  We present a model for the broad morphological distinction between the disk
and spheroidal components of galaxies. Elaborating on the hierarchical
clustering scheme of galaxy formation proposed by Cole et al., we assume that
galaxies form stars quiescently in a disk until they are disrupted into a
spheroidal configuration by mergers. We calculate formation and merging
histories, and the evolution in colour, luminosity and morphology of the galaxy
populations in different environments. Roughly $ 50\ %$ of ellipticals in our
model have undergone a major merger since $z = 0.5$, yet in spite of this we
find that cluster ellipticals have colour-magnitude diagrams with remarkably
small scatter. The morphological mix of galaxies that become rich cluster
members at high redshift is dominated by spiral galaxies, due to the long
timescale for galaxy mergers compared with the timescale for cluster assembly
at high redshift. The assembly of low redshift clusters is slower, allowing
more galaxy mergers to occur in the progenitor halos. As a result $z=0$ rich
clusters become E/S0 dominated and we find a ``Butcher-Oemler'' effect that
becomes weaker for poorer groups at high redshift. The field luminosity
function of red galaxies shows little evolution out to $z\simeq 1$ and the
reddest galaxies at these redshifts are as bright as their local counterparts.
The blue luminosity function, on the other hand, evolves rapidly with redshift,
increasing its characteristic luminosity and becoming steeper at the faint end.
These trends are similar to those recently observed in the Canada-France
Redshift Survey.
