"But observing the effects of gravity doesn't explain what it is, or why it is."
You're finally coming around! So can we now agree that evolution is observable?
"The effects of gravity can be explained mathematically. Can the effects of evolution? The analogy doesn't fit."
The Theory of Gravity is much more tenuous than the theory of evolution. There is more evidence supporting the THeory of Evolution than there is supporting the various Theories of Gravity. There is no overwhelming scientific consensus as to how gravity really works because there are too many variables. The effects of gravity can be explained mathematically but the phenomenon of Gravity cannot. Of course there are no strident opponents of Gravity so that is often overlooked.
What is observable is the great diversity of species on the earth. What is also observable are modifications or adaptation within a specie -- micro changes. What is not observable is the mechanism of "evolution" giving rise to new species.
Only a fool would deny that the concept of gravity is bogus. Step off a tall building, and "theory" is confirmed. Shoot a manned spaceship out into space with the assumption that gravity is a bogus idea, and we'll never hear from that ship's crew again as it will be flung out into the far reaches of space...by gravity. But if I say that "evolution is bogus," nobody, including myself, dies. What does it matter? Darwinism may be an interesting speculation, but as an solid explanation or a "law" to base other disciplines on, it's hardly critical to anything. David Berlinsky mused that, "The problem facing us at the [beginning of a new century] with a magnificent body of theoretical accomplishments in physics and mathematics, and a very rich body of descriptive material in biology, is to come to an understanding that when it comes to the large global issues that Darwin's theory is intended to address, we simply do not have a clue. This is a daunting admission to make, but if we're intellectually honest, we should make it. The mechanism that Darwin proposed, that of random search or a stochastic shuffle is known to be inadequate in every domain in which it's applied. It's known to be inadequate in linguistics, and it's certainly inadequate when it comes to the overwhelming complexity of living forms. There is no reason on earth to believe that this mechanism is adequate to the task that it sets itself." So why do people still hold onto it? I can understand scientists still seeking proofs for the theory. Fine. That's the nature of science. But to make grandiose claims about how it is a "fact," and the "debate has been settled," is, as I characterized it in an earlier comment, simply arrogance.