We examine the origins of the bimodality observed in the global properties of
galaxies by comparing the environmental dependencies of star-formation for
giant and dwarf galaxy populations. Using Sloan Digital Sky Survey (SDSS) DR4
spectroscopic data to create a volume-limited sample complete to M*+3, we find
that the environmental dependences of giant and dwarf galaxies are quite
different, implying fundamental differences in their evolution. Whereas the
star-formation histories of giant galaxies are determined primarily by their
merger history, resulting in passively-evolving giant galaxies being found in
all environments, we show that this is not the case for dwarf galaxies. In
particular, we find that old or passive dwarf galaxies are only found as
satellites within massive halos (clusters, groups or giant galaxies), with none
in the lowest density regions. This implies that star-formation in dwarf
galaxies must be much more resilient to the effects of mergers, and that the
evolution of dwarf galaxies is primarily driven by the mass of their host halo,
through effects such as suffocation, ram-pressure stripping or galaxy
harassment.Comment: 4 pages, 1 figure, to appear in the proceedings of "Cosmic
Frontiers", Durham, August 200
