Previous thinking on Darwinian evolution has assumed that changes in nucleic acid sequences, whether that is DNA or RNA, brings about the mutations that enable evolutionary variation.

But prions noticably lacking in nucleic acid still evolve in the same way as other infectious micro-organisms.

"On the face of it, you have exactly the same process of mutation and adaptive change in prions as you see in viruses," said study leader Dr Charles Weissmann, head of The Scripps Institute 's Department of Infectology in Florida.

"This means that this pattern of Darwinian evolution appears to be universally active."

"In viruses, mutation is linked to changes in nucleic acid sequence that leads to resistance."

"Now, this adaptability has moved one level down -- to prions and protein folding -- and it's clear that you do not need nucleic acid for the process of evolution."

"It was generally thought that once cellular prion protein was converted into the abnormal form, there was no further change."

"But there have been hints that something was happening."

"When you transmit prions from sheep to mice, they become more virulent over time."

"Now we know that the abnormal prions replicate, and create variants, perhaps at a low level initially."

'But once they are transferred to a new host, natural selection will eventually choose the more virulent and aggressive variants."

Although prion-related diseases are currently untreatable, the findings may indicate new avenues of research.

Future therapies may try to limit the production of the naturally occuring prions before they can mutate rather than try to target the already abnormal prions.

"It will likely be very difficult to inhibit the production of a specific natural protein pharmacologically," said Dr Weissmann said.

"You may end up interfering with some other critical physiological process, but nonetheless, finding a way to inhibit the production of normal prion protein is a project currently being pursued in collaboration with Scripps Florida Professor Corinne Lasmezas in our department."