The most widely accepted theory of evolution states that organisms are modified by natural selection where changes caused by mutations improve their chance of reproductive success.
[3] Adaptive mutation is a controversial claim leading to a series of experiments designed to test the idea.
Due to a frameshift mutation, a change in the sequence that causes the DNA to code for something different, FC40 is unable to process lactose.
The responses to stress are not in current DNA, but the change is made during DNA replication through recombination and the replication process itself, meaning that the adaptive mutation occurs in the current bacteria and will be inherited by the next generations because the mutation becomes part of the genetic code in the bacteria.
[5] This is particularly obvious in a study by Cairns, which demonstrated that even after moving E. coli back to a medium with minimal levels of lactose, Lac+ mutants continued to be produced as a response to the previous environment.
On the other hand, RecG mutants were key to the expression of RecA-dependent mutations, which were a major portion of study in the SOS response experiments, such as the ability to utilize lactose.
[6] Adaptive mutation was re-proposed in 1988[7] by John Cairns who was studying Escherichia coli that lacked the ability to metabolize lactose.
In doing so, he found that the rate at which the bacteria evolved the ability to metabolize lactose was many orders of magnitude higher than would be expected if the mutations were truly random.
[8] Later support for this hypothesis came from Susan Rosenberg, then at the University of Alberta, who found that an enzyme involved in DNA recombinational repair, recBCD, was necessary for the directed mutagenesis observed by Cairns and colleagues in 1989.
[9][10] Later research from 2007 however, concluded that amplification could not account for the adaptive mutation and that "mutants that appear during the first few days of lactose selection are true revertants that arise in a single step".
He found that when yeast colonies were moved from a tryptophan-rich medium to a minimal one, revertants continued to appear for several days.