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Living organisms are not precision devices. Most biological processes are inherently stochastic - whether or not a gene is transcribed is non-deterministic. As a result, the number of molecules of any given type (for instance) is not usually tightly regulated (you could postulate the existence of mechanisms for counting the number of molecules produced, but remember that for every extra mechanism you're sticking in a cell you're increasing the amount of work that that cell has to do). At the same time, the environment in which the organism is living is unpredictable. Faced with these problems, a pathway that is highly dependent on the number of molecules of something present or which depends on a particular precise set of conditions is unlikely to be a great deal of use. As a result of this, robustness is a hallmark of many biological systems - that is, they are capable of functioning even when quite badly perturbed by things such as molecular concentration or external factors not under the control of the organism. This may reduce efficiency, but it increases the probability of surviving and passing on your genes to your offspring and being fit in the biological sense.

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