Anyway, it's changes in the genome that make all the difference, and "natural selection" doesn't really explain that ~ it just describes it.
For all anyone knows the bird flu may be the prime mover for transporting genes between species, although some lotharios who post here regularly are of a different opinion (at least concerning their place in the grand scheme).
I think the Darwinian answer is, of course, that irrespective of how the genes get passed aroun, only those which benefit the species (if not the individual) give a survival advantage to the species, and that, in turn, is the element of ultimate importance.
No doubt having teeth in a world without teeth probably allows you to eat more, and presumably breed more, but that's more an "individual" survival advantage and not necessarily of great utility to your species. After all, everybody else may be getting all they really need sucking in through a reverse nephritic process managed by tiny cells in their skin.
The survival of numerous species, apparantly unchanged, for tens of millions of years, suggests that some "designs" have ultimate advantage and enable a species to beat the point spread of the probabilities surrounding natural selection, or, that they have developed an immunity to changes in the genome and are able to ward off all gene changes from whatever source.