So God creates Adam & Eve. Who did their offspring mate with? The parents? Each other? Everyone knows that inbreeding is an evolutionary dead end. This isn't even taking into account the lack of genetic diversity in the creation story of genesis.
It's important to note that the majority of Christians don't interpret Genesis literally. It's impossible given what we know about human biology today.
Finally, I think its equally important to understand that evolution, at it's core, is adaptation over long stretches of time. It does not explain where the universe came from, nor does it tell us why there's something rather than nothing.
God, as an explanation, is just as good as saying that the universe spontaneously popped into existence without a prime mover. From our perspective, the former would look a lot like, if not identical, to the latter.
The genetics were pure back then so no danger with incest. And only later did the Lord give laws against incest.
Brief videos.