All organisms make heat shock proteins, also known as chaperones, in response to high temperatures. Once this happens, the misshaped protein can no longer function, and regular cellular metabolism cannot proceed. Hsp 90's charges are special. They are proteins called signal transducers, which regulate development and cell differentiation. Hsp 90 keeps them poised and ready for action.
Signal transducers control morphological development.
They make sure the right part grows at the right spot at the right time. During times of stress, however, all sorts of proteins start to unfold, and Hsp 90 is recruited away from its normal duties to help out. The signal transduction pathways are then 'sensitized' to small genetic variations that otherwise go unnoticed, and development takes a twist. Then the natural variations inherent in an organism's DNA can produce major changes in body plan, which can be passed on to offspring. But for some, the changes might be beneficial for adapting to a new environment.