  We present predictions for the evolution of the galaxy luminosity function,
number counts and redshift distributions in the IR based on the Lambda-CDM
cosmological model. We use the combined GALFORM semi-analytical galaxy
formation model and GRASIL spectrophotometric code to compute galaxy SEDs
including the reprocessing of radiation by dust. The model, which is the same
as that in Baugh et al (2005), assumes two different IMFs: a normal solar
neighbourhood IMF for quiescent star formation in disks, and a very top-heavy
IMF in starbursts triggered by galaxy mergers. We have shown previously that
the top-heavy IMF seems to be necessary to explain the number counts of faint
sub-mm galaxies. We compare the model with observational data from the SPITZER
Space Telescope, with the model parameters fixed at values chosen before
SPITZER data became available. We find that the model matches the observed
evolution in the IR remarkably well over the whole range of wavelengths probed
by SPITZER. In particular, the SPITZER data show that there is strong evolution
in the mid-IR galaxy luminosity function over the redshift range z ~ 0-2, and
this is reproduced by our model without requiring any adjustment of parameters.
On the other hand, a model with a normal IMF in starbursts predicts far too
little evolution in the mid-IR luminosity function, and is therefore excluded.
