Just to stand up for the opposing side in @worriedguy‘s analysis: Unless men stop dating and mating with airheaded women, we’ll be adding to the supply of stupid from that side, too.
But I agree with @tedd here. The process (for the most part) is so slow and happens on such a sub-microscopic level that we won’t “see” changes until long after the fact.
I read once that the primary difference between American eels and European eels, for example (which are now separate species) had to do with the widening of the mid-Atlantic rift, which widens at about 1.5 – 2 inches per year. At one time it was thought that all of the Atlantic eels were a single species, meeting at mid-ocean for mating purposes once a year. Over time, the widening gap in the rift, which favored the European side, meant that American eels were having to travel longer and longer distances to mate. (That 2” per year, over tens of thousands and then hundreds of thousands of years, adds up to a lot more travel for their nookie.) This started to select for bigger and stronger American eels, since it took that extra bit of strength to be more successful at mating.
No one would have seen that “as it happened” (and it’s a change that one might have expected anyway – presumably modern European eels are also more successful at making that migration than ancient eels, too – at least, more successful given today’s conditions). I don’t think we’ll be able to see the changes in our own species, either.