Optimization of the frequency of spontaneous mutations
Organisms adjust of the frequency of mutations through a number of mechanisms, mostly connected with the intensity of repair processes, some of which are even capable of effectively reacting to changes in the environment (Echols & Goodman 1991). For example, if a population of bacteria finds itself in a stressing situation and is in danger of extinction, for example after transfer to an environment with an abnormal temperature, the bacteria undergo an SOS reaction. The individual bacteria begin to mutate faster, increasing the probability that a mutant that will be resistant to this stress factor will occur in the population (Taddei et al. 1997). It happens very frequently that turning off the process of repair of unpaired bases (the mismatch repair system) contributes to increasing the frequency of mutations. Turning off this process in bacteria increases the probability of interspecies recombination by more than an order of magnitude (Velkov 2002).
Experimental results have shown that evolution has “tuned” the ratio of the repair and replication activities of DNA-polymerase in the individual groups of organisms to achieve the optimum frequency of mutations from an evolutionary and functional standpoint (Cox 1976).The consequent frequency of mutations is not very high, so that the organisms and populations are not exposed to an excessive mutation burden, but not too low, so that species do not stagnate evolutionarily and can adapt to changing conditions.Experimentally, it is possible to select a line of bacteria with a much lower mutation rate (Radman, Taddei, & Matic 2000).
It is interesting and very important from a theoretical standpoint that the number of nonsynonymous substitutions/genomes related to the generation time is very similar for the most varied groups of organisms (Drost & Lee 1995)and is not related to the size or metabolic activity of the particular species.As the number of synonymous substitutions/genomes related to the generation time or the number of nonsynonymous substitutions/genomes related to the number of divisions or to the time, to the contrary, differs substantially for various types of organisms (Bromham, Rambaut, & Harvey 1996), it is probable that the present-day frequency of mutations was optimized from the standpoint of the rate of evolution and not from the standpoint of (energetic) “costs” and “profits” (Sniegowski et al. 2000). It is most certainly not determined only by physical or chemical laws valid for DNA replication, for example, the number of tautomeric transitions.