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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?

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    $\begingroup$ I think you are neglecting the rate of environmental change here. With zero environmental change, the optimal rate of evolution is zero assuming you're already adapted. There's also a difference between mutation and just exchanging different genes, both of which are part of evolution. $\endgroup$ – DKNguyen Jul 22 at 19:43
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    $\begingroup$ @DKNguyen if you include other animals in the environment then yes. Otherwise it's not weird that things keep changing bit by bit. Like macrophages changingevery day to attack or defend differently, trying to still outcompete in an otherwise static environment (in some cases static). $\endgroup$ – Trioxidane Jul 22 at 21:33
  • $\begingroup$ @Trioxidane True. $\endgroup$ – DKNguyen Jul 22 at 21:39
  • $\begingroup$ I don't have a specific answer, but the ideas I think you should focus on are having enough time between mutations to have bad mutations be selected out and having some generations for beneficial mutations to spread out among the species. How many generations it takes for each to happen? I do not know. $\endgroup$ – Arvex Jul 23 at 3:04
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    $\begingroup$ "Is the mutation rate on Earth perfectly optimized?": What does this even mean? Optimized with respect to what? That is, what is it what you want to maximize, or maybe to minimize? What does "maximum adaptability" mean? What do the words "better" and "worse" mean? What do you mean by "the mutation rate on Earth", the mutation rate of a virus or the mutation rate of a shark? $\endgroup$ – AlexP Jul 23 at 5:17
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There isn't one.

The optimal rate of mutation for a particular lineage at a particular time depends on multiple different factors, like

  1. How stable is the environment in which they live? How stable is the niche they exploit in that environment?
  2. How long do they live?
  3. How frequently do they reproduce?
  4. 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.

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  • $\begingroup$ The fact entire branches of organisms have become extinct while other have not points to the rate not only being unoptimized, but different between lineages. $\endgroup$ – DKNguyen Jul 22 at 19:46
  • $\begingroup$ @DKNguyen Being different between lineages is the whole point of my answer. $\endgroup$ – Logan R. Kearsley Jul 22 at 20:20
  • $\begingroup$ I'm not arguing anything in your answer. $\endgroup$ – DKNguyen Jul 22 at 20:20
  • $\begingroup$ I think this is a great answer. But I am interested in how life could mutate in the absence of mutagenic radiation. Is it self-imposed mutation (e.g. purposeful errors in DNA replication)? If so, how could it be made sufficiently random to allow for "never-before-seen" benefits? $\endgroup$ – cowlinator Jul 22 at 22:00
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    $\begingroup$ @cowlinator DNA replication isn't perfect. Errors happen randomly "just because", absent any interference, without having to do it on purpose. Mutations can also happen spontaneously in-place independent of replication, simply because DNA is not completely chemically stable. And chemical mutagens, freezing, and desiccation damage can all replace the role of radiation as external mutation triggers. $\endgroup$ – Logan R. Kearsley Jul 22 at 22:12
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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.

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