My point about gravity is that currently there is no accepted explanation of gravity. Gravity works, we can see that through cause and effect, experiments etc, but we don't understand why it works.
In evolution, we can observe mutations in genes on a daily basis, see how some mutations make for better chances of survival (most of the ones we observe make for less chance of survival). We can even observe some causes of gene mutation (e.g. radiation). I don't think we have yet "observed" the case where a mutation in a species prevents successful reproduction with the non-mutated population but allows reproduction with the mutated population, nor understand why this would occur.