Gregory Cochran and Henry Harpending have an interesting post at their blog West Hunter on human assortative mating. This means the tendency for like to marry like (i.e. mate with & produce offspring) on some trait. The heritable traits people care about tend to be those which reproduce a pleasant, technologically-advanced civilization: attributes such as an IQ above 105, openness, agreeableness and conscientiousness.
The authors have a blog post on this topic from a few years back and a new PDF in draft (which uses some technical language but is not too technical in content). The results are that if the right and left population-segments of the bell curve (for that heritable trait) mate assortatively - largely amongst themselves - then a caste system (high-merit vs. low-merit) emerges within one generation and then consolidates. They speculate that with mass university-level education, we're already doing this experiment for the trait of intelligence.
The result? Perhaps a hollowed out middle in our society.