  The evolution of optically selected quasars is usually supposed to be well
described by a single constant parameter, either k_L or k_D, depending whether
we refer to luminosity or density evolution. In this paper we present a study
of the variations of the evolution parameters with redshift, for different
cosmological models, in order to probe the differential evolution with
redshift. Two different quasar samples have been analyzed, the AAT Boyle's et
al. and the LBQS catalogues. Basically, these samples are divided in redshift
intervals and in each of them k_L and k_D are estimated by forcing that
<V/Vmax>=0.5. The dependence with respect to the cosmological parameters is
small. Both AAT and LBQS show roughly the same tendencies. LBQS, however, shows
strong fluctuations, whose origin is not statistical but rather due to the
selection criteria. A discussion on selection techniques, biases and binning
effects explains the differences between these results. We finally conclude
that the evolution parameter is almost constant in the redshift range 0.7 < z <
1.7, at least within 2 sigma, while it decreases slightly afterwards. Results
depend on the binning chosen (but not in a very significant way). The method
has been tested with Monte-Carlo simulated catalogues in order to give a better
understanding of the results coming from the real catalogues. A correlation
between k_L (k_D) and <V/Vmax> is also derived and is used for the calculation
of the error bars on the evolution parameter.
