So, at some time in history an ape had a genetic advancement in its species progression and became a human.
Its dna defect resulted in a new form of animal. It was of one sex or another and couldnt reproduce. At the same time and in the same county
another exact same type genetic mutation occured and another same type animal was created which could only reproduce with its counterpart across the river.
They got to gether and viola- Adam and Eve. The rest is history.
Populations become isolated from other populations of the same species, and you get genetic drift for the entire population, not just one or two individuals, and this happens over hundreds of thousands of years. The unique environmental pressures determine the direction of the drift. Eventually, the two groups of the same species become sufficiently different as to (first) form a distinct subspecies (still capable, theoretically, of interbreeding). Then, given enough time apart, they become two distinct species, sufficiently dissimilar that a mating between them could not even theoretically produce fertile offspring. At that point, they are said to be (like donkeys and horses) two distinct (though closely related) species. Given more time apart, and no live young (fertile or not) can be preduced between the two populations that were once the same species (they become genetically isolated from one another). Should something bring them together again, one species may out-compete the other, causing the other to eventually disappear from the planet. Most species that have ever existed no longer exist.
PS That evolution occurs isn't a theory. That's a mere scientific observation (like the observation that massive objects attract one another), accepted universally by science. The theory relates to the mechanisms by which it occurs (like the theories of gravitation that
explain why massive objects attract one another).
