Citrate Evolution Explained

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Citrate evolution explained, as reported in Nature, 2012; DOI: 10.1038/nature11514. In 1988 Richard Lenski and his team at Michigan State University began culturing bacteria named E. coli in a medium containing the sugar glucose and a natural food acid named citrate. Normally E. coli do not eat citrate when oxygen is present in the environment they are growing in. After 31,500 generations Lenski found one culture of E. coli was able to eat citrate when oxygen was present. This has been claimed to be proof that bacteria have evolved a new function, and that useful information can be generated by naturalistic processes. According to Richard Dawkins, “the Lenski experiments are distressing to creationists, and for a very good reason. They are a beautiful demonstration of evolution in action”. (Richard Dawkins, The Greatest Show on Earth, Bantam Press, 2009, p117)

Further research by Lenski’s team has now revealed how this change happened. A part of the bacterial genome was duplicated and the extra piece was inserted back into the genome, but at an abnormal place. This resulted in a control gene ending up near to a citrate transporter gene, which meant the citrate transporter became active under conditions when it would have otherwise been inactive. Lenski’s team described this change as “promoter capture” and claim it is an important process in evolution of new traits.

Editorial Comment: When the citrate eating bacteria were first reported in 2008, Lenski suggested: “… an existing transporter has been coopted for citrate transport under oxic conditions. This transporter may previously have transported citrate under anoxic conditions or, alternatively, it may have transported another substrate in the presence of oxygen. The evolved changes might involve gene regulation, protein structure, or both”. (PNAS, vol. 105 pp7899-7906 10 June 2008) When Lenski made this suggestion we agreed with him because a) no new genes would have appeared and b) change in gene regulation is not evolution – it just means a gene is turned on when it previously had been turned off. Now the Lenski team’s new research has revealed how this happened, and no evolution is involved. An extra copy of the “on” switch for citrate transport has been accidently made and has ended in a place where it turns on the citrate transporter all the time, instead of only some of the time. No new genes have been made. The extra copy of the promoter gene is the result of the genome being garbled by a bad copying process, and the result is inappropriate activation of citrate transporting genes. We also predict that as in the case of all wrongly duplicated/inserted copies, there should be a yet to be discovered downside to this “mistake”, just as there is when people are born with extra fingers or toes.

We also hate to disappoint Dawkins, but we were never stressed by Lenski’s experiment. After multiplying for 25 years Lenski’s bacteria are still E. coli, i.e. they have multiplied after their kind. Furthermore, the fact that the bacteria could already eat citrate under some conditions meant they already had a means of transporting it into the cell – it just wasn’t turned on all the time. Therefore, no evolution was needed to produce a citrate transporter. All that needed to happen was that it be activated under conditions when it was normally closed. (Ref. microbiology, genetics, biochemistry)

Evidence News 24 April 2013

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