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The science of the classification of organisms

Many different organisms exist here on this planet, with it estimated that there is at least 5 million species. In order for everyone to know which one you are talking about each on has to be given a name. With such a large number of organisms, how is this achieved in such a way that it is easy to understand and follow? Well, this is where the field of taxonomy comes in. Taxonomy deals with the classification of organisms in a logical manner.

So how are organisms classified? Originally people grouped organisms into basic groups, such as fish, horses, cats etc. During the Middle Ages these were written in Latin as this was the language of the scholars of that period. This works fine unless you are talking about a specific type of fish or want to know how each is related to each other. So up until around 1750 a whole series of additional terms were used, each one describing something in particluar about that organism. These string of terms could be up to 12 words in length, and each scholar would make up their own name or alter a previous on to suit themselves. This method as you can see is rather cumbersome and makes it difficult to know that you are talking about the same organism.

In the 1750's a biologist, Carl Linnaeus, introduced a shorthand way of naming the organisms. This consisted of two parts, and was found to be a much more convenient method that is was very quickly adopted as the standard. A precise set of rules were established such that no matter where or what language is spoken a uniform method of communication is used. The first name is the Genus of the organism, with the second designating the Species.

Now that there is a uniform method of naming organisms, how are these classified, showing the relationship between them all? This is achieved by grouping organisms with similar characteristics under the one group. And each group is contained within another along with several others with similar characteristics and split internally into more groups. Therefore there are 'levels' to this classification, if you start at the top, this group consists of some further groups. Then each of these groups are further broken up as you go down the levels, until the species level is reached. Each one of these taxonomic categories or groups is called a taxon.

On the first level is Kingdoms. Five currently are recognised; Prokaryotae(single celled bacteria and blue-green algae), Protista(protozoa, nucleated algae, slime molds), Fungi(absorbers), Plantae(producers), and Animalia(consumers). Each of these is then divided into Phyla, or Divisions for the plants, fungi and algae, of which there is currently 92 in the five kingdoms; such as Arthropoda, Cnidaria, Mollusca, Anthophyta, Chlorophyta. These are then further divided into; Classes, Orders, Families, Genus then Species. There are seven levels present here, but taxonomists also add additional levels such as suborders, infaorders. These are used to clearly define the evolutionary separation of each of the groups. As an example the classification for humans is as follows: Kindom Animalia, Phylum Chordata, Class Mammalia, Order Primates, Family Hominidae, Genus Homo, Species Homo sapiens.

On the final level, species, these are separated based on the following criteria: Species differ from one another in at least on characteristic and generally do not interbreed freely with one another where their ranges overlap in nature.

The good thing about this level based, or hierarchical, system is that if you know which group an organism belongs to, on any level, then you know some of the important characteristics. For example if a organism belongs to the Kingdom Plantae, then it is know it; is a eukaryotic cell, a nuclear envelope is present, mitiochondria is present, chloroplasts are present, cell wall consists of cellulose and other polysaccharides, fertilisation and meiosis is the means of genetic recombination, nutrition obtained via photosynthetic chlorophulls a and b, and has no nervous system.

References

Moe, Martin A., The marine aquarium reference: systems and invertibrates, Updated Ed, Green Turtle:Plantation, 1993.
Raven, Peter H., and Johnson, George B., Biology, 2nd, Times Mirror/Morsby College:St. Louis, 1989.