In brief, Stephen Jay Gould proposed the idea that evolution is truly stochastic (a particular technical kind of random), that if we started evolution over as of a million years ago, we probably wouldn’t be here in our current form. That is, any evolutionary step is contingent on the history of steps that went before, each based on a combination of random mutation and environment.
I’ve read several posts about the new discovery today, and the best summary with accurate excerpts and clear analysis is this one from Pharyngula (PZ Meyers Myers) [1].
In brief: A single experiment ran over 20 years, or 33,000 generations of bacterial cultures, where they froze a sample every 500 generations from each of 20 separate populations, all nurtured identically over the entire time with a particular set of stressful conditions. When a particular beneficial change occurred to the population, they could track back genetically and see what the genetic change was, and what probably allowed it to manifest in a visible way. Then they tried to get the same thing to happen again starting from various suspected branching points. In some cases, the same mutation happened again.
Of course, Michael Behe of the Discovery Institute quickly posted a sort of rebuttal [2] to the idea that yet another piece of evolutionary theory has been proven, so Meyers took him to task. Behe claims that the experiment proves how incredibly unlikely such changes are, and therefore they need an Intelligent Designer to guide them. Apparently he missed the point that the complex series of changes did happen, and were repeatable, but only in a statistical manner. As opposed to in a pre-ordained, designed sort of way.
Or possibly his point is that God individually guides the evolution of laboratory E. Coli to fool scientists into thinking that supernatural intervention is unnecessary. It’s hard to tell.
[Admin note: here is the description of the experiment by New Scientist [3]]
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