We outline the revised physical assumption which have been incorporated into our previously published evolutionary model for stellar population synthesis in galaxies. Applying the present version of the model to well-defined zones on the nucleus and along a spiral arm of NGC 2903 we show that the star formation rate has increased during the lifetime of the galaxy, steadily but only slightly in the arm, and spectacularly in the nucleus, where there is current starburst activity. We also infer that considerable gaseous inflow has occurred in the arm during the lifetime of the disc.