I think it's a combination of both isolation and environmental change. With only isolation, it still might occur, but it would take a lot longer, since most variations would not produce much of an improved reproductive success, nor would they result in much "culling" by the environment. But when the enivronment changes drastically and there are isolated populatons and thus no mixing, at some point the differences between the two (or more) populations may become large enough that they can no longer interbreed, as each population takes a different path to adapting to the new environment.
An exception might be if the "environmental effect" kills off so many species that there are lots of open "slots" in the new ecology, in which case mulitiple species may result.
Of course who/what drives the environmental change?
Sure...and do not underestimate the founder effect.
*jumps up and down frantically waving my hand in the air* I know! I know!!!