Rather than targeting specific traits, as Lamarck’s theory would have predicted, the mutations struck random genes, with some good outcomes and some bad. However, the process wasn’t completely random. Rosenberg’s findings suggested that bacteria were capable of increasing their mutation rates, which might in turn produce strains capable of surviving new conditions.

“Cells are able to adapt to stress not by knowing exactly what they need to do, but by throwing the dice as a population and making random changes to the genome,” said James Broach, a biologist at Pennsylvania State University’s College of Medicine in Hershey who studies a similar phenomenon in yeast. “That will allow stressed progeny to find an escape route.”

Does Evolution Evolve Under Pressure? – Wired Science.