Check the links in the earlier posting. I don't know. But I doubt that there is any such completely unchanging environment. No environment is completely isolated from the rest of the world. No, unchanging means unchanging.
Populations are variable.
If no environmental factor favours some variations over others, then everything stays the same, including the variability. I don't know why you say this. But it shows that the idea of a completely unchanging environment doesn't really make sense, because the variation in the genes, and hence the traits of these imagined organisms form the ebvironment for one another. So if there was no change in climate, say, there could still be changes in combinations of organisms. But most dog breeds do have hereditary problems, some with hips, some with breathing.