Trajectories of expression levels (population averages) for populations adapting in constant environments. Open symbols indicate the expression level induced with 220 μM IPTG, as assayed with a sample isolated from the evolving population. Solid symbols indicate the uninduced expression level, as assayed with a sample isolated from the population. Curves are fits based on growth rate differences under exponential growth (Additional file 1). Where induction levels are the same as in the environment to which the populations adapted, the curves are solid. Conditions during the evolutionary experiments: a) 0 μM IPTG, 350 μM Pgal. Two populations evolved in parallel are shown to yield the same adaptation dynamics (triangles and squares). b) 220 μM IPTG, 350 μM Pgal. c) 0 μM IPTG, 0 μM Pgal. d) 220 μM IPTG, 0 μM Pgal. e) 2 μM IPTG, 350 μM Pgal. f) 2 μM IPTG, 0 μM Pgal.