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Do-it-yourself evolution appears unlikely.

Do-it-yourself evolution appears unlikely

Evolutionary biologists John E. Mittler and Richard E. Lenski performed a few straightforward experiments and got the kind of results that pretty much everyone expected. In this case, that's news.

The University of California, Irvine, researchers set out to test the validity of a controversial report suggesting that bacteria can direct their evolutionary development in ways best suited to their particular needs. That radical proposal, made by John Cairns of the Harvard University School of Public Health in Boston (SN: 9/10/88, p. 166), ran counter to traditional Darwinian thought. Darwin held that mutations occur randomly in nature and that helpful mutations simply outsurvive harmful ones when subjected to selective environmental pressures.

Cairns based his conclusion on experiments he performed on a strain of the common gut bacteria Escherichia coli that contains a little piece of viral DNA. The so-called Lac- strain cannot metabolize the sugar lactose, so does not grow on media with only lactose as a nutrient. But if a Lac- bacterium mutates in a way that kicks out the viral DNA, it becomes Lac+, regaining the ability to metabolize lactose and triggering growth. Cairns' research suggested that compared to bacteria who are under no pressure to do so, Lac- bacteria placed in an environment where lactose is the only food available are much more likely to mutate into Lac+ variants.

The new work by Mittler and Lenski provides strong evidence that this mutation results not from any process of self-directed mutation, but because bacteria placed in an environment with no available food tend to eject viral DNA insertions more frequently than do well-fed bacteria. "The rate of excision mutation per viable cell per day increases by orders of magnitude as cells sit starving for several days," irrespective of whether lactose is present, they report in the March 8 NATURE.

But the issue remains far from settled. Cairns says he and others have been unable to duplicate the California researchers' findings. And both groups agree that the particular bacteria they've been using may be a less than perfect experimental system, as the viral sequence itself may be responding independently to the pressures of starvation. "In short, it's a bit of a mess," Cairns says.

It's impossible for now to completely rule out the possibility that some degree of directed mutation occurs in nature, says Bruce R. Levin, a population geneticist at the University of Massachusetts in Amherst. However, he adds, the new work "very clearly shows that the observation Cairns made can be explained by more mundane processes."
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Author:Weiss, R.
Publication:Science News
Date:Mar 10, 1990
Words:425
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