Mutations can be beneficial, deleterious, or neutral. In organisms with large genomes there are many more neutral mutations than the other two classes but in organisms with smaller genomes a higher percentage of mutations are either beneficial or deleterious. In all cases, there are more deleterious mutations than beneficial ones.
If deleterious mutations are harmful to the individual then natural selection should favor a low mutation rate in order to minimize that effect. This is especially true in large multicellular organisms where somatic cell mutations cause cancer and other problems. It seems logical that the optimal mutation rate should be zero in order to maximize the survival of the individual and its offspring.
Nothing in biology makes sense except in the light of population genetics.
But even though the number of beneficial mutations is low compared to those that are deleterious, this is the stuff of adaptive evolution. In the long run the population will become more fit if beneficial mutations occur and become fixed by natural selection. Eliminating mutations might provide a short-term advantage but eventually the population will go extinct if it can't adapt to new environments. (Neutral and deleterious mutations can also contribute to adaptation over the long term.)
The simplest explanation for this apparent paradox is that there's a trade-off between selection to minimize deleterious mutations and selection for long-term evolutionary advantage. The problem with that explanation is that it is very difficult to show how you can select for the future benefit of mutations to the species (population). It seems as though you have to invoke two bogeymen; group selection and teleology.
Maybe there's a better explanation?
The irony of natural selection. He concludes that there's some constraint that limits the ability of natural selection to achieve a zero mutation rate.
The most probable explanation is that evolution does not produce perfect adaptations. In the case of mutations, though natural selection favors individuals most able to repair any changes in DNA (although a small percentage of these might be adaptive), this level of perfection cannot be achieved because of constraints: the cost of achieving perfection, the fact that all errors are impossible to detect or remove, or that some cells (i.e., sperm or eggs) may not even have DNA-repair mechanisms because of genetic or physiological constraints.I used to think that this was the best explanation. I taught my students that the accuracy of DNA replication, for example, comes at the cost of speed. The more accurate the polymerization process, the slower it takes. This makes a lot of sense and there's experimental support for the claim. Slowing down the time it takes to replicate the genome will affect the time it takes for cell divisions and that could be harmful ... or so the argument goes.
When you think about it, there doesn't seem to be any biochemical or physiological constraints that could prevent the mutation rate from getting to zero ... or at least a lot closer than it is now.
Michael Lynch has a better answer and he explains it in a paper titled: "The Lower Bound to the Evolution of Mutation Rates" (Lynch, 2011).
As the mutation rate is driven to lower and lower levels by selection, a point must eventually be reached where the advantage of any further increase in replication fidelity is smaller than the power of random genetic drift (Lynch 2008, 2010). The goal here is to evaluate the extent to which such an intrinsic barrier can provide an adequate explanation for the patterns of mutation rates known to have evolved in natural populations.The main "constraint" is the limited power of natural selection in the presence of random genetic drift. This will depend to some extent of the size of the population.
This idea is called the "drift-barrier hypothesis. It is described in Sung et al. (2012):
... the drift-barrier hypothesis predicts that the level of refinement of molecular attributes, including DNA replication fidelity and repair, that can be accomplished by natural selection will be negatively correlated with the effective population size (Ne) of a species. Under this hypothesis, as natural selection pushes a trait toward perfection, further improvements are expected to have diminishing fitness advantages. Once the point is reached beyond which the effects of subsequent beneficial mutations are unlikely to be large enough to overcome the power of random genetic drift, adaptive progress is expected to come to a standstill. Because selection is generally expected to favor lower mutation rates as a result of the associated load of deleterious mutations, and because the power of drift is inversely proportional to Ne, lower mutation rates are expected in species with larger Ne.The Lynch lab has produced lots of evidence in support of the hypothesis although there may be some confounding factors in some populations.
The bottom line is that the real irony of natural selection is that it's just not powerful enough to reduce the error rate of replication and repair below the values we currently see.
In a sense, it's the "error rate" of fixation by natural selection in the face of random genetic drift that allows evolution to occur.
The more we learn about biology the more we learn that it's messy and sloppy at every level. Evolution is not a watchmaker and it's not even a blind watchmaker. It's a tinkerer1 and the "watch" barely keeps time.
Image Credit: The Mendel's traits image is from Wikispaces Classroom.
1. Jacob, F. (1977) Evolution and tinkering. Science (New York, NY), 196:1161. [PDF]
Lynch, M. (2011) The lower bound to the evolution of mutation rates. Genome Biology and Evolution, 3:1107. [doi: 10.1093/gbe/evr066]
Sung, W., Ackerman, M.S., Miller, S.F., Doak, T.G., and Lynch, M. (2012) Drift-barrier hypothesis and mutation-rate evolution. Proc. Natl. Acad. Sci. (USA) 109:18488-18492. [doi: 10.1073/pnas.1216223109]