  We compare the luminosity functions for red galaxies lying on the restframe
(U-V) color-magnitude sequence in a homogeneous sample of ten X-ray luminous
clusters from the MACS survey at z~0.5 to a similarly selected X-ray cluster
sample at z~0.1. We exploit deep Hubble Space Telescope ACS imaging in the
F555W and F814W passbands of the central 1.2Mpc diameter regions of the distant
clusters to measure precise colors for the galaxies in these regions and
statistically correct for contamination by field galaxies using observations of
blank fields. We apply an identical analysis to ground-based photometry of the
z~0.1 sample. This comparison demonstrates that the number of faint, Mv~ -19,
red galaxies relative to the bright population seen in the central regions of
massive clusters has roughly doubled over the 4 Gyrs between z~0.5 and z~0.1.
We quantify this difference by measuring the dwarf to giant ratio on the red
sequence which increases by a factor of at least 2.2+/- 0.4 since z~0.5. This
is consistent with the idea that many faint, blue star-forming galaxies in high
density environments are transforming onto the red sequence in the last half of
the Hubble time.
