  We examine the evolution of the galaxy luminosity function (LF) using a
sample of over 2000 galaxies, with 0.12 < z < 0.55 and 17.0 < Rc < 21.5, drawn
from the Canadian Network for Observational Cosmology Field Galaxy Redshift
Survey (CNOC2), at present the largest such sample at intermediate redshifts.
We use UBVRcIc photometry to classify our galaxies into early, intermediate,
and late types, for which we compute luminosity functions in the rest-frame B,
Rc, and U bandpasses. In particular, we adopt a parameterization of LF
evolution including luminosity and number density evolution, and take care to
quantify correlations among our LF evolution parameters. Our principal result
is a clear quantitative separation of luminosity and density evolution for
different galaxy populations, and the finding that the character of the LF
evolution is strongly dependent on galaxy type. Specifically, the early- and
intermediate-type LF's show primarily brightening at higher redshifts and only
modest density evolution, while the late-type LF is best fit by strong number
density increases at higher z, with little luminosity evolution. We also
carefully measure and account for sample selection effects as functions of
galaxy magnitude and color, and show that our results are not significantly
affected by potential systematic effects, such as surface brightness selection,
photometric errors, or redshift incompleteness. (abstract abridged)
