Contingent evolution (Introduction)

by David Turell , Sunday, June 22, 2014, 15:42 (3807 days ago)

Shades of Stephan Jay Gould. How chance mutations, against the odds, advance evolution:-"Tracing these alternative evolutionary paths, the researchers discovered that the protein - the cellular receptor for the stress hormone cortisol - could not have evolved its modern-day function unless two extremely unlikely mutations happened to evolve first. These "permissive" mutations had no effect on the protein's function, but without them the protein could not tolerate the later mutations that caused it to evolve its sensitivity to cortisol. In screening thousands of alternative histories, the researchers found no alternative permissive mutations that could have allowed the protein's modern-day form to evolve. The researchers describe their findings June 16, online in Nature.-"Thornton and Harms tested many thousands of variants but found none that restored the function of GR other than the historical mutations that occurred in actuality. "Among the huge numbers of alternate possible histories, there were no other permissive mutations that could have opened an evolutionary path to the modern-day GR," Thornton said.
 
"By studying the effects of mutations on the ancient protein's physical architecture, Harms and Thornton also showed why permissive mutations are so rare. To exert a permissive effect, a mutation had to stabilize a specific portion of the protein - the same part destabilized by the function-switching mutations - without stabilizing other regions or otherwise disrupting the structure. Very few mutations, they showed, can satisfy all these narrow constraints."-http://www.sciencedaily.com/releases/2014/06/140618220554.htm-And fits my idea of pre-planning