Dispersive line shapes have been observed in the population of the intermediate level of a three-level configuration in a fast beam of metastable Ca*40 atoms. A steady-state calculation in the weak-probe approximation is used to identify the main physical processes in the interaction of the system with a weakly saturating pump field and a weak probe field. It is shown how optical-pumping effects between the lower levels can be used to select one particular two-photon process. The selection is enhanced by the narrow Doppler profile of the fast beam. Our simple analysis explains the resulting dispersive line shapes both qualitatively and quantitatively.