If you search for decent definitions of evolution, the chances are that you’ll see genes mentioned somewhere. The American Heritage Dictionary talks about natural selection acting on “genetic variation”, Wikipedia discusses “change in the genetic material of a population… through successive generations”, and TalkOrigins talks about changes that are inherited “via the genetic material”. But, as the Year of Darwin draws to a close, a new study suggests that all of these definitions are too narrow.
Jiali Li from the Scripps Institute in Florida has found that prions – the infectious proteins behind mad cow disease, CJD and kuru – are capable of Darwinian evolution, all without a single strand of DNA or its sister molecule RNA.
Prions are rogue version of a protein called PrP. Like all proteins, they are made up of chains of amino acids that fold into a complex three-dimensional structure. Prions are versions of PrP that have folded incorrectly and this misfolded form, called PrPSc, is social, evangelical and murderous. It converts normal prion proteins into a likeness of its abnormal self, and it rapidly gathers together in large clumps that damage and kill surrounding tissues.
Li has found that variation can creep into populations of initially identical prions. Their amino acid sequence stays the same but their already abnormal structures become increasingly twisted. These “mutant” forms have varying degrees of success in different environments. Some do well in brain tissue; others thrive in other types of cell. In each case, natural selection culls the least successful ones. The survivors pass on their structure to the “next generation”, by altering the folds of normal prion proteins.
This process follows the principles of Darwinian evolution, the same principles that shape the genetic material of viruses, bacteria and other living things. In DNA, mutations manifest as changes in the bases that line the famous double helix. In prions, mutations are essentially different styles of molecular origami. In both cases, they are selectively inherited and they can lead to adaptations such as drug resistance. In prions, it happens in the absence of any genetic material.