Saturday, January 9, 2010
The evolutionary concept of quasi-species
About a week ago I read a really interesting post on ScienceDaily.com.
(Which you can find here -> http://www.sciencedaily.com/releases/2009/12/091231164747.htm)
It was about the darwinian evolution of so called "quasi-species", that is species who go outside the usual definition of what constitute as "life" or "alive". In this case some scientists at The Scripps Institute have found that prions, little bits of infectious proteins, are capable of evolution even though they lack DNA and RNA. Prions are what cause Mad Cows disease, which is called Creutzfeldt-Jakob when it occurs in people. Every mammalian species produces normal prion proteins. The little infectious prions simply destroy healthy prion proteins in the brain by converting them into toxic ones. These toxic cells don't function as needed for a healthy animal and also destroy surrounding tissue. All diseases known to be caused by prions are untreatable to date, and therefore eventually fatal. Another interesting disease caused by prions is the Kuru-disease which was encountered among the Fore peoples of Papua New Guinea. When it was first examined in the late 50's, one thought the causes lay in cannibalistic behaviour among the Fore. By eating the brains of diseased people one would also contract the disease. The likelihood of this explanation is still being debated though.
The "evolution" part of the prions came when the scientists found out that these prions are affected by natural selection. Apparently it was quite similar to the way viruses "reproduce" although as said, prions lack both DNA and RNA. As far as I've understood it, normally evolution means mutation, or change, of the nucleic acid (DNA). These changes occur because some DNA survive the environment and some don't. The looks of the DNA that survives is what will contribute to the next set of DNA. These are the simple genetics I learned about it school.
Originally the scientists thought that when a prion had converted, or folded as its called, a cell into its toxic form, no more changes would occur. There would always be the same folding of cells, similar perhaps to the way rock changes in cycles - lava, mountains, big rocks, smaller rocks, sand, lava and so on. The way rock changes is quite predictable and hasn't changed since the beginning of things, as far as we know. There had long been hints though, that this wasn't the case with infectious prions since they acted somewhat differently when set in different environments. When looking more closely, one found that the infectious prions change the way they fold proteins, to a way more suitable and effective of the current environment. Something that has been interpreted as a form of evolution.
One doesn't yet know however how these changes occur, only that they do occur. The article mentions that the abnormal, infectious prions replicate, but doesn't explain further what it means by "replication". The leader of the scientist team, Charlers Weissman, concludes;
"But we know that mutation and natural selection occur in living organisms and now we know that they also occur in a non-living organism. I suppose anything that can't do that wouldn't stand much of a chance of survival".
So, anything that needs to influence its surroundings to keep on existing (i.e survive) has to be able to adapt to said surroundings. Perhaps this could say to broaden the concept of "life" from everything with DNA to everything that "changes in a non-cyclic pattern to influence its surroundings in a non-predictable way".