  We have constructed a family of simple models for spiral galaxy evolution to
allow us to investigate observational trends in star formation history with
galaxy parameters. The models are used to generate broad band colours from
which ages and metallicities are derived in the same way as the data. We
generate a grid of model galaxies and select only those which lie in regions of
parameter space covered by the sample. The data are consistent with the
proposition that the star formation history of a region within a galaxy depends
primarily on the local surface density of the gas but that one or two
additional ingredients are required to fully explain the observational data.
The observed age gradients appear steeper than those produced by the density
dependent star formation law, indicating that the star formation law or infall
history must vary with galactocentric radius. Furthermore, the
metallicity-magnitude and age-magnitude correlations are not reproduced by a
local density dependence alone. These correlations require one or both of the
following: (i) a combination of mass dependent infall and metal enriched
outflow, or (ii) a mass dependent galaxy formation epoch. Distinguishing these
possibilities on the basis of current data is extremely difficult.
