First, NS is always specific in nature, meaning there is always a selection pressure. There has never been a case where natural selection doesn’t involve specific selection. It is intrinsic to NS. Take the guppies experiment without predators. The new selection pressure is now sex. Get rid of sex and the selection pressure might be how fast one can convert nutrients, take that away and there will be something else. There is never a static system (which you seem to imply there is) where specific selection is not on-going.

Second, (I use this phrase loosely) we can not know the mind of nature. To expand on that, it is impossible to know what selection pressure will be greatest each season. We can only evaluate specific traits on a per period basis under certain conditions. We can then use those results to make assumptions on the whole and provide evidence for theories. This is the very nature of science. By your line of reasoning all of science is tautology due to the nature of the universe.