  We study the chemical and spectro-photometric evolution of galactic disks
with detailed models calibrated on the Milky Way and using simple scaling
relations, based on currently popular semi-analytic models of galaxy formation.
We compare our results to a large body of observational data on present day
galactic disks, including: disk sizes and central surface brightness,
Tully-Fisher relations in various wavelength bands, colour-colour and
colour-magnitude relations, gas fractions vs. magnitudes and colours,
abundances vs. local and integrated properties, as well as spectra for
different galactic rotational velocities. Despite the extremely simple nature
of our models, we find satisfactory agreement with all those observables,
provided the timescale for star formation in low mass disks is longer than for
more massive ones. This assumption is apparently in contradiction with the
standard picture of hierarchical cosmology. We find, however, that it is
extremely successfull in reproducing major features of present day disks, like
the change in the slope of the Tully-Fisher relation with wavelength, the fact
that more massive galaxies are on average ``redder'' than low mass ones (a
generic problem of standard hierarchical models) and the metallicity-luminosity
relation for spirals. It is concluded that, on a purely empirical basis, this
new picture at least as successful as the standard one. Observations at high
redshifts could help to distinguish between the two possibilities.
