Human microevolution, ooh. That sounds like a good guess. Google is showing me some results... it will take a while to parse them.
I would be very interested to hear evidence to the contrary, though.
Well the first thing that comes to mind is the incredibly horrible failure rate of common contraceptives, and the unplanned pregnancy rate and birth rate that goes with them.
Evidence:
In not even four years, about 25% of people using condoms became pregnant. Birth control pills were similar. http://www.jfponline.com/Pages.asp?AID=2603
"49% of pregnancies in the United States were unintended" http://www.cdc.gov/reproductivehealth/UnintendedPregnancy/index.htm
"These pregnancies result in 42 million induced abortions and 34 million unintended births" (world population growth was 78 million for contrast) http://www.arhp.org/publications-and-resources/contraception-journal/september-2008
If there's any trait at all that's connected with this - inability to afford more expensive methods, not caring about reliability enough to get an IUD or something more effective, dexterity level too low to correctly apply the product, impulse control issues / inability to think under pressure or when excited, forgetfulness, inability to resist temptation, etc. those traits are likely to reproduce faster than their counterparts. Considering that half our population growth is unintended, I'm pretty concerned about it.
The situation could be that (if a genetic irresponsibility trait exists and is responsible for a large portion of unintended pregnancies that go full term) even if the responsible portion of the population is larger, that the irresponsible portion begins it's generations sooner, and it's growth outstrips that of the responsible portion of the population, overpowering it in a short time.
We're also doing things like removing sociopaths out of the population and putting them into jails. This probably reduces the rate at which they reproduce, though I'd expect far slower evolution there, if any, than I would with something that influences contraceptive failure.
We select certain types of people (or they select themselves) for the military. When they go off to war, they're more likely to die before reproducing. Since Americans tend to send their soldiers away, they're also a lot less likely to reproduce before dying in a war than soldiers defending a home territory where they have access to lovers.
If welfare creates a perverse incentive to have more children, any trait that might make welfare appealing to a person could end up being reproduced.
People who get a 2 or 4 year degree have more free evenings in which to find a lover and take care of a child. Contrast that with people who get a higher level degree. They have to wait longer before they'll be ready.
People in certain industries work very long hours. They might not get a chance to meet someone or might decide they can't have kids working as many hours as they do.
For these last two groups, if they're determined to have kids, they'll probably find a way to do it -- but they may be significantly delayed compared with someone who gets a 4-year degree, works a 40 hour week and can start having kids when they're still in their early 20's. The delay of a few years probably wouldn't make much of a difference one or two generations away, but if there are any traits that result in one getting a higher level degree or working longer hours, those people probably won't reproduce as fast as others.
Well the first thing that comes to mind is the incredibly horrible failure rate of common contraceptives, and the unplanned pregnancy rate and birth rate that goes with them.
By "evidence" I mean evidence that allele frequencies have noticeably changed. These are all hypotheses about things that might be affecting allele frequencies but, again, my standing assumption is that the timescales are too short.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.