  We compare environmental effects in two analogous samples of galaxies, one
from the Sloan Digital Sky Survey (SDSS) and the other from a semi-analytic
model (SAM) based on the Millennium Simulation (MS), to test to what extent
current SAMs of galaxy formation are reproducing environmental effects. We
estimate the large-scale environment of each galaxy using a Bayesian density
estimator based on distances to all ten nearest neighbors and compare
broad-band photometric properties of the two samples as a function of
environment. The feedbacks implemented in the semi-analytic model produce a
qualitatively correct galaxy population with similar environmental dependence
as that seen in SDSS galaxies. In detail, however, the colors of MS galaxies
exhibit an exaggerated dependence on environment: the field contains too many
blue galaxies while clusters contain too many red galaxies, compared to the
SDSS sample. We also find that the MS contains a population of highly
clustered, relatively faint red galaxies with velocity dispersions comparable
to their Hubble flow. Such high-density galaxies, if they exist, would be
overlooked in any low-redshift survey since their membership to a cluster
cannot be determined due to the "Fingers of God" effect.
