A heterarchy is a system of organization where the elements of the organization are unranked (non-hierarchical) or where they possess the potential to be ranked a number of different ways.
Definitions of the term vary among the disciplines: in social and information sciences, heterarchies are networks of elements in which each element shares the same “horizontal” position of power and authority, each playing a theoretically equal role. But in biological taxonomy, the requisite features of heterarchy involve, for example, a species sharing, with a species in a different family, a common ancestor which it does not share with members of its own family. This is theoretically possible under principles of “horizontal gene transfer.”
A heterarchy may be parallel to a hierarchy, subsumed to a hierarchy, or it may contain hierarchies; the two kinds of structure are not mutually exclusive. In fact, each level in a hierarchical system is composed of a potentially heterarchical group which contains its constituent elements.
The concept of heterarchy was first employed in a modern context by Warren McCulloch in 1945. As Carole L. Crumley has summarised, “[h]e examined alternative cognitive structure(s), the collective organization of which he termed heterarchy. He demonstrated that the human brain, while reasonably orderly was not organized hierarchically. This understanding revolutionized the neural study of the brain and solved major problems in the fields of artificial intelligence and computer design.