Evolution Is As Certain As Gravity
Whenever critics of Evolution point out that Evolution is merely a theory, proponents are quick to point out that Gravity is a theory too (The Theory Of Gravity). This is completely correct. Both of these theories describe abstract, unobserved ideas, and they both also describe experimentally-verified, easily observable concepts.
For example, Evolution describes dinosaurs evolving into birds (something nobody has ever observed) and Gravity describes Dark Matter (another thing nobody has ever observed).
They also speak of easily observable material: Evolution predicts that moths which mutate into colors that offer better camouflage will survive better than those that don't (observed), and Gravity predicts that pencils will fall to the ground when dropped (observed).
There is not necessarily a problem with using Gravity to demonstrate the certainty of Evolution. The problem arises when the evolution advocate attempts to equivocate, in his opponent's mind, the certainty of observable aspects of Gravity with the certainty of unobservable aspects of Evolution.
In summary, the analogy itself is a valid one, but the conclusion that tends to be derived from it is a product of illogical, albeit accidental, equivocation.