The dependence of the average spectral index of extragalactic sources on luminosity in complete samples selected at different frequencies is shown to vary in a manner that directly reflects their evolutionary properties. In particular, easily recognizable features are introduced by a luminosity dependent evolution and by the presence of a cutoff in the redshift distribution. The expected dependence on luminosity of the average optical to X-ray energy index αox for X-ray selected active galactic nuclei is discussed in some detail.
