I always thought it was reproductive isolation of a population that is the main factor in speciation (or in the production of sub-species, for that matter). But then again, I could be wrong...
That is a position also taken by creationists like those at AiG.
They contend, as I understand it, that because life was created with perfect, functional DNA, (no junk) that as populations became isolated, certain traits were lost, allowing others to express. That would be a loss of information.
The ToE says that information is gained.
However, even if this new concept takes hold that it’s through reproductive isolation and information is indeed lost through that, there still arises the question of where the information came from in the first place.
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?