Science alert! If you are utterly disinterested in the world around you, don’t bother to read this post as it will undoubtedly bore you.
I love evolutionary biology and genetics. The intricacies of life never cease to fascinate me, so when I read of studies like this I am so filled with wonder and excitement that I want to share it with everyone. To sum up this link for those of you not inclined to read it themselves (tsk tsk), this experiment, conducted over the course of 20 years, provided supporting evidence to Gould’s hypothesis of evolutionary contingency, in which he puts forth the idea that
…if the tape of life were rewound to the time of the organisms found in the Canadian outcrop known as the Burgess Shale, dated to about 530 million years ago, and replayed with a few contingencies tweaked here and there, humans would most likely never have evolved (source link here).
In other words, Gould suggested that you cannot re-evolve the same end result from the same starting point.
This particular experiment took 12 colonies of identical E. coli bacteria and let them flourish in identical environments for 20 years, or 40,000 generations. Every 500 generations a sample from each of the 12 colonies was frozen, to be thawed out and to resume growth at a later date, for comparison.
At around generation 31,500, something interesting happened. From the article:
After about 31,500 generations, one colony of bacteria evolved the novel ability to use a nutrient that E. coli normally can’t absorb from its environment. Thawed-out samples from after the 20,000-generation mark were much more likely to re-evolve this trait than earlier samples, which suggests that an unnoticed mutation that occurred around the 20,000th generation enabled the microbes to later evolve the nutrient-absorption ability through a second mutation…In the 11 other colonies, this earlier mutation didn’t occur, so the evolution of this novel ability never happened.
“I would argue that this is a direct empirical demonstration of Gould-like contingency in evolution,” Lenski [the lead researcher] says. “You can’t do an exact replay in nature, but we were able to literally put all these populations in virtually identical environments and show that contingency is really what had occurred.”
The next step will be to determine what that earlier mutation was and how it made the later change possible, Lenski says. If the first mutation didn’t offer any survival advantage to the microbes on its own, it would make the case airtight that Gould was right. That’s because a mutation that doesn’t improve an organism’s ability to survive and reproduce can’t be favored by evolution, so whether the microbe happens to have that necessary mutation when the second evolutionary change occurs becomes purely a matter of chance.
“I don’t think they’ve necessarily shown” that the first mutation gave the microbes no survival advantage, comments Christopher Dascher, a microbiologist at Mount Sinai School of Medicine in New York. “But they certainly point very strongly in that direction.”
Lenski notes that the growth rate and the density of bacteria in the colony jumped up after the second mutation, but not after the first one.
This is the statement that gets me excited: “a mutation that doesn’t improve an organism’s ability to survive and reproduce can’t be favored by evolution, so whether the microbe happens to have that necessary mutation when the second evolutionary change occurs becomes purely a matter of chance.”
I know its completely nerdy and geeky of me, but I find this to be totally awesome. And a definite rejection of the creationist assertion that there is no proof of evolution in science. So to those individuals, I say “take that!”
Lessons Learned 