...I seem to have assumed the number of BP changed by small or point mutations would make up the majority of all BP changed by mutations. (I was probably primed because you started out by talking about the energy cost per BP.) Now that you’ve pointed that out, I have no good reason for that belief. I should look for quantified sources of information.
OK, so now we need to know 1) what metabolic energy order of magnitude is big enough for selection to work, and 2) the distribution of mutation sizes. I don’t feel like looking for this info right now, maybe later. It does seem plausible that for the right values of these two variables, the metabolic costs would be big enough for selection to act against random nonfunctional mutations.
But apparently there is a large amount of nonfunctional DNA, and also I’ve read that some nonfunctional mutations are fixated by drift (i.e. selection is zero on net). That’s some evidence for my guess that some (many?) nonfunctional mutations, maybe only small ones, are too small for selection pressure due to metabolic costs to have much effect.
Yeah, I will definitely concede small ones have negligible costs. And I’m not sure the answer to 1) is known, and I doubt 2) is well quantified. A good rule of thumb for 2) though is that “if you’re asking whether or not it’s possible, it probably is”. At least that’s the rule of thumb I’ve developed from asking questions in classes.
...I seem to have assumed the number of BP changed by small or point mutations would make up the majority of all BP changed by mutations. (I was probably primed because you started out by talking about the energy cost per BP.) Now that you’ve pointed that out, I have no good reason for that belief. I should look for quantified sources of information.
OK, so now we need to know 1) what metabolic energy order of magnitude is big enough for selection to work, and 2) the distribution of mutation sizes. I don’t feel like looking for this info right now, maybe later. It does seem plausible that for the right values of these two variables, the metabolic costs would be big enough for selection to act against random nonfunctional mutations.
But apparently there is a large amount of nonfunctional DNA, and also I’ve read that some nonfunctional mutations are fixated by drift (i.e. selection is zero on net). That’s some evidence for my guess that some (many?) nonfunctional mutations, maybe only small ones, are too small for selection pressure due to metabolic costs to have much effect.
Yeah, I will definitely concede small ones have negligible costs. And I’m not sure the answer to 1) is known, and I doubt 2) is well quantified. A good rule of thumb for 2) though is that “if you’re asking whether or not it’s possible, it probably is”. At least that’s the rule of thumb I’ve developed from asking questions in classes.