Trilobyte, it seems the thing you don't understand about evolution is that it is actually a complex blend of inbreeding and hybrid vigour.
We know from Dairy cattle that most mutations on the DNA are harmful and they lead to recessive genes. I'll work on finding a site. But BLAD is the first one I think of. It causes deformities in calves with a double recessive and they die some months before they're born. Not an advantage for either the calf or the farmer. Scientists have even worked out which bull spread BLAD around the world. Selection acts on these mutations. But selection doesn't act on the DNA or the genes. It can only act on individuals the express the gene.
To get a double recessive gene you normally have to have inbreeding within an isolated population. Genes advantageous to the species as a whole therefore usually first appear in isolated populations. Their spread into the wider population can be aided by hybrid vigour as the gene spreads through a hybrid zone. The postinf Out of Asia not Africa shows this process was inportant during the development of Homo erectus. Presumably it's still important for our evolution.
Hope this explanation is simple enough.
