WHEN DOES NATURAL SELECTION FAVOR HIGH MUTATION RATES?
The mutation rate itself is a trait that can evolve by natural selection, provided the existence of genetic variation for the character. Given that most mutations have deleterious fitness effects (9, 15, 17, 18, 31, 38), having a too high mutation rate would be prejudicial in the short term simply because (in a first approach) the population equilibrium fitness for a haploid asexual population decreases exponentially with mutation rate (8). In the long term, however, it can be argued that the higher the mutation rate, the more likely it is that beneficial mutations will be produced. An optimal mutation rate, which maximizes the rate of adaptation, is reached when these opposing factors are balanced (3, 25, 27, 34). Hypotheses about the high adaptability of RNA viruses should take into account this trade-off and address why the balance between beneficial and deleterious mutational effects leads to different outcomes in RNA viruses and DNA organisms, including other viruses. Implicitly,
