When it comes to speciation, all you need is two populations that do not
have a genetic interchange. This can be because of geological reasons. It can be also because of diet.
Although I can not find the source, there was an article about a species of insect that in one location , had two seperate populations that do not interbreed. The reason for the difference was one starting having a diet from one source, in a tree, and the other one stayed onthe ground and ate something else. They are still the same species.. yet the two populations don't interbreed, based on their diet. This is going through the same process that it is believed the birds in the galopagos islands did.
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