In order for the cell to function properly, proteins must be robust to both changes in their environment and errors made during their synthesis. At the same time, proteins also need to be able to evolve novel functions to survive on long evolutionary timescales. The very same processes, i.e. genetic and phenotypic mutations, generate the diversity that leads to functional innovations and broken proteins, ultimately resulting in novel organisms, diseases and in some cases extinction.
In short, proteins exhibit evolutionary plasticity. But how do proteins remain robust and facilitate innovations at the same time? How can we distinguish variations, genetic and phenotypic, that are good or bad?