  We are interested in the invasion phase for stochastic processes with
interactions when a single mutant with positive fitness arrives in a resident
population at equilibrium. By a now classic approach, the first stage of the
invasion is well approximated by a branching process. The macroscopic phase,
when the mutant population is of the same order of the resident population, is
described by the limiting dynamical system. We obtain sharper estimates and
capture the intermediate mesoscopic phase for the invasive population. It
allows us to characterize the hitting times of thresholds, which inherit a
large variance from the first stages. These issues are motivated in particular
by quantifying times to reach critical values for cancer population or
epidemics.
