  It has been recently shown that molecular clouds do not exhibit a unique
shape for the column density probability distribution function (Npdf). Instead,
clouds without star formation seem to possess a lognormal distribution, while
clouds with active star formation develope a power-law tail at high column
densities. The lognormal behavior of the Npdf has been interpreted in terms of
turbulent motions dominating the dynamics of the clouds, while the power-law
behavior occurs when the cloud is dominated by gravity. In the present
contribution we use thermally bi-stable numerical simulations of cloud
formation and evolution to show that, indeed, these two regimes can be
understood in terms of the formation and evolution of molecular clouds: a very
narrow lognormal regime appears when the cloud is being assembled. However, as
the global gravitational contraction occurs, the initial density fluctuations
are enhanced, resulting, first, in a wider lognormal Npdf, and later, in a
power-law Npdf. We thus suggest that the observed Npdf of molecular clouds are
a manifestation of their global gravitationally contracting state. We also show
that, contrary to recent suggestions, the exact value of the power-law slope is
not unique, as it depends on the projection in which the cloud is being
observed.
