Recent studies have shown that the star cluster initial mass
function (CIMF) can be well approximated by a power law, with
indications for a steepening or truncation at high masses. This
contribution considers the evolution of a Schechter type CIMF due to
cluster disruption, with emphasis on the part of the mass function
that is observable in the first ~1 Gyr. Cluster disruption due to the
tidal field of the host galaxy and encounters with giant molecular
clouds flattens the low-mass end of the mass function, but there is
always a part of the `evolved Schechter function' that can be
approximated by a power law with index -2. The mass range for which
this holds shifts to higher masses with increasing age. Mean cluster
masses derived from luminosity limited samples increase with age very
similarly (due to the evolutionary fading of clusters). Empirical mass
functions are, therefore, approximately power laws with index -2, or
slightly steeper, at all ages. The results are illustrated by an
application to the star cluster population of the interacting galaxy
M51.