[VadeRetro]

Other proteins act as chaperones, bringing molecules to their proper locations in the cell, and so on. In short, with that kind of protection from change, it is very difficult for mutations, of whatever kind, to gain a foothold. But extreme stress pushes cells beyond their capacity to produce protective proteins, and then mutation can occur.

Interesting point here. It is environmental stress which interferes with the genetic "proofreading" mechanism and allows the effective mutation rate to climb in times of deep crisis.

[Rudder]

...it is environmental stress which interferes with the genetic "proofreading" mechanism and allows the effective mutation rate to climb in times of deep crisis.

mutation rates not only climb, but the such mutations (non-lethal) can also accumulate in the gene pool over successive generations.