Taxonomic categories and hierarchy

Classification is not a single step. It is a hierarchy of steps, and each step is a rank or category.

Because each category is part of the overall arrangement it is called a taxonomic category, and all the categories together make up the taxonomic hierarchy.

Each category is a unit of classification and represents a rank; it is commonly called a taxon (plural taxa).

Insects are a concrete example: they share features such as three pairs of jointed legs, so they can be recognised and given a rank.

Taxonomic groups are distinct biological entities, not just collections of organisms that happen to look alike.

The common categories, from lowest to highest, are species, genus, family, order, class, phylum (called division in plants) and kingdom. Species is the lowest category for all organisms.

To place an organism in these categories you need to know its characters, which reveal its similarities to and differences from individuals of its own kind and of other kinds.

Taxonomists have also created sub-categories within this hierarchy so that taxa can be placed more soundly.