Prions are the infective agents that cause transmissible spongiform encephalopathies such as Mad Cow Disease in humans. All prions affect the brain or neural tissues and are currently untreatable. What makes them particularly fascinating is that unlike other infective agents such as bacteria, protozoa, and viruses, they don't contain any genetic material. No DNA or RNA. Prions are just misfolded proteins but they are capable of spreading, causing disease, and evolving.
Prions spread disease a little like zombies. The prions themselves are misfolded proteins, and when they come into contact with correctly folded versions of the protein they cause them to misfold as well. Once these proteins become misfolded they can go on and convert further proteins to the misfolded form. This misfolded protein accumulates in neural cells and tissues causing damage to the brain. In mammals, all prion diseases are caused by a protein known as PrP (prion protein). The correctly folded form is referred to as PrPC and the misfolded form as PrPSc.
Not only do the prions spread they also change and evolve as they go. There are various different theories as to how a prion, a piece of folded protein with no associated DNA, can evolve. The 'cloud hypothesis' is that different variants of PrPSc are present within the organism. Depending on outside environmental pressures, one form may be able to spread quicker or more effectively than others and therefore will be selected for.
A second hypothesis is the 'deformed template' model. This puts forward that changes in the environment can themselves cause changes to the prions and generate new PrPSc variants. This expands the pool of PrPSc variants that can be selected for, making it more likely that a successful protein will be able to spread and propagate. It may be that this form of evolution only happens when the environment is not favourable for exact duplication of the misfolded states, so some prions are misfolded slightly differently.
It's difficult to prove experimentally that the prions are changing due to the environment, and prion research is not my area, but the reference points to some recent research which seems to show prions changing form in response to different environments in hamster brain homogenates. It's also interesting to consider changes already seen in prions as they cross the species barrier. It could be that as well as providing new evolutionary pressures for more efficient folds, the environment of a new species may change the variety of protein folding.
As well as being exciting for prion researchers, this kind of research raises all sorts of possibilities for the behaviours of proteins in normal situations, inside cells. Prions can evolve as they change proteins around them at a spectacular and alarming rate, but that doesn't mean that other cellular proteins can't 'evolve' in similar ways, albeit at a slower rate. If the protein in question is one that influences DNA it may be able to stamp its own change into the genetic code, to be passed on to future generations. This may be speculation at the moment, but it opens up a range of exciting possibilities for evolution beyond the genetic.
Reference: Makarava N, Baskakov IV (2013) The Evolution of Transmissible Prions: The Role of Deformed Templating. PLoS Pathog 9(12): e1003759. doi:10.1371/journal.ppat.1003759