A barrier to macroevolution & objections to it

This is the same argument as "Can a Dog ever evolve to be a Cat..."The answer is NO in both cases.The human Eye can not become the Drosophila Eye or visa-versa. Neither can a Dog evolve to become a Cat. What could happen in both cases (in both directions) is that one element or species could, with the right evolutionary pressures, evolve to greatly resemble the other.A dog, could in time could come to look like a cat (change the size, muzzle shape ear shape, stance, claws, blah , blah) so much so that it would take a blood test to determine the difference. Does this mean that the dog has become a cat? Not at all, its genetics would be different to that of a cat and resemble more that of a dog. While it might still be able to breed with other dogs (the chances are unlikely) it would defiantly not be able to breed with a cat. It would not be a ”cat’ but would look like one.Same with the eyes. You could envision a scenario where evolutionary pressures could drive either system to become more like the other. Imagine if you will that over time the genes that define the size of the rods and cones causes a gradual increase in size. In time they will start to exert pressure on the interior of the eye ball and this will case the eye socket to changer shape to a more open dish. As the rods and cones push outward the iris expands and eventually disappears( along with the lens) leaving a spreading dome of rods and cones covered with the clear cover that once covered the iris. Eventually you would have a system very much like that in Drosophila. Would the human eye have BECOME that of the fly? NO. Does it closely resemble it? Yes.Whilst I’m at it I do have to ask the question: Since when has evolution only been about the INCREASE of variation of genes? I thought evolution was to do with the CHANGE in the frequency of these changes? (not specifying increase or decrease?)