> Yet we see big stretches of DNA mutating freely, at rates that exactly match what would be expected from accumulation of random copying errors.
But the rate of mutation is itself subject to selection. There isn't a base rate, just a setting that's different for different parts of the genome. Some parts have more copy errors than other parts. Some are hung out in the sun more often.
So you can conclude from the mutation rate of a particular stretch that it would probably be bad if it started mutating more, and that it would probably be bad if it started mutating less, but not that nothing's influencing the mutation rate.
But the rate of mutation is itself subject to selection. There isn't a base rate, just a setting that's different for different parts of the genome. Some parts have more copy errors than other parts. Some are hung out in the sun more often.
So you can conclude from the mutation rate of a particular stretch that it would probably be bad if it started mutating more, and that it would probably be bad if it started mutating less, but not that nothing's influencing the mutation rate.