  This paper presents a new spectral evolutionary model of galaxies, properly
taking the effects of nebular emission and pre-main sequence evolution into
account. The impact of these features in different photometric filters is
evaluated, along with the influence that variations in the physical conditions
of the gas may have on broadband colours, line ratios and equivalent widths.
Inclusion of nebular emission is demonstrated to radically change the predicted
ultraviolet, optical and near-infrared colours during active star formation.
Pre-main sequence evolution is also seen to give a non-negligible contribution
to the luminosity in the near-infrared during the first few millions years of
evolution and should not be omitted when very young systems are being modelled.
Finally, we present a comparison of our predictions to observations and two
other recent codes of evolutionary synthesis.
