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A Glossary for Systems Biology

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ADAPTATION

see also

robustness , integral control, differentiation, system (taxonomy)

Meaning

Biology:

• The response to an extracellular stimulus returns to its pre-stimulus value even in the continued presence of the signal. [63]
• Adaptation to changes in environmental conditions is the driving force behind the evolutionary development of all species from a common predecessor. [37]

Systems Theory:

• The ability of a system to compensate for changes in its environment. [6,5]
• Adaptive control, where controller parameters are constantly adjusted to accommodate changes in process dynamics, process parameters or disturbances. [8]
• After reacting to a persisting stimulus, the output of the systems returns to its pre-stimulus value.

Explanation

In systems theory there are a number of ways to interpret adaptation.

The first two sound very similar: to compensate for changes. The difference is in where the changes take place and what has to adapt to them. In the first case the changes are external to the system, and the systems adapts to them. In the second case they are internal to the system, and a control scheme has to adapt to them in order to be able to continue controlling the system.

In both cases the commencing change itself activates a mechanism that opposes the external disturbance. This is done to ensure that the essential variables of the system stay within acceptable (i.e. stable/non-dangerous/uncritical) limits. These limits are much narrower than they would be if the disturbance were unopposed. The narrowing is the objective manifestation of the mechanism's adaptation [6].

The third possibility concerns the reaction to a stimulus that lasts longer than a certain interval, out-lasting the 'sensitivity period' of the system. This interpretation coincides with one of the meanings of adaptation in biology.

``A hallmark of many biological sensing devices is the ability to adapt to a persistent input stimulus, thereby increasing the range of sensitivity.'' [63] To counter such persistent stimuli, adaptation can be implemented through integral feedback control (integral control), in biological as well as in technical systems. The term used for adaptation in technical systems is 'asymptotic tracking' [63]. In biology, this form of adaptation is also called desensitization [63]. It is argued that integral control is not only sufficient but also necessary for robust perfect adaptation [12].

The second biological interpretation puts adaptation in an evolutionary context. Adaptation to changing environmental conditions - also called adaptive radiation or acclimatization - resulted in the development of new species which were adapted to those conditions, thus being able to withstand selection. Successful species incorporated the adapted characteristics into their genetic code to pass it on to their offspring [37].

Regardless of context, adaptation is one possibility to make a system robust against changes in its environment. (robustness)

Examples

Biology:

• bacterial chemotaxis: changes in concentration of a substrate in the surrounding media influence propulsion activity of the cell, allowing it to move to the location of the highest (attractor)/lowest (repellent) concentration [39,63]; see also Chapter 'Recommended Readings'.
• eye: The retina works best at a certain intensity of illumination. In bright light the nervous system contracts the pupil, and in dim relaxes it. Thus the amount of light entering the eye is maintained within limits. [6,5]
• all homeostasis systems: ``For example, within the cell, the levels of important second messenger molecules such as calcium and of key metabolites such as ATP fluctuate dramatically in response to both internal and external events. Integral control operating through the enzymes that create or remove these molecules can provide a robust mechanism for restoring the concentrations of these species to their optimal steady-state level.'' [15]

Systems Theory:

• adaptive control (see Explanation), e.g. course control for a rocket which burns off fuel, thus changing its mass [20].

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