Modern classification of organism
The modern classification is simply an extension of the system of classification proposed by Corollus Linnaeus. Linnaeus introduced four categories namely class, order, genus and species in each of the two kingdoms proposed by him- the plant kingdom and the animal kingdom. The present system of classification involves a series of taxonomic ranks or taxa. In addition to the four categories introduced by Linnaeus, two more- phylum which comes prior to a class and family which comes prior to a genus have been introduced. This is now called as Linnaean hierarchy.
The Linnaean hierarchy
The Linnaean system of classification consist of a hierarchy of graded taxonomic ranks that are called taxa. Any given taxon may contain several lower taxa which can be usually distinguished based on certain common characteristics. Such lower ranks may in turn be divided into a succession of progressively smaller ranks. The lower the rank of a group, the more similar are the organisms grouped in it. If any two given organisms can be grouped under the same lower rank of taxon, it implies that the two organisms are structurally, functionally, embryo logically similar and that they have had a comparable evolutionary history.
Within the living world as a whole, the biggest taxonomic rank is kingdom. The next higher rank within a kingdom is the phylum or division. It is customary to use the term phylum for major groups in the animal kingdom and the term division for major groups in the plant kingdom. The phylum or division is a broad grouping of more or less closely related organisms, sharing certain common characteristics.
Each phylum or division has the next taxon called class. The members of each class exhibit certain distinguishing characters that are unique only to them.
In the same way, using comparable criteria of similarities and relationships, each class can be divided into orders, each order into families, each family into genera and each genus into species. Species is normally the basic or fundamental unit of classification. A species is therefore the narrowest taxonomic category and kingdom is the broadest category in the Linnaean hierarchy.
A typical Linnaean hierarchy has seven taxa represented are Kingdom-Phylum-Class-Order-Family-Genus-Species.
Many a times, it may become necessary to make a distinction between two consecutive taxonomic ranks. In such a situation, an additional rank may be introduced between any two existing ranks. A prefix sub or super is added to such new ranks. For example, between a phylum and a class, there may be sub phylum or a super class. Similarly, between a class and an order there may be a sub class or super order.
Thus, the extended Linnaean hierarchy may consist of Kingdom-Sub Kingdom-Phylum-Sub Phylum-Class-Sub Class- Order- Sub Order- Family- Sub Family- Genus- Species.
In some cases, it is not universally agreed whether a given group of organisms represents a distinct phylum or sub phylum and so on. In fact, among taxonomists there is a general agreement regarding lower taxonomic ranks, rather than the higher ones. The higher rank categories are actually being re shuffled more or less constantly. This is probably for the reason that these rankings incorporate the recent knowledge on the evolutionary history of these organisms. As this knowledge improves, the rank categories are rearranged accordingly.