[mkross]

I assume the reason to stop is due to a feeling of guilt. Instead of considering human influence as a part of nature, many people consider it to be some sort of perversion. So instead of being amazed by the adaptability of life, people feel guilty about causing changes.

Plus there is the whole time scale thing. The fact that evolution took a really long time to differentiate polar and grizzly bears and in a few lifetimes you would see that split undone is concerning.

[Swizec]

If the two species can mate it was never that big a split to begin with.

Personally I'm far more intrigued by the fact evolution can take action as quickly as a few generations if need be.