peter borger writes:
If so, than it is imaginable that mutations are always introduced at the same spot.
Always? That would be wildly improbable. In the human genome there are literally thousands and thousands of places where a mutation might be more likely to occur than the average for loci across the whole genome. All you can say is that some loci are more likely than others to experience a mutation. Mutations can still occur at unlikely locations. Going back to the earthquake analogy, the stronger building is less likely to fall than other buildings, but it is still possible for it to fall while other weaker buildings remain standing. There are no certainties with earthquakes and radiation, only probabilities.
And as observed in mtDNA of distinct primates and human subpopulations usually the same nucleotides are introduced.
There are a wide variety of mutation types, and this only covers one of them. Limiting my comments only to nucleotide substitution, I could accept a tendency toward the same nucleotides being introduced, but saying "usually the same nucleotides are introduced" feels a bit too strong to me. This tendency has two natural origins:
- Biochemical. Nucleotide substitutions are affected or limited by the rules governing biochemical reactions.
- Filtering. Some nucleotide substitutions result in expression within the organism of characteristics inconsistent with survival and/or reproduction.
In order to add a supernatural possibility to this list you have to show that at least some mutational tendencies are not explained by these two possibilities.
Thus, mutation is non-random with respect to these features and that would bring down the strongest argument for molecular evolution: sequence similarity.
I don't think you mean to say that it "brings down the strongest argument for molecular evolution." Didn't you mean to say it brings down the strongest argument for common descent?
Anyway, it is only non-random in the sense that what happens is limited by natural laws. When I throw a rock repeatedly into the air at precisely the same angle and speed, the time it takes to fall is non-random, because gravity always acts the same way. When you subject the same biochemical repeatedly to the identical situation, what happens is always the same thing and is non-random, because the laws governing chemical reactions are always the same. Only if you repeated the same biochemical experiment over and over again getting different results each time might it be considered evidence of divine direction. And of course, radiation is the wildcard in this mix. How it "knocks out" a chemcial bond is random, dependent upon precisely where the radiation strikes.
So, how can we discriminate between common descent and common mechanism (whether or not DNA structure related is irrelevant to this observation)?
I hope that the inductions made about the descent of human sub-groups were not made by following single mutational lines. The fewer mutational lines used to determine descent pathways, the less likely they are to be correct, precisely for the reasons you mention, that for a single mutational line it would not necessarily be possible to differentiate between common descent and common mechanism.
But I don't want to lose sight of your original point when you began this thread, which cited an article about radiation causing increased mutational rates at genomic "hotspots". There is nothing non-random about this. Once again revisiting the earthquake scenario, you shake a city harder and the rate at which the weaker buildings fall increases. You radiationally bombard a genome harder, and the rate at which the weaker loci mutate increases.
--Percy
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