Recovery from costs associated with adaptation will often lead to loss
of adaptive alleles once selection in their favor stops

Bacteria can rapidly adapt to almost any imaginable selective
pressure. Many rapid adaptations display antagonistic pleiotropy,
meaning that while they are adaptive to certain bacterial traits they
harm others. This raises the question of whether such costly
adaptations will tend to persist once selection in their favor is no
longer exerted. Bacteria can compensate for many of the harmful
effects of adaptive alleles through compensatory mutations. Such
compensatory mutations can occur at various different loci and
therefore likely occur more frequently than reversion mutations that
revert an adaptive allele back to its original form. It is thus
thought that costly adaptations will more likely persist, due to their
deleterious effects being compensated for, rather than revert back to
their original form. Here we demonstrate that once selection in favor
of a costly adaptation stops, such adaptations are likely to be much
more frequently lost than was previously thought. We show that due to
their ability to maintain high levels of standing genetic variation
during adaptation, bacterial populations can rapidly fluctuate the
frequencies of their genotypes. This in turn leads to the rapid loss
of costly adaptations, once they are no longer useful. Furthermore,
during adaptation, mutators, defective in their mismatch repair often
emerge. We show that in such mutators, reversion of costly alleles
back to their ancestral form tends to occur, again leading to the loss
of costly adaptations.