  We consider several mechanisms that possibly affect the evolution of disk
galaxies in clusters using analytical models based on a hierarchical clustering
scenario. We especially focus on the evolution of disk galaxies in subclusters
located around a main cluster. We show that ram-pressure stripping cannot be
always ignored in subclusters, although their masses are much smaller than that
of the main cluster. The star-formation rate of a galaxy may gradually decrease
by the stripping of warm gas (`strangulation') in a main cluster. However, we
find that ram-pressure stripping could start before the strangulation is
completed, if a field galaxy directly falls into the main cluster. Since this
conflicts with some recent observations, many galaxies might have been affected
by some environmental effects when they were in subclusters before they fell
into the main cluster (`pre-processing'). We show that strangulation and
evaporation of the cold gas by the surrounding hot ICM in subclusters are the
possible candidates. We also show that the observed morphological
transformation of disk galaxies at z ~< 1 is not chiefly due to galaxy mergers.
