What about the dinosaurs that developed flights twice in its evolutionary life? Just because all the non-spike rabbits were eaten or killed doesn't mean the DNA is gone. The spikes allows the rabbits to survive, despite having a significant cost on the species. As soon as the threat is gone, the spikes should disappear gradually as the less-spiked rabbits breed faster because they have more energy to throw around.
Yes the pelt was an advantage... when it was NEEDED. We don't NEED it anymore. You made my argument, and the argument of the TFA!
We lost our thick body hair because we learned fire and clothing. By taking control of our external temperature away from our body, it stopped developing thick body hair. Letting go of the cost of generating the pelt.
Our jaws are shrinking because we no longer eat by tearing raw flesh from the bones. Our meals are prepared and easy to consume. Our wisdom teeth (and little finger) are the first sacrifices to our readily available food sources. Our mouths will get smaller, we will lose more teeth. Because it costs less to have less teeth and a smaller jawbone.
If the world was suddenly thrown into an ice age. And we lost all our technology and all our of higher knowledge. Eventually we would re-evolve our thick body hair, our jaws would extend, our average height would shrink dramatically. Because the cumulative information is stored in our DNA. And the cost of surviving.
If you want to say that a spiked rabbit and a speedy rabbit are branches of a non-spiked, non-speedy rabbit. And so if all the speedy rabbits die, the spiked rabbit is all that's left. I'm amiable to that. Though the original rabbit sucked ass, in my opinion. But even so, the spiked rabbit would eventually evolve the spikes away because the spikes cost too much. Just because the speedy rabbit died off, doesn't mean the spiked rabbit can't evolve out of spikes and into speediness on its own.
And the ability to track other creatures is not a genetic trait, but a result of higher brain functionality. Though you are correct, partly. The higher brain function is a costly feature. The moment we had a free thought, we clung to it. To justify the larger, stronger brain, we've evolved a weaker body. Less hair, no claws, no spikes, no thick skin, nothing. The bigger our brains become through non-selective breeding, the less our bodies will become to compensate.
You are also right about the timescale. To completely remove the thorns on the trees, it will probably takes 1000s of years, if not much more. But evolution is a constant balance with nature. Change nature and evolution adjusts. So even though its only been 500 years, SOME change in the plant should be seen.
And there are thornless rose bushes, very many species of them, in fact. http://answers.yahoo.com/question/index?qid=20080221015317AAxLtH3