[trimethylpurine]

I might be missing it.

>or it might not

To determine that it is not, would by definition imply an observation that it is no longer fit.

In other words, how do we know that it is not adaptable? We'd see that it no longer propagates, and is therefore unfit.

Can you think of a case where these two synonyms are not inseparable in meaning as pertaining to evolution?

I can't. I think Darwin meant exactly this.