Five kingdom classification:
In biology, kingdom is the highest taxonomic group of living organisms. Classification of organisms can be explained by following categories:
1. Two kingdom classification:
Carolus Linnaeus first came up with two kingdom classification which includes only plantae and animalia. There was no differentiation of the eukaryotes from the prokaryotes; neither unicellular from multicellular, nor photosynthetic from non-photosynthetic.
2. Three kingdom classification:
German biologist Ernst Haeckel (1866) in his book Generalla Morphologie de Organismen suggested a three kingdom classification which he includes Protista, Plantae and Animalia. In the third kingdom protista he grouped all the single celled organisms that are intermediate in many respects between plants and animals.
3. Four kingdom classification:
Herbert Copeland in 1956 have suggested a four kingdom classification, originally called mycota but later referred to as monera including the prokaryotes like bacteria and BGA, which have many characteristics in common like they have a single membrane system without a nucleus and membrane bounded sub- cellular organelles such as mitochondria and chloroplast. All other organisms are eukaryotes and have complex structure with a nucleus. Other organelles are divided by intracellular membrane.
4. Five kingdom classification :
R.H. Whittaker (1969) recognized an additional kingdom for the fungi. This kingdom has received wide acceptance. Thus all the organisms were classified into five kingdom starting with Monera ( where all prokaryotic unicellular organisms were placed). Following that all the eukaryotic unicellular organisms were placed under the kingdom Protista. The organisms were then classified based on the presence or absence of cell wall. The ones without cell wall were the animalia and the ones with cell wall were again classified into photosynthetic and non- photosynthetic which includes plantae and fungi respectively. This system of classification of living organisms is better than following the older classification of plants and animals because it did not match up the confusion of putting one species in two different kingdoms.
5. Six kingdom classification:
Currently most biologists recognize six kingdoms in which two prokaryotic kingdoms (Archebacteria and Bacteria), a large unicellular eukaryotic kingdom ( Protista) and three multicellular eukaryotic kingdom ( Fungi, plantae and animalia).
Viruses are not included in any of the present five kingdoms, mainly due to their non- living characterstics.
Demerits of two kingdom classification:
Ø This system did not indicate any evolutionary relationship between plants and animals.
Ø It grouped together the prokaryotes with eukaryotes and unicellular with multicellular organisms.
Ø This system did not distinguish between the heterotrophic and autotrophic organisms.
Ø It did not mention some acellular organisms like virus and viroids.
Ø Dual organisms like Euglena and Lichens did not fall in either kingdom.
Five kingdom classification was given by R.H. Whittaker. He placed monera, protista, fungi,plantae and fungi in separate groups. Monera includes bacteria, protista includes eukaryotic organisms, fungi includes eukaryotic and heterotrophic organisms, plantae includes eukaryotic and multicellular organisms, animalia includes multicellular eukaryotic heterotrophic organisms.
Viruses are non cellular organisms and act as a connecting link between plants and animals. Lichens are symbiotic association between algae and fungi. Viroids are infectious agents that are smaller than virus.