Taxonomy: Biological Classification of Organisms
When we classify the organisms into hierarchical series of groups on the basis of their evolutionary relationships, it is called Systematics. Classification is a subtopic of Systematics which deals with ordering of organisms into groups and taxonomy is the study of principles and procedures of classification. Nomenclature is the process of naming an organism so that this particular organism is known by same name all over the world. Currently, the scientists follow binomial nomenclature in which any organism is denotes by a name with two components viz. Genus and Species. For example, Mango is named as Mangifera indica, whereby, Mangifera is its Genus and indica is its species. While first letter of Genus is always capitalized, first letter of species is always in lower case. For example:
- Tomato → Solanum lycopersicum
- Potato → Solanum tuberosum
- Brinjal →Solanum melongena
In the above example, Tomato, Potato and Brinjal belong to same genus while they are different species. We note here that for plants, scientific names are based on agreed principles and criteria, which are provided in International code for Botanical Nomenclature (ICBN). Animal taxonomists have evolved International Code of Zoological Nomenclature (ICZN).
Taxonomical Hierarchy
Species is the smallest taxonomical unit and refers to a group of individual organisms which interbred among themselves and produce fertile offspring when they interbred. The group of related species is called Genus. Related Genera {General is plural of Genus} are kept in a family, related families are kept in Order. Related Orders are kept in classes. Classes comprising animals like fishes, amphibians, reptiles, birds, mammals etc. constitute the next higher category called Phylum. Generally animals are subdivided into phyla, while plants are subdivided into Divisions. All animals/plants belonging to various phyla/divisions are assigned to the highest category called Kingdom. The below graphic shows position of humans in above taxonomic ranks:
Five Kingdom Classification
Initially, the scientists had put all the living organisms into two Kingdoms viz. Plantae and Animalia. However, there were some problems such as – this classification did not distinguish between Eukaryotes / Prokaryotes, unicellular / multicellular, photosynthetic / non-photo synthetic organisms. Later, they divided the entire living world into five Kingdoms viz. Monera, Protista, Fungi, Plantae and Animalia. This five kingdom classification was based on several features such as cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationships.
- All the prokaryotes were kept in Monera. This Kingdom comprises mainly Bacteria and blue green algae.
- All unicellular eukaryotes were kept in Protista. This kingdom comprised of Algae and Protozoa.
- All fungi were kept in Kingdom Fungi while multicellular plants and animals were kept in Plantae and Animalia respectively.
In the above classification, Viruses have not been included because of their pseudo-living nature.