include all single-celled organisms. Some taxonomists also placed simple multicellular organisms, such as seaweeds, in Kingdom Protista. Bacteria, which lack nuclei, were placed in a separate group within Protista called Monera.

Kingdom Protista - includes all single-celled organisms, such as amoebas and diatoms, and sometimes simple multicellular organisms such as seaweeds.
Kingdom Plantae - Plants
Kingdom Animalia - Animals

In 1938 American biologist Herbert Copeland proposed a fourth kingdom, Monera, to include only bacteria. This was the first classification proposal to separate organisms without nuclei, called prokaryotes, from organisms with nuclei, called eukaryotes, at the kingdom level.

PROKARYOTES

Kingdom Monera (Prokaryote) - Bacteria

EUKARYOTES

Kingdom Protista - includes Bacteria Amoebas, diatoms, and other single-celled eukaryotes, and sometimes simple multicellular organisms, such as seaweeds.
Kingdom Plantae - includes Plants and Fungi.
Kingdom Animalia - Animals

In 1957 American biologist Robert H. Whittaker proposed a fifth kingdom, Fungi, based on fungi’s unique structure and method of obtaining food. Fungi do not ingest food as animals do, nor do they make their own food, as plants do; rather, they secrete digestive enzymes around their food and then absorb it into their cells.

Kingdom Monera (Prokaryote) - Bacteria
Kingdom Protista - includes amoebas, diatoms, and other single-celled eukaryotes, and sometimes simple multicellular organisms, such as seaweeds.
Kingdom Fungi - includes multicellular, filamentous organisms that absorb food.
Kingdom Plantae - includes multicellular organisms that obtain food through photosynthesis.
Kingdom Animalia - includes multicellular organisms that ingest food.

In 1990 American molecular biologist Carl Woese proposed a new category, called a Domain, to reflect evidence from nucleic acid studies that more precisely reveal evolutionary, or family, relationships. He suggested three domains, Archaea, Bacteria, and Eucarya, based largely on the type of ribonucleic acid (RNA) in cells.

PROKARYOTES
Domain: Archaea
Kigdoms:

Crenarchaeota - includes ancient bacteria that produce methane.
Euryachaeota - includes ancient bacteria that grow in high temperatures.

Domain: Bacteria

EUKARYOTES
Domain: Eucarya
Kingdoms:

Protista
Fungi
Plantae
Animalia