  Using the L-Galaxies semi-analytic model we simultaneously fit the HI mass
function, stellar mass function and galaxy colours. We find good fits to all
three observations at z = 0 and to the stellar mass function and galaxy colours
at z = 2. Using Markov Chain Monte Carlo (MCMC) techniques we adjust the
L-Galaxies parameters to best fit the constraining data. In order to fit the HI
mass function we must greatly reduce the gas surface density threshold for star
formation, thus lowering the number of low HI mass galaxies. A simultaneous
reduction in the star formation efficiency prevents the over production of
stellar content. A simplified model in which the surface density threshold is
eliminated altogether also provides a good fit to the data. Unfortunately,
these changes weaken the fit to the Kennicutt-Schmidt relation and raise the
star-formation rate density at recent times, suggesting that a change to the
model is required to prevent accumulation of gas onto dwarf galaxies in the
local universe.
