Consider a small organisim, say, a guppy. Presumably, this species has a single "genomic potential". So, I get a few dozen of these little guys and I set up shop.
I do some selective breeding of these fish, which is just another way of saying that I impose an arbitrary means of selecting the ones which will be allowed to survive and breed. The ones in the first tank will face an arbitrary selective pressure for big, colorful tails. The fish in the second tank will face a selective pressure for normal tails, but large size.
Eventually I get a strain of this fish with a big, colorful tail, and a second strain that that is larger than before. Which of these strains represents the true "genomic potential" of this species?
Obviously, each fish is well-adapted to the particular selective pressures in its particular tank. The "ultimate expression" of this evolutionary path depends both upon the fish, and on the environment in which the fish lives.
This guy seems to suggest that there is some latent form that each species is destined to evolve into, regardless of the environment - that selective pressures do not drive evolution. That's a really hard argument to take seriously.
But they're all still guppies. At the end of your "experiment" you mix the two populations and after a while the differences you achieved by your selective breeding will disappear.
ML/NJ