  We study the accumulation of deleterious mutations in a haploid, asexually
reproducing population, using analytical models and computer simulations. We
find that Muller's ratchet can come to a halt in small populations as a
consequence of a finite genome size only, in the complete absence of backward
or compensatory mutations, epistasis, or recombination. The origin of this
effect lies in the fact that the number of loci at which mutations can create
considerable damage decreases with every turn of the ratchet, while the total
number of mutations per genome and generation remains constant. Whether the
ratchet will come to a halt eventually depends on the ratio of the per-locus
deleterious mutation rate $u$ and the selection strength $s$. For sufficiently
small $u/s$, the ratchet halts after only a few clicks. We discuss the
implications of our results for bacterial and virus evolution.
