  We present photometric and spectroscopic measurements of the galaxy
populations in clusters from the CNOC1 sample of rich, X-ray luminous clusters
at $0.18 < z < 0.55$. A classical measure of the galaxy blue fraction for
spectroscopically confirmed cluster members shows a significant Butcher-Oemler
effect for the sample, but only when radii larger than 0.5$r_{200}$ are
considered. We perform a principal component analysis of galaxy spectra to
divide the total cluster light into contributions from stellar populations of
different ages. Composite radial distributions of different stellar populations
show strong gradients as a function of cluster-centric radius. The composite
population is dominated by evolved populations in the core, and gradually
changes at radii greater than the virial radius to one which is similar to
coeval field galaxies. We do not see evidence at any radius within the clusters
for an excess of star formation over that seen in the coeval field. Within this
redshift range, significant evolution in the fractional population gradient is
seen. Both low and high redshift clusters have similar populations in the
cluster cores, but higher redshift clusters have steeper gradients and more
star forming galaxies at radii outside of the core region~-- in effect, a
restatement of the Butcher-Oemler effect. Luminosity density profiles are
consistent with a scenario where this phenomenon is due to a decline over time
in the infall rate of field galaxies into clusters. Depending on how long
galaxies reside in clusters before their star formation rates are diminished,
this suggests a decrease in the infall into clusters of a factor of $\sim 3$
between $z > 0.8$ and $z \sim 0.5$. We also discuss alternative scenarios for
the evolution of cluster populations.
