  The bit-string Penna Model is used to simulate the competition between an
asexual parthenogenetic and a sexual population sharing the same environment. A
new-born of either population can mutate and become a part of the other with
some probability. In a stable environment the sexual population soon dies out.
When an infestation by fastly mutating genetically coupled parasites is
introduced however, sexual reproduction prevails, as predicted by the so-called
Red Queen hypothesis for the evolution of sex.
