Why organisms are classified

Classification organises biological diversity into groups based on shared features. It helps biologists identify organisms, communicate accurately and predict characteristics. Modern classification also reflects evolutionary relationships, although the syllabus mainly requires observable structural features and the five-kingdom model.

The species concept

A species is a group of organisms that can reproduce with one another to produce fertile offspring. Members may vary in colour, size or behaviour while still belonging to the same species. Organisms from different species may occasionally produce hybrid offspring, but the offspring are commonly sterile, which maintains separation between the species.

Hierarchy

The major ranks are kingdom, phylum, class, order, family, genus and species. Each lower rank contains fewer organisms that share more features. The genus and species together provide the scientific name. Although learners are not usually required to memorise full hierarchies for many organisms, understanding the nested pattern helps interpret unfamiliar data.

Binomial nomenclature

The binomial system gives every species a two-part scientific name. The first word is the genus and begins with a capital letter; the second is the species name and begins with a lower-case letter. Both words are italicised when typed or underlined separately when handwritten. For example, humans are Homo sapiens.

Choosing classification features

Useful features are observable, consistent and not dependent on temporary conditions. Number of legs, presence of wings and leaf-vein pattern are more useful than size or colour when those vary with age or environment. A good classification statement names a feature and its alternative states.

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