Nested hierarchy

A nested hierarchy is the name given to the hierarchical structure of "groups within groups" or "branches from a trunk" used to classify organisms. For example, a species is a subset of a genus, which is a subset of a family, which is a subset of an order - and so on.
This structure is assumed to reflect the evolutionary pattern of descent with modification.

The system is hierarchical and tree-like because it consists of a single all-inclusive category (the tree trunk) progressively dividing into non-overlapping categories (the branches). It will not permit, for example, the genera in one family being members of another family. Such straddling would be an example of heterarchy which is more like a network than a tree.

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See also