 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.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.This would not be possible if adaptive mutation was not at work because natural selection would not favor this mutation in the new environment.