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- The modern classification is an extension of the system of classification proposed by Carolus Linnaeus.
- The present system of classification involves a series of graded taxonomic ranks called taxa which include phylum and family in addition to the four taxa introduced by Linnaeus.
- The hierarchy of taxonomic ranks is known as Linnaean hierarchy. In this any given taxon may contain several lower taxa that can be distinguished on the basis of common characteristics.
- The biggest taxonomic rank is Kingdom. It is classified into Phyla, phyla into Classes, classes into Orders, order into Families, families into Genera and genera into Species.
- Sometimes there may be additional ranks between two consecutive taxa, usually with a prefix sub or super. There may be sub kingdoms, sub phyla, super classes, sub classes and so on.
- Species is the lowest unit of classification. It is a group of closely related organisms which can breed among themselves producing fertile young.
- Binomial nomenclature is the system of giving a scientific name to an animal or a plant. The name consists of two words, the first one referring to the name of the genus and the second to the name of the species.
- Binomial nomenclature avoids the confusion created by using common names.
- Certain specific guidelines issued by the International Council of Binomial Nomenclature (ICBN) are followed in the use of binomial nomenclature. The most important among these is that name of the genus should start with a capital letter and name of the species with a small letter.
- One of the earliest scheme of classification in the natural system is the two-kingdom classification proposed by Linnaeus in 1758.
- Linnaeus placed living organisms into two kingdoms - Plantae and Animalia.
- The plant kingdom proposed by Linnaeus and subsequent works of that period comprised of bacteria, fungi, algae, liverworts, masses, ferns, conifers and flowering plants.
- The animal kingdom proposed by Linnaeus and others included the unicellular and multicellular animals.
- The two-kingdom classification has certain demerits, particularly with reference to lower forms of life which show common features with both plants and animals.
- In 1858 Earnt Haeckel proposed the three-kingdom classification in which apart from Plantae and Animalia, a new kingdom Protista was created to include all the unicellular organisms, both from plants and animals.
- The advent of electron microscope helped scientists in discovering the existence of two forms of cells in organisms - prokaryotic and eukaryotic.
- In 1966, Copeland suggested the creation of a fourth kingdom-Monera to include the prokaryotic organisms like bacteria and blue-green alga. This led to a four kingdom classification.
- In 1960 Robert H. Whittaker proposed the creation of a new kingdom called Mycota to include the fungi exclusively. This led to a five kingdom classification which is in practice today.