science

I'm pretty sure "can produce fertile offspring with each other" is a pretty common definition of a species. At least I've seen it before.

"Fertile" is key here, because while donkeys and horses can produce offspring (mules), mules are infertile, so donkeys and horses are different species.

So pretty much yes: While no single generation will differ enough from its parents to be a new species (at least very rarely), once you move a couple thousand generations, you can have a new species. However the point at which the new species "came into being" is very fuzzy.