166
The Mystery of the Genome
unit, being eliminated one at a time. We now know that mutations are numerous and diffuse, thwarting any possible “one-at-a-time” elimination mechanism. If we combine Muller’s recognition of near-neutral (i.e. un-selectable) mutations (above), with his recognition of mutational advance (above), we see that no selection system can stop “Muller’s Ratchet” - even in sexual species. Even if selection could eliminate every mutation for numerous generations, eventually a new mutation would always “sneak through”. In asexual species, once a mutation is fixed in a genome, there is no way to go back - to get rid of it. Hence the information can only degenerate - it is a unidirectional (ratcheted) process. Each new generation will have to have more mutations than the last, and so will be inferior to the last. Ironically, if we extend our analysis, we realize that this is not just true in asexual species, but is also true in sexual species. The “ratchet” works because a certain fraction of the deleterious mutations will always “sneak past” selection, and become fixed in the genome. These will still vastly outnumber any possible beneficial fixations. Selection cannot separate the few good from the many bad, because they are in large linkage blocks - they cannot be teased apart. Therefore each part of the genome (each linkage block) must individually degenerate due to Muller’s ratchet.