Four different Types of Hierarchies used in Biology. A) A constitutive hierarchy of molecules, organelles, cells, and organs of a multicellular organism, in which all molecules are contained in organelles, all organelles in cells, and all cells in organs. B) The more realistic case of a cumulative constitutive hierarchy of molecules, organelles, cells and organs of a multicellular organism, in which not all molecules are contained in organelles or cells, and not all cells in organs. C) An aggregative hierarchy of species being aggregated to genera, genera to families and families to orders, just like in a traditional Linnaean taxonomy. D) A cumulative aggregative hierarchy, as it can be found in the hierarchical organization of military stuff, in which individuals with higher ranks (i.e. sergeants, lieutenants, captains) 'emerge' in aggregates of higher order (i.e. squads, platoons, companies).
Vogt BMC Bioinformatics 2010 11:289 doi:10.1186/1471-2105-11-289