The earth came into existence 4 to 5 billion years ago and life originated around 3.4 billion years ago. In these many years, approximately 15 million different kinds of organisms have evolved. The wide variety of organisms is termed biodiversity.
Various kinds of organisms differ from each other structurally. R. Whittaker classified all the organisms into five major groups, called five kingdom classification. All living organisms are further classified on the basis of their similarities and differences into different categories such as:
kingdom → phylum → class → order → family → genus → species
Classification shows evolutionary relationships between organisms and is also termed Systematics. The science of classification or systematics is termed Taxonomy. Common names are confusing and variable, so each kind of organism has been given a scientific name. A scientific name includes the names of the genus and species.
1. Monera: All bacteria, are the only prokaryotes, i.e. their hereditary material is not enclosed in a nucleus.
2. Protoctista: Single celled nucleated plant-like algae and animal like protozoans.
3. Fungi: Multi-cellular nucleated organisms which are saprotrophs.
4. Plantae: Includes plants which are autotrophs and manufacture food by photosynthesis. Their cells possess a cell wall.
5. Animalia: Includes animals which are heterotrophs and are made of cells without a cell wall.
Every kind of organism has a scientific name made of two parts: (i) name of the genus and (ii) name of the species. The initial letter of the genus is written with a capital letter and species with a small letter. The scientific name is written in italics or underlined.
Biodiversity has three levels namely: