Contrary to what people seem to think,
natural selection is
not just about making the
human race smarter. It's about making us,
as a whole, more able to survive and
adapt to change. That means not only that some of us need to be "smarter" (more effective
communication, better manipulation of abstract
symbols, quicker
processing time, etc.), but that others of us need also to stay at a level that we don't mind doing
tedious, but necessary, duties.
Let me clarify. I'll use humanity's current favorite toy, the computer, as an example because I'm reasonably familiar with it. That said, this illustration could be done with the loom, the sailing ship, the bow and arrow, the man-made dwelling, the knapped obsidian blade, or whatever other technological innovation you can think of. The computer took a lot of genius (over a lot of time) to finally make real, from pure mathematics and boolean logic, to the vacuum tube, to the transistor, and so forth. A great deal of terrifically bright people worked on ideas that would eventually make the computer possible. For each of their innovations, though, there had to be all kinds of societal infrastructure supporting them. People to make the paper that mathematicians would write on, to mine the raw materials to be made into vacuum tubes, to keep all the labs and offices clean and usable, etc. So many people completing tasks that the geniuses wouldn't have wanted to do themselves, while making it possible for those same geniuses to do their work.
Intelligence evolved to fall in a bell curve for a reason, as society doesn't need, and couldn't function properly with, an abundance of overly smart people. If everyone engineered their children to have IQs of 175, we'd die back just as surely -- though for different reasons -- as if epidemic disease made all of our children be born with 50s and 60s. It's a mistake to have things like The Darwin Awards trumpeting deaths caused by stupid mistakes and calling it evolution. Natural selection is a hell of a lot more complex than that, and its end goal (if that's what you want to call it) is more interesting than a race of super-smart beings that popped into existence when all the "dumb" ones were killed off.