Through self-organization, a system becomes ordered in space and/or time, often leading to emergent properties that qualitatively differ from those of its individual units.
The reductionist approach — systematically dismantling complex systems to examine individual components — has been successful for the sciences over the past few centuries, from the isolation of the elements in chemistry, the discovery of atomic and subatomic particles in physics, to the purification and study of proteins, DNA and RNA in biology.
Although this reductionist approach in biology will continue, there is increasing interest in determining the properties of systems of interacting biomolecules. How do networks of proteins and genes integrate and respond to signals? How do dynamic organelle structures, such as the mitotic spindle, form? What controls growth and division? How does the genome create an organism? Self-organization is central in these processes1, at various sizes (Fig. 2).
Self-organized systems differ from self-assembled ones as they rely on a continuous input of energy for maintenance and are far from thermal equilibrium. Classical thermodynamics — successful in the physical sciences — does not apply. Instead of self-assembling into the lowest energy state, such as a crystal, energy-dissipating components self-organize into highly complex structures through which there is a constant flux of energy and material.
Established theories, such as those of dynamical systems and control (from physics and engineering) can provide a basis for understanding self-organization in biology; however, the unique properties of biological systems — their multiple components and energy-dissipation mechanisms, and wide ranges in time and space — pose practical and intellectual challenges.