The modern "underclass" has probably only existed for at most, 100-200 years in various places. That's 3-10 generations with plenty of gene-movement in and out of the group in the meantime.
It seems a bit more plausible to me to argue that humans already have a variety of strategies for reproduction and different circumstances bring out different strategies.
My impression is that in peasant societies, teenage pregnancy is very common and families try to maximize the production of children. However, in hunter-gather societies, which pre-date peasant societies, my vague understanding that is teen-pregnancy is less common and there is less maximization of reproduction since population is often consciously consciously kept in the bounds of the land's productivity.