Muller's ratchet is a principle of evolutionary genetics describing mutant 
accumulation in populations that are repeatedly subjected to genetic bottleneck. 
The immediate effect of Muller's ratchet, overall loss of fitness, has been 
confirmed in several viral systems belonging to different groups. This report 
shows that in addition to fitness loss, genetic bottlenecks also have 
longer-term effects, namely changes in the capacity of viral populations to 
adapt. Thus, vesicular stomatitis virus strains with a history of genetic 
bottleneck have lower adaptability than strains maintained at relatively large 
population sizes. This lower adaptability is illustrated by their reduced 
ability to regain fitness and by their inability to outcompete wild-type 
populations in situations where the initial fitness of the bottlenecked mutant 
is the same or even higher than the initial fitness of the wild-type.