fertility
warning: unconventional possibly offensive thoughts ahead. bacteria happily gobble up all resources as fast as they can. then hibernate as spores when they're in danger of starving. until such time as conditions become good again. higher life forms don't really have that luxury. when times are lean the death rate's high. so fertility rates have to be high just to keep the species going. but times get good, death rate drops, and we start consuming resources. which is fine as long as the good times last. but generally they don't. so we need some way to lower the fertility rate. otherwise a big time boom could lead to an equally big time bust. which if big enough might take a very long time to recover from. even when the good times return. so here's what i was thinking. marginal individuals are those that would die during tough times but live during good times. suppose they had some sort of unspecified fertility limiting gene. they still contribute to the good of the species. but they either don't reproduce at all. or pass the limiter along. and as the good times continue the limiter gets wider distribution. so more of the population has reduced fertility. and when the inevitable bust comes it's not so bad. anywho, not so controversial. until you start listing what the limiters might be: obesity, impotence, intelligence, autism, homosexuality, etc.