Natural Selection among Humans
It appears there is an opposite selection process. Once again, birth control and intelligence, family planning will have interesting undesireable effects.
Selective pressure will favore drug and alcohol and other less than responsible type behavior because they result in not only unintended pregnancy, but also earlier (shorter generations) births as well.
In the past, the irresponsible parents had children with lower survival rates. Now that we have a society that at least minimally ensures the survival of every child, that selective pressure is now gone. Also, the more responsible and intelligent people now have birth control to reduce their percentage of the genetic mix. I call this extreme selective pressure which has a shorter time span for change than other factors that may take a much longer time.
I am not advocating the end to birth control. On the contrary! Our population is at a crisis level and any humane way to slow that growth must be used. I am just posting a warning to what may happen in the next 200 to 500 years if we succeed in 100% available voluntary birth control over a 200 to 500 year timeframe. And that to reverse this effect we may have to consider society adapting a licensing program for chidren with upper limits for all, and strict age, minimal income standards, and education standards before licence for reproduction is granted, along with genetic screening for known genetic diseases.
Also, and probably more importantly, cultural evolution along this matter is probably more significant. Those culture that advocate higher population growth through religion or what ever means will displace the rest of the cultures. But these cultures are very hostile to generic education and the mixing of cultures, as this tends to dilute the pronatalist stand.
Of course, this is a moot subject; humanity is facing a severe bottleneck this century, and it is highly unpredictable what the mix of humans and culture will be on the other side (the survivors) of the energy crunch and eco-disasters.