So if a person can't change the fly, change the definitions.
I am seeing a common theme in these threads. It is in those areas of willful and woeful ignorance that those who choose to be anti-Catholics rail the most against that of which they know nothing. God gave you the gift of intellect. You do not worship Him by refusing to use it.
We are dealing with science here so definitions actually serve as boundary conditions. A common species must, by definition, have the ability to successfully breed with other members of the same species. New "fruit fly" species did evolve in the lab that were unable to successfully reproduce with their antecedent species. And, for the record, Drosophila is a genus, not a species, containing many, many "fruit fly" species.
Not only have you failed the science, but your logic is highly flawed as well. You are claiming that because fruit flies did not become mice or clams that evolution was not demonstrated, when the Theory of Evolution contents that gradual changes occur such that the most well suited variant survives to reproduce and the least suited die out and that over time the cumulative changes are significant enough to form new genus, orders, families, etc.