  We propose a hybrid dynamical system approach to model the evolution of a
pathogen that experiences different selective pressures according to a
stochastic process. In every environment, the evolution of the pathogen is
described by a version of the Fisher-Haldane-Wright equation while the
switching between environments follows a Markov jump process. We investigate
how the qualitative behavior of a simple single-host deterministic system
changes when the stochastic switching process is added. In particular, we study
the stability in probability of monomorphic equilibria. We prove that in a
"constantly" fluctuating environment, the genotype with the highest mean
fitness is asymptotically stable in probability. However, if the probability of
host switching depends on the genotype composition of the population,
polymorphism can be stably maintained.
  This is a corrected version of the paper that appeared in Mathematical
Biosciences 240 (2012), p. 70-75. A corrigendum has appeared in the same
journal.
