Diversity in the Living World

Look around and you will find life in astonishing variety — potted plants, insects, birds, pets and other animals, alongside countless organisms too small for the naked eye to see. The more ground your observations cover, the wider the range of living things you meet: a patch of garden reveals far fewer kinds of organisms than a dense forest, where both the number and the types of living things multiply dramatically.

Every distinct kind of plant, animal or other organism represents a species. When scientists count all the species that have been recognised and formally described, the total falls somewhere between 1.7 and 1.8 million. This vast catalogue of different life forms is what we mean by biodiversity — the number and variety of organisms found on Earth.

The need to name organisms

Because there are millions of plants and animals, and because people everywhere refer to them by different local names that shift from place to place even within one country, communication becomes confusing. To talk clearly about a particular organism anywhere in the world, its name must be standardised so that one organism carries the same name everywhere. Assigning such standard names is called nomenclature.

Naming, however, is only possible once an organism has been correctly described and we know exactly which organism a name belongs to — this is identification. To make study manageable, scientists have established agreed procedures for giving every known organism a scientific name accepted by biologists worldwide:

• Scientific names for plants follow the International Code for Botanical Nomenclature (ICBN).
• Scientific names for animals follow the International Code of Zoological Nomenclature (ICZN).

These codes ensure each organism has just one name, that a correct description leads everyone to the same name, and that a name is never reused for a different known organism.

Binomial nomenclature

Every scientific name has two parts — the generic name (genus) and the specific epithet (species). This two-part naming system is called binomial nomenclature, devised by Carolus Linnaeus and used by biologists everywhere because of its convenience. The scientific name of mango, for example, is written Mangifera indica: Mangifera is the genus, and indica is the specific epithet.

The universally followed rules of nomenclature include:

• Biological names are generally in Latin and written in italics; they are Latinised regardless of their original language.
• The first word denotes the genus and the second denotes the specific epithet.
• When handwritten, both words are underlined separately; in print they appear in italics, marking their Latin origin.
• The genus name begins with a capital letter, while the specific epithet begins with a small letter — as in Mangifera indica.
• The author's name, in abbreviated form, follows the specific epithet at the end of the name, for example Mangifera indica Linn., showing that Linnaeus first described the species.

Classification, taxonomy and systematics

Since studying every organism individually is impossible, we group them. Classification is the process of placing things into convenient categories based on easily observable characters. When we say "dog", "cat", "insect", "mammal", "wheat" or "plant", each term immediately calls up a set of associated features. The scientific term for such categories is taxa (singular: taxon). …