According to the authors of the paper, research has uncovered "the evolution of a key innovation"; the New Scientist report refers to "a major evolutionary innovation" and "a rare and complex new trait"; The Scientist calls it a "big evolutionary jump". They are referring to the "ability to metabolise citrate, a [. . .] nutrient in their culture medium that E. coli normally cannot use."
Escherichia coli (source here)
The experimental work involved keeping 12 strains of the bacterium E. coli in a medium where there was glucose for them to metabolise, but also a plentiful supply of citrate. "Examining E. coli cultures that his lab has maintained since 1988, Lenski found that one population of the bacterium had evolved the ability to metabolize citrate - an unprecedented trait - after more than 30,000 generations, or approximately 15 years." Using samples preserved from earlier stages of this extended experiment, the research team established that one of the 12 populations gained a hidden mutation after about 20,000 generations. Only this population later developed the ability to metabolise citate. Thus, it can be inferred that "this evolutionary novelty grew from the accumulation of unpredictable, chance events."
"It's a very elegant demonstration that major changes may depend on accretion of minor changes before hand," said Albert Bennett, a University of California, Irvine evolutionary physiologist who gave Lenski feedback on the study before it was published in PNAS. "What's really demonstrated here is that the way has to be paved before hand."
This research is being hailed as a vindication of evolutionary theory against the sceptics:
"Lenski's experiment is also yet another poke in the eye for anti-evolutionists, notes Jerry Coyne, an evolutionary biologist at the University of Chicago. "The thing I like most is it says you can get these complex traits evolving by a combination of unlikely events," he says. "That's just what creationists say can't happen."
There are several observation that should be made before reaching general conclusions. The first relates to the machinery needed to metabolise citrate. The system to do this is already largely in place, but one enzyme is lacking. This is the comment from Mike Behe:
"Now, wild E. coli already has a number of enzymes that normally use citrate and can digest it (it's not some exotic chemical the bacterium has never seen before). However, the wild bacterium lacks an enzyme called a "citrate permease" which can transport citrate from outside the cell through the cell's membrane into its interior. So all the bacterium needed to do to use citrate was to find a way to get it into the cell. The rest of the machinery for its metabolism was already there. As Lenski put it, "The only known barrier to aerobic growth on citrate is its inability to transport citrate under oxic conditions."Consequently, it is at least worth asking the question whether the E.coli bacterium had, in the past, lost the ability to metabolise citrate and what we are now seeing is a restoration of that damaged system. If this were the case, we should not be talking about "a major evolutionary innovation" but rather about the way complex systems can be impaired by mutations.
As yet, it is not known what mutations were involved. But clearly, if there were two, and if the first was needed before the second could complete the job, the experiments demonstrate how difficult it is to achieve orchestrated changes. This was exactly the point of Behe's study of ways of achieving resistance to malaria. In his words:
"I think the results fit a lot more easily into the viewpoint of The Edge of Evolution. One of the major points of the book was that if only one mutation is needed to confer some ability, then Darwinian evolution has little problem finding it. But if more than one is needed, the probability of getting all the right ones grows exponentially worse. "If two mutations have to occur before there is a net beneficial effect - if an intermediate state is harmful, or less fit than the starting state - then there is already a big evolutionary problem." And what if more than two are needed? The task quickly gets out of reach of random mutation."
So, far from this research being "another poke in the eye for anti-evolutionists", it demonstrates a major problem for those evolutionists who want to claim Darwinism can achieve major transformations. These mutations are not only rare, they are also useless without the pre-existence of a biochemical system that can turn the products of mutation into something beneficial. Behe writes:
"If the development of many of the features of the cell required multiple mutations during the course of evolution, then the cell is beyond Darwinian explanation. I show in The Edge of Evolution that it is very reasonable to conclude they did."
Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli
Zachary D. Blount, Christina Z. Borland, and Richard E. Lenski
Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0803151105
Abstract: The role of historical contingency in evolution has been much debated, but rarely tested. Twelve initially identical populations of Escherichia coli were founded in 1988 to investigate this issue. They have since evolved in a glucose-limited medium that also contains citrate, which E. coli cannot use as a carbon source under oxic conditions. No population evolved the capacity to exploit citrate for >30,000 generations, although each population tested billions of mutations. A citrate-using (Cit+) variant finally evolved in one population by 31,500 generations, causing an increase in population size and diversity. The long-delayed and unique evolution of this function might indicate the involvement of some extremely rare mutation. Alternately, it may involve an ordinary mutation, but one whose physical occurrence or phenotypic expression is contingent on prior mutations in that population. We tested these hypotheses in experiments that "replayed" evolution from different points in that population's history. We observed no Cit+ mutants among 8.4 x 1012 ancestral cells, nor among 9 x 1012 cells from 60 clones sampled in the first 15,000 generations. However, we observed a significantly greater tendency for later clones to evolve Cit+, indicating that some potentiating mutation arose by 20,000 generations. This potentiating change increased the mutation rate to Cit+ but did not cause generalized hypermutability. Thus, the evolution of this phenotype was contingent on the particular history of that population. More generally, we suggest that historical contingency is especially important when it facilitates the evolution of key innovations that are not easily evolved by gradual, cumulative selection.
Behe, M. Multiple mutations needed for E. coli, Amazon Blog, 6 June 2008
Grant, B. Evolution loves history, The Scientist Newsblog, 2 June 2008
Holmes, B. Bacteria make major evolutionary shift in the lab, New Scientist, 09 June 2008
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