  We present N-body simulations of Low Surface Brightness (LSB) galaxies and
their Interstellar Medium to investigate the cause for their low star formation
rates (SFR).Due to their massive halos, stellar disks of LSB galaxies are very
stable and thin. Lack of dust makes the projected edge-on surface brightness of
LSB galaxies comparable to the projected edge-on surface brightness of
dust-rich High Surface Brightness (HSB) galaxies of similar size.
  We show that the low surface densities found in LSB galaxies are by
themselves not enough to explain the slow evolution of LSB galaxies. A low
metal content of the gas is essential. As a consequence the gas cools
inefficiently, resulting in an almost negligible cold gas fraction. We show
that LSB galaxies must have molecular gas fractions of less than 5 percent.
  Our best model has a SFR which is on average low but fluctuates strongly.
This causes the large spread in colors of LSB galaxies. From a distribution of
birthrate parameters we conclude that the presently-known and modeled gas-rich
blue LSB galaxies constitute the majority of the total population of gas-rich
LSB disk galaxies. We deduce the existence of an additional red, quiescent and
gas-rich population which constitutes less than 20 percent of the total
population. This does not rule out the existence of a large number of gas-poor
LSB galaxies. These must however have had an evolutionary history dramatically
different from that of the gas-rich galaxies.
