  We compute the evolution of different abundance ratios in the Milky Way (MW)
for two different sets of stellar yields. In one of them stellar rotation is
taken into account and we investigate its effects on the chemical evolution
model predictions. Moreover, we show that some abundance ratios offer an
important tool to investigate the halo-disk discontinuity. For the first time
it is shown that the effect of a halt in the star formation between the
halo/thick disk and thin disk phases, already suggested from studies based both
on Fe/O vs. O/H and Fe/Mg vs. Mg/H, should also be seen in a C/O versus O/H
plot if C is produced mainly by low- and intermediate-mass stars (LIMS). The
idea that C originates mainly from LIMS is suggested by the flat behavior of
the [C/Fe] ratio as a function of metallicity, from [Fe/H]=-2.2 to solar, and
by the fact that very recent C/O measurements for stars in the MW halo and disk
seem to show a discontinuity around log(O/H)+12=8.4. Finally, a more gentle
increase of N abundance with metallicity (or time), relative to models adopting
the yields of van den Hoek and Groenewegen (1997), is predicted by using the
stellar yields of Meynet and Maeder (2002 - which include stellar rotation but
not hot-bottom burning) for intermediate mass stars. This fact has some
implications for the timescales of N enrichment and thus for the interpretation
of the nature of Damped Lyman Alpha Systems.
