Environmental mechanisms shaping the nature of dwarf spheroidal
  galaxies; the view of computer simulations
  We review numerical work carried out over the last decade on the role of
environmental mechanisms in shaping the nature of the faintest galaxies known,
dwarf spheroidals (dSphs). In particular we discuss a model in which dSphs
originated from gas dominated, disky dwarfs that were accreted by massive
galaxies several billions of years ago. Dwarfs accreting at $z > 1$, when the
cosmic ultraviolet ionizing flux was much higher than today and was thus able
to keep the gas in the dwarfs warm and diffuse, were rapidly stripped of their
baryons via ram pressure and tidal forces, producing very dark matter dominated
objects with truncated star formation histories, such as the Draco dSph. The
low star formation efficiency expected in such low-metallicity objects prior to
their infall was crucial for keeping their disks gas dominated until stripping
took over. {\it Therefore gas stripping along with inefficient star formation
provides a new feedback mechanism, alternative to photoevaporation or
supernovae feedback, playing a crucial role in dwarf galaxy formation and
evolution}. Tidally induced instabilities, termed "tidal stirring", turned the
surviving stellar disk into a spheroid. Moreover, we examine the properties of
the newly discovered ultra-faint dSphs in light of this scenario and argue that
they likely belong to a different population of lower mass dwarf satellites.
These were mostly shaped by reionization rather than by environmental
mechanisms and are thus good candidates for being "reionization fossils". We
discuss implications of the morphological transformation scenario on the
substructure problem. (Abridged)
