This seems like quite a stretch: humans aren’t fruit flies, and something as complicated as reproduction is not a simple trait which overrides socialization and intelligence.
This is especially true when you look at why people aren’t having as many kids as previous generations: the quality of life impacts are significant and people are willing to go to considerable lengths to reduce them. If you have genes which make you extra horny or foil certain birth control techniques, you can switch strategies and if you don’t, your children will have a stronger incentive to do so after growing up with a cautionary example.
You can’t meaningfully reason about that complicated situation as if it were something like moth coloration.
Some people just won't care very much. People like this exist in the world today.
We've found genetic influences for lots of behavioral traits, including ones related to politics and religion. (and more commonly known things like: depression, autism, intelligence, etc.)
There probably will be some reproductive organ changes to sometimes defeat some forms of birth control, but mostly this will be brain changes. If people actually want birth control they can invent things faster than we can evolve past them, but if people just want kids then it doesn't matter how effective birth control might be.
Non-voluntary measures are possible, but again the evolutionary winners will be the ones who evade that. If the desire for kids is strong enough, people will bribe their way out of mandatory birth control.
I'm an example: I wanted a large amount of children, and taught my children the joys of a larger family. So far, they think along the same lines.
I don't think one can determine if the trait is or isn't cultural by guessing. How could you possibly know the origin of your own mental preferences? Even political affiliation, which sure looks cultural at first glance, has been shown to be under genetic influence.
There is evidence that there are certain genes which promote religiosity. They don't guarantee it – somewhere out there, there is probably an atheist with lots of religiosity promoting genes, and a devout believer with none of them – but they do make religious belief more likely.
People who have large families for religious reasons will pass on both their religious beliefs to their children, and also in many cases their genetic predispositions to greater religiosity. Of course, there is no guarantee that the child will stay in the parent's religion, but so long as the retention rate is decent (say >= 70%) the religious belief (and the genetic predisposition to adopt it) will be spread widely through the population.
Even though organised religion isn't looking very healthy at the moment (at least in developed countries), this is reason to believe that its long-term prospects are actually quite bright.
It's well accepted that many childless women in their 30s will start to crave having children. Someone with a mutation that causes this craving to happen in their 20s as well will have more kids on average.
Admittedly, for some of those traits the mutation required might be too complicated to be on a relevant time scale. But things like poor impulse control could easily be caused by a mutation (brain injuries can cause poor impulse control, so a mutation that damages those same circuits also could), which would be transmitted and potentially selected for.
A smart person with a burning desire to produce offspring will have the greatest advantage. A smart person who dislikes children will have the greatest disadvantage. In between those people are the people who screw up birth control.
If the human population contains a million people with the trait for wanting lots of kids and a billion people who are just dumb about birth control, both will be selected by evolution. Once those traits are the norm, it would likely turn out that wanting kids is the more successful strategy. We could thus see intelligence go both directions, first dropping and then rising.
Since then the useful selective function for individual survival has been the ability to operate in a tribe and make the tribe successful.
Who's to say what that has really been selecting for? Intelligence may very well have almost no correlation with individual genes, and simply be an emergent phenomenon optimized for by pro-social behaviors, or activated by being in the presence of people who are already fulfilling that role. Maybe what we regard as individual intellect is repressed pathways only chemically triggered in childhood after the development cycle casts around to try and figure out the status of the tribe and specialize into the optimum addition?
50% of what? (which is to say, you picked that number because 50% genetics 50% environment feels like a good split, and how do you even measure that?)
It means that if you plotted a graph of orphaned children IQs on the X axis and their biological parents IQs on the Y axis you'll get a messy and very bulgy line going up and to the right.
And this kind of thing is discovered by noticing that the IQs of identical twins are much more similar than those of non-identical twins.
Wikipedia has a fairly good overview: https://en.m.wikipedia.org/wiki/Heritability_of_IQ
Huh, guess I missed the news that we'd discovered human level problem solving abilities in sheep.
People with lower intelligence may be just as likely to adopt a short term hedonic lifestyle and use contraception more. Totally depends on cultural trends.
What we are selecting for is any heritable trait that might durably make people more likely to intentionally reproduce. That could be a lot of things: a stronger biological clock, later menopause for women, changes in cognitive patterns to the degree that they are heritable, maybe even longevity or longer healthspan. Its complex and hard to predict.
The desire to have kids is a social / cultural trait. We have plenty of evidence that it is fluid over time and changes with economic opportunities and whatever. It is very hard to maintain cultural cohesion in groups as they get larger. Even the Mormons would break ranks past some critical mass. The drop in fertility rate is due to changing culture and economic incentives. We have tons of data to support this and more of the world is headed in this direction economically and culturally.
Edit: if so, it seems we can get rid of plastics and pollutants like PFOA's in 500 year cycles by constructing condoms of the biggest pollutants...
The point of my example is to crush the mistaken notion that evolution is inherently slow.
Our environment now includes birth control. Most people in the current population do not feature traits to overcome birth control. There is a subset of the population which does have the needed traits. This well-adapted subpopulation will grow rapidly because birth control is a very strong selection factor. From an evolutionary perspective, the correct use of birth control is equivalent to the death of offspring.
Remember that evolution is not inherently slow. It is usually slow because the existing population is typically well-adapted. A simple thought experiment demonstrates that evolution can be fast: we would all have blue eyes if the others were executed.
Given the severe selection caused by loss of offspring, evolution will rapidly overcome birth control. Just a few generations might mostly do the job. Within 500 years, birth control will simply not be a factor in human population.