  Using wide-field $BVR_cI$ imaging for a sample of 16 intermediate redshift
($0.17 < z < 0.55$) galaxy clusters from the Canadian Network for Observational
Cosmology (CNOC1) Survey, we investigate the dependence of cluster galaxy
populations and their evolution on environment. Galaxy photometric redshifts
are estimated using an empirical photometric redshift technique and galaxy
groups are identified using a modified friends-of-friends algorithm in
photometric redshift space.We utilize the red galaxy fraction (\fred) to infer
the evolutionary status of galaxies in clusters, using both individual galaxies
and galaxies in groups. We apply the local galaxy density, \sig5, derived using
the fifth nearest-neighbor distance, as a measure of local environment, and the
cluster-centric radius, \rCL, as a proxy for global cluster environment. Our
cluster sample exhibits a Butcher-Oemler effect in both luminosity-selected and
stellar-mass-selected samples. We find that \fred depends strongly on \sig5 and
\rCL, and the Butcher-Oemler effect is observed in all \sig5 and \rCL bins.
However, when the cluster galaxies are separated into \rCL bins, or into group
and non-group subsamples, the dependence on local galaxy density becomes much
weaker. This suggests that the properties of the dark matter halo in which the
galaxy resides have a dominant effect on its galaxy population and evolutionary
history. We find that our data are consistent with the scenario that cluster
galaxies situated in successively richer groups (i.e., more massive dark matter
halos) reach a high \fred value at earlier redshifts. Associated with this, we
observe a clear signature of `pre-processing', in which ... <and more>
