Classification is the arrangement of organisms into groups based on shared characteristics. It makes the millions of species on Earth easier to study, identify and compare.
Hierarchy of classification
Organisms are grouped in a series of ranks from the largest and most general to the smallest and most specific:
- Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.
- As we move down the ranks, organisms share more characteristics and the groups contain fewer members.
- The species is the basic unit — a group of organisms that can interbreed to produce fertile offspring.
Domains and kingdoms
The three-domain system divides life into Archaea, Bacteria and Eukarya. The older five-kingdom system recognises Monera, Protista, Fungi, Plantae and Animalia. Classification is based on features such as cell type (prokaryote or eukaryote), number of cells and mode of nutrition.
Binomial nomenclature
Binomial nomenclature, introduced by Carolus Linnaeus, gives every species a two-part Latin name so that scientists worldwide use one agreed name. The rules are:
- The first word is the genus (capital letter); the second is the species (small letter).
- The name is written in italics (or underlined when handwritten).
Example
Humans are named Homo sapiens: Homo is the genus and sapiens is the species. The oil palm is Elaeis guineensis.
Key idea
A scientific name = Genus (capital, italic) + species (small, italic), e.g. Homo sapiens.
To identify an unknown organism, biologists often use a dichotomous key — a series of paired statements about observable features. At each step you choose the statement that fits the organism, and this leads you step by step to its correct name. Classification and clear naming therefore make communication among scientists accurate and prevent confusion between organisms that look alike.
Remember
Down the hierarchy: more shared features, fewer organisms per group. Species is the smallest unit.