  The dynamics of a two-species community of $N$ competing individuals is
considered, with an emphasis on the role of environmental variations that
affect coherently the fitness of entire populations. The chance of fixation of
a mutant population (and, in particular, of a single mutant) is calculated as a
function of the mean relative fitness of the mutant, the amplitude of fitness
variations and their typical duration. We emphasize the distinction between the
case of local competition and the case of global competition; in the latter a
noise-induced stabilization mechanism yields a higher chance of fixation for a
single mutant. The distinction becomes dramatic in the weak selection phase,
where the chance of fixation for a single deleterious mutant is an
$N$-independent constant for global competition and decays like $(\ln N)^{-1}$
in the local competition case. The possibility of long-term persistence of
${\cal O} (N)$ suboptimal (and extinction-prone) populations is discussed, as
well as its relevance to stochastic tunneling between fitness peaks.
