As a further introduction into Hsp90 and its role in evolution, here are two segments cut from this review:
1. “Because signaling proteins with multiple regulatory states often undergo a conformational switch, the structural flexibility needed for these steps may render them inherently less stable and thus more likely to be recognized by Hsp90. On the other hand, under stress conditions such as heat shock Hsp90 contributes more generally to the refolding of denatured proteins (Nathan et al., 1997). Although the exact structural features recognized by Hsp90 are not yet understood, the exposure of these features to Hsp90 is likely a result of intrinsic or stress-induced structural flexibility. Thus, interactions of substrate proteins with Hsp90 arise from structural properties at the molecular level rather than biological function.”
2. “Defects in cell physiology caused by Hsp90 disruption can lead to defects at the level of tissue and organism. Interestingly, recent work connects Hsp90 function with morphological evolution, a process that often requires the effects of independent genetic changes (Rutherford and Lindquist, 1998). Hsp90-null mutants are lethal in eukaryotes, but surprisingly partial disruption of Hsp90 in Drosophila by a temperature-sensitive mutation or low amounts of GA shows a wide assortment of heritable phenotypic variations. The variations may arise from alleles of proteins, which depend on the full function of Hsp90 as a conformational buffer to maintain “wild-type” activity but whose phenotypes can be stabilized by other genes following selection. By extension, polymorphisms in all of the proteins participating in an Hsp90-dependent signaling pathway should be buffered by Hsp90 function. In the wild, overloading of the Hsp90 “capacitor” with denatured proteins under environmental stress could similarly increase the phenotypic diversity on which natural selection and ultimately evolution acts (Rutherford and Lindquist, 1998). Hsp90 with its connection to the cellular signaling network may be particularly suited to such a function. Thus, the mechanisms of chaperone-mediated protein folding at a molecular level can be integrated with cellular processes and with the development of organisms and species.”
Thus, if I understand this correctly: Under stressful conditions the Hsp90 is titrated away from its normal substrates exposing structural variations otherwise masked. Seeing that many of these normal substrates are signaling proteins (and thus affects many other proteins) this potentially, leads to extensive variations in phenotype.