The allocations of mutations as beneficial, neutral, and deleterious is for representational purposes only (not based on actual data), and the proportion of neutral mutations was held constant for all three genotypes. The genotype at the fitness peak C does not have any way to become more fit on this landscape and thus has no beneficial mutations available to it.
The genotype at B is more fit than A and is closer to a fitness peak, so it has a smaller fraction of beneficial mutations than that at A. The genotype at A is not well adapted to the environment (far from a fitness peak) so has a larger fraction of mutations that would be beneficial.
The more variable the environments an organism experiences and the lower fitness the organism is in those environments, the more an increased mutation rate would be favored since there is a greater chance per mutation of a mutation being beneficial.Ī fitness landscape showing three genotypes on different places on the landscape (A, B, and C) and a schematic pie chart of the distribution of mutations available to each genotype. At another extreme, if an organism is suddenly thrust into an environment that it’s not well adapted to (akin to being at A in Fig 2), there is a larger fraction of potentially beneficial mutations available and having a nonzero mutation rate would be preferable to all descendants always staying exactly the same. In a constant environment (one where the fitness landscape does not change), it would be best for the optimal genotype to not mutate at all.
At an extreme, an organism that’s “perfectly” adapted to its constant environment would do best to reduce its mutation rate to zero-there are no more beneficial mutations, so all mutations are likely worse than the current genotype (see C in Fig 2). In some cases, there is no benefit to mutation at all. Mutation rates are evolvable and can respond to selection That remains true whether an organism has a low mutation rate or a high mutation rate, and biological entities differ dramatically in their per-nucleotide mutation rate (over eight orders of magnitude, Fig 1). While the fraction of mutations that are harmful versus beneficial may change in different organisms, in different environments, and over time, deleterious mutations are thought to always outnumber beneficial mutations. While a small percentage of mutations are helpful and some are inconsequential (neutral or nearly neutral in effect), a large portion of mutations are harmful. Many mutations cause organisms to leave fewer descendants over time, so the action of natural selection on these mutations is to purge them from the population. However, most mutations are not beneficial for the organisms with them. Mutations are the building blocks of most of evolution-they are the variation upon which natural selection can act, and they are the cause of much of the novelty we see occur in evolution. The fabled mutation rates of RNA viruses appear to be partially a consequence of selection on another trait, not because such a high mutation rate is optimal in and of itself. Researchers often assume that natural selection has optimized the mutation rate of RNA viruses, but new data shows that, in poliovirus, selection for faster replication is stronger and faster polymerases make more mistakes. However, their mutation rates are almost disastrously high, and a small increase in mutation rate can cause RNA viruses to go locally extinct. RNA viruses have high mutation rates-up to a million times higher than their hosts-and these high rates are correlated with enhanced virulence and evolvability, traits considered beneficial for viruses.
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