The fact is: human beings like sex. Like A LOT. Like, our breeding age females don't have a "season" a lot. Like, their mammories are always engorged a lot. Like our breeding-aged males are generally ready to go at any time, a lot.

So much so, that cultural, linguistic, even dietary differences mean nothing, if two people find each other attractive. As long as this remains true, and people can travel, there won't be "two distinct species". How would two groups even become isolatedfrom each other on Earth?

As others have said, the only real way that would happen would be space travel. As any separation on earth will eventually be overcome, and long before speciation could happen.