What is the optimal rate of mutation (for evolution)?

Question

Evolution requires a process:  Mutations occur; most mutations are harmful/detrimental, but a few are beneficial.  Then, beneficial mutations are selected for.
It seems to me that there is a trade-off.  Too much mutation, and too many individuals will get harmful mutations and die off, shrinking the population and increasing the likelihood of extinction.  Too little mutation, and the species will not be able to adapt to a changing environment.
On Earth, all life has some mutation due to cosmic radiation, UV radiation, etc.  There are also certain chemicals that can cause radiation.  But many species have evolved DNA repair mechanisms to mitigate some of this mutation.
Is the mutation rate on Earth perfectly optimized?  On a planet completely protected from all forms of radiation, would life fare better or worse?  What about on a planet bombarded by very high levels of radiation?
What is the optimal rate of mutation to ensure both population growth and maximum adaptability?

John O · Answer

The optimal rate would be that which promotes evolution quickly enough that life does not become extinct. Therefor, it is local. On other worlds than Earth, environmental conditions might change so quickly and drastically that a faster rate of evolution might stave off extinction, but an Earth-like rate would stand little chance.
Given all that, Earth is pretty close to optimal... life has managed to continue to exist for nearly 4 billion years at this point.
On a longer time scale, all planetbound life is subject to grim outcomes. Stars don't last forever. It might be said that an optimal rate also permits the kind of evolution that allows life to leave its home world. This wouldn't necessarily be through intelligence and space-faring technology, though my imagination isn't quite so extravagant to be able to come up with plausible alternatives.

Logan R. Kearsley · Answer

There isn't one.
The optimal rate of mutation for a particular lineage at a particular time depends on multiple different factors, like

How stable is the environment in which they live? How stable is the niche they exploit in that environment?
How long do they live?
How frequently do they reproduce?
How many offspring do they have?

etc.
And different lineages have in fact evolved different baseline mutation rates to optimize their further evolution. How quickly populations mutate and exactly how they mutate is not simply a function of how much mutagenic radiation is in the environment--it also depends on things like how effective their radiation protection and gene repair mechanisms are, and how accurate their DNA replication machinery is, which are traits which are themselves subject to evolution. On a planet completely protected from all forms of mutagenic radiation, species would evolve to have a particular advantageous mutation rate anyway; and on a planet with much higher levels of radiation, they'd evolve more protections against said radiation, again tuning their mutation rate to be whatever it needs to be.

What is the optimal rate of mutation (for evolution)?

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