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| Five Kingdom Classification |
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| The present trend in biology is to follow the five kingdom classification proposed by R.H.Whittaker in the year 1969. Whittaker classified the living organisms into five kingdoms namely. |
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| 1. KINGDOM: MONERA (prokaryotic organisms) |
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| 2. KINGDOM: PROTISTA (primitive eukaryotic organisms) |
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| 3. KINGDOM: MYCOTA (exclusively fungi) |
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| 4. KINGDOM: METAPHYTA (advanced eukaryotic plants) |
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| 5. KINGDOM: METAZOA (all multicellular animals) |
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| According to this classification, Monera represent the earliest group of organisms. The Monera are thought to have given rise to Protista from which the three other kingdoms of organisms namely, the fungi, plants and animals evolved along separate lines. Fungi were the first to appear from Protista. Later, about a billion years ago some protists must have evolved into primitive multicellular animals. Still later, probably about 350 million years ago, some protists must have evolved into higher forms of plants. |
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| fig. 7.9 - The Five Kingdom System of Classification |
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| The five-kingdom classification has certain merits and demerits. However, it is largely the most accepted system of modern classification mainly because of the phylogenetic placing of different groups of living organisms. |
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| This system of classification looks more scientific and natural because of the following considerations: |
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 Separation of prokaryotes into an independent kingdom is justifiable because they differ from all other organisms in their general organization. |
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 Grouping of all unicellular eukaryotes under the kingdom Protista has solved many problems, particularly related to the position of organisms like Euglena. |
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 Elevation of the group fungi to the status of a kingdom is justifiable since fungi totally differ from other primitive eukaryotes like algae and protozoans. |
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 The kingdoms Metaphyta and Metazoa are now more homogenous groups than they were in the two kingdom classification as it shows the phylogeny of different life styles. |
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 The five-kingdom classification gives a clear indication of cellular organization and modes of nutrition, the characters which appeared very early in the evolution of life. However, the five-kingdom classification has certain drawbacks also, particularly with reference to the lower forms of life. |
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 The kingdoms Monera and Protista include diverse, heterogeneous forms of life. In both the kingdoms there are photosynthetic (autotrophic) as well as non-photosynthetic (heterotrophic) organisms. |
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 Both the kingdoms include organisms which have cells with cell wall as well as without cell wall. |
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 None of the three higher kingdoms include a single ancestor of all its forms. Multicellular lines have originated from protistans several times (polyphyletic). |
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 Unicellular green algae like Volvox and Chlamydomonas have not been included under Protista because of their resemblance to other green algae. |
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 Slime moulds differ totally from other members of Protista in their general organization. |
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 Viruses have not been given proper place in this system of classification. |
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| Nevertheless, the five-kingdom classification has found a wide acceptance with biologists all over the world. |
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| One of the unsolved mysteries in biology is that of viruses. Due to the absence of a cellular organization viruses cannot be placed with either prokaryotes or eukaryotes. They are considered as intermediate between living and non-living systems. Viruses are active and show reproduction only inside the host cell. In the free state they are totally inactive. They may even be purified and crystallized like chemical substances. Viruses have a genetic material represented by either DNA or RNA, surrounded by a protein sheath. Viruses reproduce by using the metabolic machinery and raw materials of the host cell. |
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| It is because of these peculiarities that viruses do not fit into any of the five kingdoms of life. |
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| Bacteria also present certain taxonomic problems. The bacterial cell has a cell wall like other plant cells. However, most bacteria show flagella which are used for locomotion. Some bacteria are autotrophic while most of them cannot prepare their own food. They have prokaryotic cells which lack a definite nucleus and cytoplasmic (membrane-bound) organelles. They also do not have a mitotic apparatus (during cell division) and do not exhibit meiosis. In view of these features, some of which they share with blue green algae, it is imminent that bacteria are to be given a separate taxonomic status. The five-kingdom classification has given a justification to bacteria by placing them in a separate kingdom called Monera. |
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