Trajectories of expression levels for the populations evolved in alternating environments. Open symbols represent induced expression levels (at 220 μM IPTG), solid symbols are uninduced expression levels. Curves are fits based on growth rate differences under exponential growth (Additional file 1). a) an environment alternating between 2 μM IPTG, 350 μM Pgal and 220 μM IPTG, 39 μM Pgal. b) an environment alternating between no IPTG, no Pgal and 2 μM IPTG, 350 μM Pgal. Both populations evolve towards a constitutive expression.