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| The Linnaean Hierarchy |
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| The Linnaean system of classification consists of a hierarchy of graded taxonomic (named) ranks that are called as taxa. Any given taxon (singular) 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 or taxon, it implies that the two organisms are structurally, functionally, embryologically similar and that they have had a comparable evolutionary history. |
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| 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. |
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| 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. |
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| 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. |
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| A typical Linnaean hierarchy has seven taxa represented as follows: |
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| fig. 7.2 - Linnaean Hierarchy |
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| 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 subphylum or a superclass. Similarly between a class and an order there may be a subclass or superorder. |
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| Thus, the extended Linnaean hierarchy may consist of the following taxonomic ranks. |
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| fig. 7.3 - Extended Linnaean Hierarchy |
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| In some cases, it is not universally agreed whether a given group of organisms represent 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 reshuffled more or less constantly. This is probably for the reason that these rankings incorporate most recent knowledge on the evolutionary history of these organisms. As this knowledge improves, the rank categories are rearranged accordingly. |
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| Species is generally the lowest taxonomic rank representing organisms that are very much closely related to one another. A species is defined as a group of closely related, structurally and functionally similar organisms which can breed among themselves, producing fertile offspring and which are reproductively isolated from such other groups. The members of a species could be spread over a wide geographical area in which considerable, constant environmental variations occur. Hence, a species is considered to be made up of different populations from different geographical areas and it is such local populations that often become the units of study rather than the entire species. |
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| All present day human beings, irrespective of the nationality, colour, caste, creed or religion, belong to a single species called Homo sapiens. |
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