I was recently asked by an e-mailer to comment on a new study about evolution of prions based via a process like Darwinian selection.
Prions are misfolded proteins (or misfolded protein complexes). They aren’t alive. They can’t replicate on their own. They require their host’s cellular machinery for producing new proteins they can “misfold” in order to propagate.
Prions can be dangerous because they propagate themselves by misfolding other properly folded proteins produced in the cell. The misfolded proteins don’t always function property, and this can disrupt activity in the cell. As the BBC article states, “Prions are associated with 20 different brain diseases in humans and animals.” The new research just shows that prions don’t always make perfect copies of themselves when misfolding other proteins. This leads to variation. Naturally, those prions that are better able to infect their hosts tend to do so, and thus they tend to “evolve” towards more “virulent” forms.
Whenever you have replication, variation, and some selection pressure (such as scarce resources), Darwinian evolution is probably going to occur. Does this mean that prions help us explain how biological organisms evolve?
In this study, the evolution of prions means their original function has been disrupted and they have become less useful in their hosts. This type of evolution shows the destruction of original biological function, not the improvement of it.
Natural selection can only preserve traits that exist — we must account for the origin of a trait before Darwinian selection can preserve it. Thus, as evolutionary biologist Rudolf Raff has observed, neo-Darwinism might model “the survival of the fittest, but not the arrival of the fittest.” And the arrival of the fittest can be problem for neo-Darwinism. Although he denies that such a structure exists, Jerry Coyne admits, “It is indeed true that natural selection cannot build any feature in which intermediate steps do not confer a net benefit on the organism.” Since these prions do not benefit their host, this research doesn’t do much to show how living organisms evolve.
Most biological systems are significantly more complex than prions. Prions have a complexity is significantly less than that of even viruses (which also aren’t alive). The observation that proteins can get increasingly screwed up via Darwinian evolution isn’t going to do much to explain how functional proteins evolved in the first place.