True but that's still Natural Selection which we both agree upon. As you say the gene pool of the moths that have larger wing span will increase and therefore larger wing span gene will become dominant. However the species is still a moth not a transitional species. Natural selection is not a theory...it is fact. We see it around us with the above being an example.

Evolution of one species in to another completely new species is what the Theory of Evolution is all about. It uses NS as a basis and stretches it to propose evolution of one species in to another. As opposed to NS, ToE still remains a theory as transitional species have never been found. There have been false alarms (most famous was Piltdown man!) and the most recent one was the discovery of human fossils on an indonesian island. The former was a fabrication and the latter was accepted to be a modern human with a brain deformity.

It's an interesting topic and I am still learning myself. With the advances in genetics and unlocking of the DNA, more evidence should come forth on this issue.