How different must two populations be to be different?
That would depend on whether you were a lumper or a splitter. An extreme splitter would argue that any variation, say a different color phase on a bear, is justification for identifying it as a separate species. This was a pretty common notion at the beginning of the 20th century, when many naturalists thought that Grisly (sic) bears and "Silver Tips" were two separate species. The recent discovery of hybridization between Grizzly and Polar bears will probably have extreme lumpers arguing that since the are genetically compatible, they are really one species, with grizzlies and polars being sub-species. The truth, as it usually always does, lies somewhere in the middle.
In the end it is all political and depends on who is trying to grind what ax.
Proponents or unrestrained development push hard for the lumper viewpoint. If you can convince a court that, for example, all snail darters are just minor variations of the same species, then you can build your hydro-electric dam without ESA issues. At the same time a protectionist is going to be an extreme splitter and claim that because the snail darter that lives in the stream that is proposed for damming has an extra spot on it's tail, it is an entirely separate species and it's habitat must be preserved.
It is a battle that will never end, with the pendulum swinging from one extreme to the other without ever stopping in the middle.
