  An empirically motivated model is presented for accretion-dominated growth of
the super massive black holes (SMBH) in galaxies, and the implications are
studied for the evolution of the quasar population in the universe. We
investigate the core aspects of the quasar population, including space density
evolution, evolution of the characteristic luminosity, plausible minimum masses
of quasars, the mass function of SMBH and their formation epoch distribution.
Our model suggests that the characteristic luminosity in the quasar luminosity
function arises primarily as a consequence of a characteristic mass scale above
which there is a systematic separation between the black hole and the halo
merging rates. At lower mass scales, black hole merging closely tracks the
merging of dark halos. When combined with a declining efficiency of black hole
formation with redshift, the model can reproduce the quasar luminosity function
over a wide range of redshifts. The observed space density evolution of quasars
is well described by formation rates of SMBH above $\sim 10^8 M_\odot$. The
inferred mass density of SMBH agrees with that found independently from
estimates of the SMBH mass function derived empirically from the quasar
luminosity function.
