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A Glossary for Systems Biology

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INTEGRAL CONTROL Glossary DYNAMIC (BEHAVIOR)

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HIERARCHY

see also

modularity

Meaning

Biology:

• Taxonomy creates a hierarchical structure of different kinds of organisms and their evolutionary connections.
• hierarchy in regulation (see Examples)

Systems Theory:

• hierarchical structures where one system controls others
• multi-level structures of systems built from systems

Explanation

Any multi-level structure can be seen as hierarchical. A good example is shown in the illustrations to modularity, where systems are repeatedly connected to form other systems.

Hierarchy can also pertain to levels of influence, for example if modules on the same level are not linked as equals. In that case some could exercise control over others, or have more influence on a higher-level functionality. More often levels of influence coincide with levels within a structural hierarchy. That way, higher-level elements exercise control over elements from lower levels.

These aspects apply in biology as well as technical systems. A special form of hierarchy in biology is taxonomy (including phylogeny), where a hierarchy of evolutionary dependencies between different species is reconstructed.

Examples

Biology:

• taxonomy: hierarchy of all species in evolution (differentiation)
• phylogenetic tree [13] (differentiation; see Illustration)
• hierarchy in regulation
  • global regulatory networks, e.g. heat shock [43]
  • genetics: cell cycle - modulon - regulon - operon [19,24]; see Figure 3.5

Systems Theory:

• hierarchical control

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