  We compare the galaxy population of an SPH simulation to those predicted by
the GalICS semi-analytic model and a stripped down version without supernova
and AGN feedback. The SPH simulation and the no-feedback GalICS model make
similar predictions for the baryonic mass functions of galaxies and for the
dependence of these mass functions on environment and redshift. The two methods
also make similar predictions for the galaxy content of dark matter haloes as a
function of halo mass and for the gas accretion history of galaxies. Both the
SPH and no-feedback GalICS models predict a bimodal galaxy population at z=0.
The "red'' sequence of gas poor, old galaxies is populated mainly by satellite
systems while, contrary to observations, the central galaxies of massive haloes
lie on the "blue'' star-forming sequence as a result of continuing hot gas
accretion at late times. Furthermore, both models overpredict the observed
baryonic mass function, especially at the high mass end. In the full GalICS
model, supernova-driven outflows reduce the masses of low and intermediate mass
galaxies by about a factor of two. AGN feedback suppresses gas cooling in large
haloes, producing a sharp cut-off in the baryonic mass function and moving the
central galaxies of these massive haloes to the red sequence. Our results imply
that the observational failings of the SPH simulation and the no-feedback
GalICS model are a consequence of missing input physics rather than
computational inaccuracies, that truncating gas accretion by satellite galaxies
automatically produces a bimodal galaxy distribution with a red sequence, but
that explaining the red colours of the most massive galaxies requires a
mechanism like AGN feedback that suppresses the accretion onto central galaxies
in large haloes.
