Why sex exists remains an unsolved problem in biology. If mutations are on the 
average deleterious, a high mutation rate can account for the evolution of sex. 
One form of this mutational hypothesis is Muller's ratchet. If the mutation rate 
is high, mutation-free individuals become rare and they can be lost by genetic 
drift in small populations. In asexual populations, as Muller noted, the loss is 
irreversible and the load of deleterious mutations increases in a ratchet-like 
manner with the successive loss of the least-mutated individuals. Sex can be 
advantageous because it increases the fitness of sexual populations by 
re-creating mutation-free individuals from mutated individuals and stops (or 
slows) Muller's ratchet. Although Muller's ratchet is an appealing hypothesis, 
it has been investigated and documented experimentally in only one group of 
organisms--ciliated protozoa. I initiated a study to examine the role of 
Muller's ratchet on the evolution of sex in RNA viruses and report here a 
significant decrease in fitness due to Muller's ratchet in 20 lineages of the 
RNA bacteriophage phi 6. These results show that deleterious mutations are 
generated at a sufficiently high rate to advance Muller's ratchet in an RNA 
virus and that beneficial, backward and compensatory mutations cannot stop the 
ratchet in the observed range of fitness decrease.