Perturbations and robustness of complex systems

The prediction of the effects of perturbations in protein networks remains a major challenge. Biological systems are constantly modified, as a result of changes in the environment, during the development of an organism, and during the evolution of a population. The time scales of these pertubations range from femtoseconds for the formation of transcient protein interactions to millions of years for the evolution of life.

Cellular networks generally function reliably and with high fidelity, even in uncertain or fluctuating environments. A crucial prerequisite for the high precision exhibited by many networks is their ability to keep the concentrations of the active components within tightly defined bounds. This can be achieved by the specific organisation of the network and the existence of compensatory mechanisms.

At the molecular level, mutations of a given amino acid in a protein are not completely independent of mutations of other amino acids within the same protein or within interacting proteins. Functional and structural requirements impose constraints on the viable mutations, and a mutation at one position must often be compensated by a complementary mutation at another position.

At the evolutionary scale, co-evolution and co-adaptation are well-documented phenomena that are important forces in the organization of biological communities, host–parasite and predator–prey interactions, symbiosis and mutualism.