Complex Systems: From chemistry to systems biology

There is great interest in complex systems in chemistry, biology, engineering, physics, and gene networks, among others. The complexity comes from the fact that in many systems there are a large number of variables, many connections among the variables including feedback loops, and many, usually nonlinear, equations of motion, or kinetic and transport equations. “Many” is a relative term; a properly interacting system of just three variables can show deterministic chaos, a complex behavior indeed. For the natural scientist and the engineer, nearly all their systems are complex. Many problems still resist the arguments of symmetry, averaging, timescale separation, and covariation that often underlie complexity reductions. New tools have allowed us to peer at even the nanometer scale of the structure of materials, explore the dynamic chemical composition of the explosions in our increasingly efficient engines, and determine the organization of the genome and architecture of the molecules and molecular networks it implicitly encodes. All these are revealing extraordinary arrangements of kinetic processes, feedback loops, and spatial organization that together create complex behaviors. The recent interest is due, in part, to the substantial advances in measurement techniques of chemical and biological species and experiments on complex systems, concurrent substantial advances in theory, and the increased urgency of analyzing and understanding complex systems of fundamental importance. Nowhere is the importance of complex dynamics and architectures clearer than in biological systems. In this issue, all of the articles address problems of complexity in organisms. Topics range from information processing in their signaling network and the organization of their metabolism, to how populations of differentiated cells communicate with one another to coordinate behavior, and to how evolution has arrived at different recurrent motifs … 1To whom correspondence should be addressed. E-mail: john.ross{at}stanford.edu