|MadSci Network: Evolution|
"Will population control have an adverse effect on natural selection?"
The question is rather fuzzy, and my attempts to sharpen it up may carry its meaning off in directions different from what you intended. I assume you're asking whether population control might prevent natural selection from further improving our species, or indeed might even cause a detrimental drift. The words "improving" and "detrimental," of course, presuppose some agreed-upon standard of goodness, for which you'll have to consult the Mad Theologian web page.
Natural Selection, being a process rather than a person, has no preferences in direction, and will continue to work just fine, in the sense that genes that promote long-term reproductive success will always continue to become more prevalent in the gene pool. For example, in the United States, higher education has been a factor in Natural Selection over the past 50 years. College graduates have fewer offspring than non-graduates; so we have unintentionally been reducing the frequency of genes that make people more likely to graduate from college. Is that good? Bad? "Adverse?" Decide for yourself; Natural Selection doesn't care.
If population control is imposed by governmental rules, any gene that predisposes its owner to evade such rules (and I don't doubt that there are such genes) will be favored over alternative "obedient" alleles. Also, the rules would very likely be written in a way that would increase the long-term reproductive success of politicians, so genes conducive to politically successful behavior would be favored. Good? Bad? Again, it's your decision.
Here's another example: I have two exceptionally conscientious sisters who have done their part to solve the population problem by having no children. If there are genes influencing this level of conscientiousness (and I don't doubt that there could be), where is our current policy of "population control by appeal to conscience" taking these genes? Answer: oblivion.
How big a selective effect could result? That depends on the duration of the controls and the strength of the selective pressure. Fortunately, political stability is so short-lived these days (recent centuries) that any system of controls is likely to be overthrown before it can have a large effect on our gene pool. (I'm assuming here that we're talking about population control by birth prevention, rather than by extermination.) So it seems unlikely that we'll see a population control program sufficiently long-lived to change our gene pool much. Of course, in the absence of just such a program, we will get the "default" result, population control by "natural" means: war, famine, and disease. Take your pick.
What about the possibility that we will all decide to be "good" and limit our reproduction to the right level? Such a world would unfortunately be unstable in the presence of any gene that made its owner less inclined to be good.
Try the links in the MadSci Library for more information on Evolution.