When exposed to certain adverse conditions, in some genetic backgrounds cells actually fare better than their prion-free siblings; this finding suggests that the ability to adopt a [PSI+ prion form may result from positive evolutionary selection. It has been speculated that the ability to convert between prion-infected and prion-free forms acts as an evolutionary capacitor to enable yeast to quickly and reversibly adapt in variable environments. Nevertheless, Reed Wickner maintains that and [PSI+ are diseases, although this claim has been challenged using theoretical population genetic models.