Saturday, March 23, 2013

The Chaotic Paths of Evolution

Evolutionary Paths. In a paper published in the November 2012 issue of BioScience (1) researchers led by Shozo Yokoyama at Emory University have labeled “highly questionable” assumptions relied upon to study the evolution of protein molecules. One of these assumptions is that changing a particular gene in a known location would affect the properties of ancestral and modern protein forms in the same way. That assumption allows computers to infer likely evolutionary paths leading to the forms of proteins found in modern organisms.

Doubts on Molecular Adaption. In experiments designed to test these assumptions, Yokoyama created hypothesized ancestral visual pigments and variants of them that might have been produced by mutation. What he found was that properties of related version of proteins often changed in different ways when the same mutation was introduced in each. This caused standard computational and statistical methods to rarely be able to identify the experimentally supported evolutionary pathway. Yokoyama concludes that his studies cast “serious doubt” on the “fundamental principles of molecular adaptation” used in thousands of papers.

No Going Back. In a related article (2), a University of Oregon team found that evolution can never go backwards. The team reconstructed the gene for a glucocorticoid receptor in the version that existed more than 400 million years ago. They found that over a rapid period of time, random mutations in other proteins caused changes in the protein’s structure which made it incompatible with the receptor’s primordial form. In other words, there is evolutionary bridge burning, which implies that the direction evolution took may be neither ideal nor inevitable.

New Pathways. In yet another study (3), Georgia Tech researchers resurrected a 500-million year-old gene from E. coli and inserted it into the modern E. coli bacterium in the exact spot on the chromosome it existed in originally. After producing eight identical bacterial strains, they allowed the strains to divide and observed their growth rate. At first the altered organisms were not as fit as the modern day versions without the ancient gene insertion, but the growth rate eventually increased. After 500 generations the researchers sequenced their genomes. What they found is that the ancient gene did not accumulate mutations. Instead it was the modern proteins that interact with the ancient protein which mutated to increase the organism’s fitness. In other words, the bacteria found a new evolutionary pathway to adapt.

Fractal Pathways. These studies support findings previously discussed in this blog that show that the structure and evolution of DNA and proteins is fractal (chaotic). This means that at any point in the evolutionary process multiple pathways exist. Another of the implications of this is that minor changes in initial conditions (random mutations) cause huge effects down the evolutionary pathway. One cannot go backwards in evolution because the pathway evolution took is only one of many it could have taken and is forever closed. Even if one interjects an ancient gene from the same bacterium, a new pathway is found for evolution, not the same one previously taken.

Multiple Paths to Adaptation. The fractal nature of the evolutionary process implies that on other planets, or even on this one if one could go back in time and reconstruct the initial conditions, evolution could take multiple pathways to adaptation, producing one of many possible solutions for each organism. This has obvious negative implications for intelligent design proponents and creationists who propose that only a designer could have created the unique complex organisms we see today.

1. Yokoyama, Shozo BioScience 62(11):939-948. 2012
2. Bridgham, JT, Ortland, EA, Thornton, JW, Nature. 2009 Sep 24;461(7263):515-9. doi: 10.1038/nature08249.
3. Georgia Institute of Technology (2012, July 11). Giving ancient life another chance to evolve: Scientists place 500-million-year-old gene in modern organism. ScienceDaily. Retrieved March 23, 2013