Taxonomic Categories

Classification is not a single step but a hierarchy of steps, each step representing a rank or category. Because a category is part of the overall taxonomic arrangement, it is called a taxonomic category, and all the categories together make up the taxonomic hierarchy. Each category is a unit of classification, a rank, commonly termed a taxon (plural: taxa).

An example makes the idea concrete. Insects form a group of organisms sharing common features such as three pairs of jointed legs — they are recognisable, concrete objects that can be classified, and were therefore given a rank of their own. In this way, a group represents a category, a category denotes a rank, and each rank is a unit of classification. Taxonomic groups of this kind are genuine biological entities, not merely loose collections of similar-looking forms.

Taxonomic study of all known organisms has produced a common set of such categories, running from the broadest at the top to the narrowest at the bottom:

• Kingdom
• Phylum (or Division, for plants)
• Class
• Order
• Family
• Genus
• Species …