Networks in molecular evolution

Networks are a common theme at all levels of molecular evolution: Networks of metastable states and their connecting saddle points determine structure and folding kinetics of biopoly- mers. Neutral networks in sequence space explain the evolvability of both nucleic acids and polypeptides by linking Darwinian selection with neutral drift. Interacting replicators, be they simple molecules or highly complex mammals, form intricate ecological networks that are cru- cial for their survival. Chemical reactions are collected in extensive metabolic networks by means of specic enzymes; both the enzymes and the chemical reaction network that they gov- ern undergoes evolutionary changes. Networks of gene regulation, protein-protein interaction, and cell signaling form the physico-chemical basis for morphogenesis and development. The nervous systems of higher animals form another distinct level of network architecture. We are beginning to understand the structure and function of each of the individual levels in some detail. Yet, their interplay largely remains still in the dark.