  Using a semi-analytical model of the evolution of the Milky Way, we show how
secular evolution can create distinct overdensities in the phase space of
various properties (e.g. age vs metallicity or abundance ratios vs age)
corresponding to the thin and thick discs. In particular, we show how key
properties of the Solar vicinity can be obtained by secular evolution, with no
need for external or special events, like galaxy mergers or paucity in star
formation. This concerns the long established double-branch behaviour of
[alpha/Fe] vs metallicity and the recently found non-monotonic evolution of the
stellar abundance gradient, evaluated at the birth radii of stars. We extend
the discussion to other abundance ratios and we suggest a classification
scheme, based on the nature of the corresponding yields (primary vs secondary
or odd elements) and on the lifetimes of their sources (short-lived vs
long-lived ones). The latter property is critical in determining the single- or
double- branch behavior of an elementary abundance ratio in the Solar
neighborhood. We underline the high diagnostic potential of this finding, which
can help to separate clearly elements with sources evolving on different
timescales and help determining the site of e.g. the r-process(es). We define
the "abundance distance" between the thin and thick disc sequences as an
important element for such a separation. We also show how the inside-out
evolution of the Milky Way disc leads rather to a single-branch behavior in
other disc regions.
