Are you serious?
Two populations of ancient Great Apes live on each side of a great river in Africa.
The apes on the west side are all fat, dumb and happy swinging from the branches of their trees, eating their fruits & nuts, and having a good old time with the Mrs.
But those poor suckers on the east side of the river -- well the climate changed, the land dried up, the trees died off, and now those sorry apes had to climb down out of their trees, and either learn to make it on the ground or die.
And, most of them died.
But a few found a way, and the more they worked at living on the ground, the better things got for them.
However, slowly, slowly they changed -- stood more upright, walked on two feet, used their hands for clubbing and carrying stuff back, so they could have a good time with the Mrs.
Now, after many, many generations those poor suckers on the dry side of the river no longer looked like, or smelled like their fat-dumb-and-happy "relatives" still up in their wet-side trees.
So if, by chance, it ever did happen that the male of one side, ahem, "bumped into" a female from the other, well, you know... she just didn't look right, didn't smell right, didn't eat the same food, and being around her actually made him sick.
So, horny as young males are known to be, in that case he just didn't feel like it, and couldn't do it.
And that precise moment in time is what scientists call "speciation".
Does that answer your question?
but no proof of that happening has ever been found?