I've thought about this too. I have a severe stutter, and I looked at my own DNA, and apparently I have some sort of sequence that correlates with stuttering or speech impediments, my mom doesn't have it, so presumably it comes from my father. I only met him once, and I remember him saying something like "I had a stutter when I was younger, but then it just went away" My theory is that if stuttering is primarily genetic, actual expression of that gene, or severe stuttering is quite rare even among carriers. If its being selected for, then maybe the stuttering gene has some other effect then just stuttering? I read that stutterers generally have above average IQ, so maybe it's related to that. Anyhow, I can't really think of any way severe stuttering is conducive to reproduction, but it would make sense that some other side effect must have had a good enough benefit to outweigh the potential risk of severe impediment of communication.