  We examine a model of biological evolution of Eigen's quasispecies in a holey
fitness landscape, where the fitness of a site is either 0 (lethal site) or a
uniform positive constant (viable site). So, the evolution dynamics is
determined by the topology of the genome space. It is modeled by the random
Bethe lattice. We use the effective medium and single-defect approximations to
find the criteria, under which the localized quasispecies cloud is created. We
find that shorter genomes, which are more robust to random mutations than
average, represent a selective advantage which we call ``topological''. A way
of assessing empirically the relative importance of reproductive success and
topological advantage is suggested.
