ISN'T IT LUCKY THAT SOME PROTO giraffe, eons ago, developed a random mutation or two that produced a long neck? And that moths in 19th-century England spontaneously developed a mutation that made them as dark as the soot-covered trees where they alit (and could then be camouflaged)? Darwin thought that such random mutations, combined with nature selecting the "genetically fit," fully explained evolution. In other words, "mutations arise independently of biological needs," explains James Shapiro of the University of Chicago in the current issue of the journal Science. "The evolutionary watchmaker is blind." But for about a decade there have been hints that the watchmaker may, in fact, see. In experiments with bacteria, mutations that are useful arise more frequently than mutations that are neutral or deleterious-almost as if the microbes "know" which novel traits it would be good to have. To biologists, this unDarwinian notion has been borderline heresy. But now two papers in Science go a long way toward making "adaptive mutations" respectable.
The researchers, one team at MIT and one at the University of Utah, examined E. coli bacteria, the kind that live in the human gut. The bacteria lacked a gene that digests lactose-milk sugar-so the researchers put the microbes in a dish containing nothing but. Rather than dying of starvation, the colony of E. coli developed, 100 times more quickly than Darwinian evolution allows, a mutation that enabled them to eat lactose. That much had been reported last year, too. But the latest experiments give skeptics an,explanation of how the bacteria seem to anticipate desirable mutations. It has to do with what passes for sex in the microscopic world: the transfer of a circle of DNA called a plasmid, in this case containing a gene that digests lactose. It happens that the systems controlling bacterial sex rev up when a bug is starving, the researchers report-exactly the situation of bacteria unable to digest lactose. Adaptive mutations probably don't work this way in higher organisms like giraffes and moths. But if primitive organisms preferentially acquire traits that help them survive, biologists will have to admit that, 136 years after "The Origin of Species," evolution still holds surprises.