Nobody has observed gravity either, only its effects.

Evolution is simply the change in allele frequency in populations of organisms over time. This is most certainly observable.

That is by not metrics what evolution simply is.

The present-day biosphere is an effect, as is the fossil record and DNA relations between species.

That does not prove evolution the way gravity is proven.

How so? Evolution is just change in populations over time with common descent, and that's pretty well demonstrated through the things I mentioned, about the same as gravity is demonstrated by things falling. What's different?

I've never seen it demonstrated definitively through the fossil record. There are a lot of assumptions and best-guesses leading back to common ancestors.

I've never seen gravity demonstrated definitively either. Sure, they say that there's this force that makes stuff fall, but I've seen plenty of things go up instead of down when released.

That is farcical. If something is thrown in the air, you can demonstrate the force of Newton's constant, thereby definitively demonstrating gravity. Actually calculating the gravitational constant with precision is the only difficulty there. Nothing about this is assumed. Which back to the point, is nothing like the assumptions of hereditary lines in the so-called "fossil record".

I've had a party balloon on my ceiling for the past month. Clearly there are some holes in this "gravity" theory of yours.

Evolution is observable.

I think it is comparable. We do have the fossil record, after all.