I read a sci-fi short story quite a while ago that addressed this question. In this story human intelligence had progressively declined to the point that most of the human race were basically idiots only capable of menial tasks, eating, and reproducing. The small percentage of humans still mentally astute worked at producing and distributing food and maintaining the cities to provide food and shelter for the rest. I took the story as a comment that we have reached the point that natural selection no longer weeds out the stupid, weak, and infirm.
So you're going to answer a science question with... science fiction? And science fiction that sounds like a libertarian fantasy at that?
Yeesh.
If this outcome were remotely plausible, it would imply that, right now,
Homo Sapiens would be spending most of their brain-power and intelligence on providing for teeming cities of
Pan Troglodytes. In reality, Humans are numbered in their billions, while Chimpanzees are endangered.
A putative future speciation event, with selection both for and against intelligence, would presumably produce an intelligent post-human animal that was as selfish and disinterested in the less intelligent post-human species as we are disinterested in the fate of the other apes. If the less intelligent ones don't like that, their revolution will no doubt be as effective as a revolution of Chimpanzees with rocks would be against our current, well armed, Human race. Being able to tear a man in half with your bare hands does you know good if you are gunned down from a kilometre away. But all of this is fantasy - it might make a good parable, but it isn't a realistic prospect for our future.
More plausibly, given that gene mixing in current human populations is higher than ever (due to the availability of rapid transport, which reduces geographic separation); and given that there seems to be some barrier to further increases in intelligence, through freedom of reproductive choice, and the greater uptake of that freedom by the intelligent, there may be little increase in intelligence beyond our current level. Equally, given the disadvantages inherent in reduced intelligence in a technological world, there is little pressure for reduced intelligence. An equlibrium somewhere around the status quo for intelligence seems fairly likely. Large movements in either direction are, perhaps, less favoured.
On the other hand, while not having kids is an intelligent choice today, it would not necessarily be so in a world with a smaller population - and population is likely to fall in the next few centuries, once it stabilises in the middle of this one. The invention of reliable, safe and effective contraception in our generation has totally changed the game; it is far too soon to say how evolution will be affected by this, but anything that has a dramatic effect on reproduction will surely have an impact if it is sustained for the long term. In ten thousand years, our descendants may well be quite different from what we are today, simply because of the invention of the contraceptive pill. How those differences will manifest can only be speculation at this early stage.
We are in the first few generations of the first species on the planet to have intellectual, rather than hormonal or emotional, control of our reproduction.
