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Classification, in biology, systematic arrangement of the world's organisms into categories based on their characteristics. This science, sometimes called taxonomy, is often aimed at establishing evolutionary relationships. The system of classification comprises 7 major categories. These are, from most to least inclusive: kingdom; phylum (called division in botany); class; order; family; genus; species. Traditionally, there were only 2 kingdoms, plants and animals. Today, however, most biologists would recognize 3 others: the Protista (1-celled organisms and some forms of algae), the Monera (bacteria and blue-green algae), and the Fungi (mushrooms and molds). The species is the basic unit of classification. As a rule of thumb, 2 animals belong to the same species if they are capable of mating to produce fertile offspring.

See also: Biology.

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