I have a long-standing theory that modern medicine is actually screwing with evolution. Back in the day of the cavemen (and women, for you feminists out there), it was survival of the fittest. You have crappy eyesight? You get trampled by an elephant. You have a week immune system? You probably don't make it past your 30s. Depressing, but true.
Flash forward to the 21st century. In our modern existence, we have such goodies as antibiotics to thwart the likes of bronchitis, corrective eyewear for those not blessed with good eyesight, cholesterol drugs for those with a family history, hearing aids for the hard of hearing, and of course medication for the mentally unstable.
Modern medicine has basically allowed for the mentally unstable to co-exist with the so-called "normal" (although who knows what the definition of "normal" really is) population. As a result, the mentally unstable are able to lead relatively normal lives: hold down jobs, get married, and even raise a family. As much as medication will keep them at bay, it won't prevent their bad genes from being passed on to their kids. The result? Their kids may end up with mental instabilities as well. The instabilities may be milder, or more severe than that of their parent(s). And this can go on for generations to come. Evolution is beaten. It's depressing to think about this. One may even call me cold-hearted to say this, but it IS a fact.