We present a new approach to the virtual-topology reconfiguration problem for wavelength-routed, optical wide-area networks under dynamic traffic demand. By utilizing the measured Internet backbone traffic characteristics, an adaptation mechanism is proposed to follow the changes in traffic without assuming that the future traffic pattern is known. In that sense, our work differs from the previous studies which redesign the virtual-topology according to an expected (or known) traffic pattern, and then modify the connectivity to reach the target topology. The key idea of our approach is to adapt the underlying optical connectivity by measuring the actual traffic load on lightpaths continuously (periodically based on a measurement period) and reacting promptly to the imbalances caused by fluctuations on the traffic by adding or deleting one lightpath at a time. We aim to correct the encountered load imbalance directly, either by tearing down a lightpath that is lightly loaded or by setting up a new lightpath when congestion occurs. We introduce high and low watermark parameters on lightpath loads to detect any over/underutilized lightpath, and to trigger an adaptation step. The adaptation method is evaluated through simulations and the effect of system parameters (high and low watermarks, length of the measurement period) are investigated.