Well, we have noticed that whenever we know the explanation for a biological phenomenon, that explanation is always natural and never supernatural. So unless and until we find an exception to that, we're going to work on the theory that that's the case --- just as we're going to work on the theory that there are no flying pigs until we find a flying pig.
So it's not a premise in biology. It's a conclusion. If research had pointed to a metaphysical elan vital (for example), biologists would believe in that instead, it would be in all the textbooks.
As indicated, supernatural is not-natural. If science is limited to the testing and observation of natural things (matter and energy), then by that definition, the testing and observational techniques cannot be used to validate or negate anything supernatural ...
But it is a premise. You limit your testing to natural things only, do not have any tests outside of natural processes, and thus you cannot, by definition, have any other conclusions beyond something natural.
But this isn't so. Think of any miracle in the Bible — let's say the bush in Exodus that "burned with fire, and was not consumed". Scientifically we could verify that there were indeed flames, and that the bush was not being consumed. And science would tell us that this was a a miracle, being a local violation of the laws of nature.
Indeed, the more scientific we are, the more clearly we can perceive this. Someone who didn't know very much about fire might think, "yes, it's an oddity, but is it a miracle"? He might class it with other oddities like Old Faithful or an eclipse of the sun. But our scientific knowledge of fire would make us absolutely certain that we were in the presence of the supernatural. It is exactly scientific knowledge — knowledge of the natural order of things — that would allow us to detect the supernatural — a violation of that order.
Yeah, it's easy to see where you're going. You're going to declare that (1) information is immaterial, and (2) that it is therefore out of reach of "naturalism".
But the thing is, Richard, scientists do in fact believe in, record, and study, the information in DNA. (They also believe that there's information in a dictionary, or a gif of a kitten if they come to that.) So if we grant you those two things, it will be necessary for you to concede that scientists are not naturalists, since they believe in a supernatural entity (the information in DNA) and, indeed, know a lot more about the supernatural than you do.
You're building an impossible triangle here, something's got to give.