Posted on 05/18/2006 11:16:00 AM PDT by PatrickHenry
Forcing Darwin's hand: capturing natural selection in a flask
Even with modern genomic tools, it's a daunting task to find a smoking gun for Darwinian evolution. The problem lies in being able to say not just when and how a specific gene mutated but also how that one genetic change translated into real-world dominance of one population over another.
Rice University biologists, using an ingenious experiment that forced bacteria to compete in a head-to-head contest for evolutionary dominance, today offer the first glimpse of how individual genetic-level adaptations play out as Darwinian natural selection in large populations. The results appear in the May 19 issue of Molecular Cell.
"One of our most surprising findings is that an estimated 20 million point mutations gave rise to just six populations that were capable of vying for dominance," said lead researcher Yousif Shamoo, associate professor of biochemistry and cell biology. "This suggests that very few molecular pathways are available for a specific molecular response, and it points to the intriguing possibility of developing a system to predict the specific mutations that pathogens will use in order to become resistant to antibiotics."
Rice's study involved the heat-loving bacteria G. stearothermophilus, which thrives at up to 73 degrees Celsius (163 F). Shamoo and graduate students Rafael Couñago and undergraduate Stephen Chen used a mutant strain of the microbe that was unable to make a key protein that the bacteria needed to regulate its metabolism at high temperatures. They grew the bacteria for one month in fermentor, raising the temperature a half degree Celsius each day.
Over a span of 1,500 generations, the percentage of mutant strains inside the fermentor ebbed and flowed as the single-celled microbes competed for dominance. Eventually, one strain squeezed out almost all the competition by virtue of its ability to most efficiently metabolize food at high temperature.
The metabolic protein required to thrive at high-temperature could only be made in one genetic region of the bacteria's DNA, meaning the researchers had only to characterize that small region of the genome for each new strain in order to measure evolutionary progress.
The researchers sampled the fermentor for new strains every other day. Though millions of mutations in the target gene are believed to have occurred, only about 700 of those were capable of creating a new variant of the target gene. In all, the researchers identified 343 unique strains, each of which contained one of just six variants of the critical gene.
The first of the six, dubbed Q199R, arose almost immediately, and was the dominant strain through the 500 th generation. Around 62 degrees Celsius, the Q199R was unable to further cope with the rising temperature, and a new round of mutations occurred. Five new varieties - themselves mutant forms of Q199R - vied for final domination of the fermentor. Three of the five were driven to extinction within a couple of days, and the final two fought it out over the remaining three weeks of the test.
The research included a raft of additional experiments as well. The team characterized each of the mutant proteins to document precisely how it aided in metabolic regulation. The fermentor experiment was repeated and the same mutations - and no others - were observed to develop again. Three of the six genes - the "winner," it's closest competitor and Q199R - were spliced back into the original form of the bacteria and studied, to rule out the possibility that mutations in other genes were responsible for the competitive advantage.
Shamoo said it's significant that the mutations didn't arise where expected within the gene. Four of the six occurred in regions of the gene that are identical in both heat-resistant and non-heat-resistant forms of G. stearothermophilus . Shamoo said this strongly shows the dynamic nature of evolution at the molecular and atomic level.
Shamoo said the most promising finding is the fact that the follow-up test produced precisely the same mutant genes.
"The duplicate study suggests that the pathways of molecular adaptation are reproducible and not highly variable under identical conditions," Shamoo said.
The research was funded by the National Science Foundation, the Welch Foundation and the Keck Center for Computational and Structural Biology.
The mutations are there, hanging around - just in case. It is a way of "hedging bets". Stay loose and diverse.
Call it an as yet unsupported hypothesis. ;)
Well, yeah. I would be too. But you don't know either or you wouldn't say that you were *reasonably sure*. Did you read the whole study then?
And how does asking for the method of how they determined the number of mutations indicate that I don't understand the underlying science? It was a question concerning *knowing* how they got to the estimate, not *understanding* the science.
Should nobody, then, dare to question how science was done without fear of being accused of ignorance of the science? If another *real scientist* questioned it, would he get the same *learn what you're talking about first* kind of response that creationists/IDers get? I think not.
Don't you have a sense of humor?
Yes ;)
"Did you read the whole study then?"
Nope. I read the press release. I trust the scientific process enough to know that every expert in the field will question the experiment and its outcome, and they DO know the science well enough to call BS if it's wrong.
"And how does asking for the method of how they determined the number of mutations indicate that I don't understand the underlying science? It was a question concerning *knowing* how they got to the estimate, not *understanding* the science."
So do you understand the science? I don't - I have already said that I'm not a molecular biologist; are you? I have enough understanding to grasp the basic premise, I believe.
"Should nobody, then, dare to question how science was done without fear of being accused of ignorance of the science? If another *real scientist* questioned it, would he get the same *learn what you're talking about first* kind of response that creationists/IDers get? I think not."
Absolutely another scientist, would not get told to learn what he's talking about. If you are a molecular biologist, working in the study on mutations in DNA within bacteria, I will retract my contention that you question the research because you are clueless of the science.
"It was a question concerning *knowing* how they got to the estimate, not *understanding* the science."
Actually, I can't let this rest...
My issue with your post was not that you asked the question about where the estimate came from. My issue was with your blanket statement that "there isn't much to substantiate this", without knowing the underlying science.
But then, I suspect that you knew that.
People who don't accept the ToE are generally considered not *real scientists* and regularly told to get an education and come back when they know what they are talking about, regardless of their scientific background. Even if a physicist questioned the molecular biologist's experiment, he would get more deference than is given almost any non-evo on this forum. The underlying assumption is that one who rejects evo rejects all science and is ignorant of how it and science works and that is not true. Rejecting the ToE and the interpretation of the fossil record is not a blanket rejection of science as a whole.
Thanks, I got it. The article said the study's results were reproducible. That's what got me to wonder about this. But, the study also started with a large number of bacteria, so I don't know that it was comparable in the way I was thinking.
Interesting! Thank you.
An isolated environment would help keep that from happening.
Cheers!
So this adds a little noise. It doesn't change the relative selective advantage.
So this adds a little noise. It doesn't change the relative selective advantage.
That's my point, that it does add noise. So while it does a great job of illustrating the processes involved, it might give an erroneous estimate of the rate at which mutations take hold and propagate.
The next step would be to apply find a separate second stressor on the population, and systematically vary that to see the mutation rate for selection involving only the second stressor.
Then perform an experiment where both things are varied simultaneously, to see how multiple selection pressures interfere one with another...
Cheers!
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.