Finite genome size can halt Muller's ratchet

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.