Functional cartography of complex metabolic networks:
We find that, typically, 80% of the nodes are only connected to other nodes within their respective modules, and that nodes with different roles are affected by different evolutionary constraints and pressures. Remarkably, we find that metabolites that participate in only a few reactions but that connect different modules are more conserved than hubs whose links are mostly within a single module.
This is exactly what evolutionary biology would predict. The more networks a molecule is involved in, the more evolutionary constraints it experiences.