Earlier quoted context omitted.
Did the movie really contain genetic claims? I took the sequence at the beginning to be more a criticism of Cleavon's cultural traits. (example: "I thought you was on the pill or some shit!") In fact, most of the movie's humor centered around a downward cultural slide. Kids do pick up their parents memes more readily, after all.
Yes. IIRC, the "opening bit" that camiller was referencing above was about how intelligent people were continually having fewer children while less intelligent people bred as quickly as they could. (ninja edit: here ya go: http://www.youtube.com/watch?v=PSROlfR7WTo )
I don't recall the movie taking a stance towards nature. In fact, the moral of the whole story is to try to solve problems, and the great speech at the end envisioned a time where people cared whose ass it was and why it was farting. These are messages about cultural priorities, not genetic limitations.