If there are heritable traits that lead to people having more kids in a developed world environment then at some point evolution will take over and we'll start to see exponential growth again.
This assumes that biological evolution is the only systematic effect on population growth rate. I would argue that the fact that we have decreased infant mortality at the same time as decreased population growth is evidence that there are other systematic pressures limiting population growth.
There are certainly things about that modern world that are pushing population growth rates lower. The availability of birth control and more legal rights for women prominently among them. That's why the population growth rate should be leveling out in 80 years or so.
But looking forward maybe 800 years, if there are alleles which tend to make people more likely to want and have kids then I would expect the prevalence of those alleles to increase and the birth rate to rise again. When I look around at my social groups most of the people I know enjoy sex but there's a lot of diversity in how much people are interested in having kids. I don't have any basis for saying if that's a heritable trait or not. But if it is heritable then I'd expect people like that to be the majority before all that long and in the long term population growth to become exponential again.
And in the very long speed of light limitations mean that the resources available to humanity can grow cubicly at best so if exponential growth resumes Malthus will be with us again at some point.
That 800 year extrapolation assumes that humans won't have access to genomic modification though, which is unlikely. In any case, the number of kids one has is highly culturally and environmentally dependent--the same people whose great grandparents had 4-6 children now routinely have 1-2. Even in a purely natural-selection sense, if "having more kids" were significantly heritable, the process should have been running for all of human history, and it's not like we're having 400 kids per woman these days. Besides, r-strategists don't always beat out k-strategists.
That's possible but I'm not sure why someone who would prefer more descendants would genetically modify their progeny to not prefer more descendants. If this is a government imposed eugenics plan it could certainly work, though.
For most of human history a desire for sex was quite sufficient to ensure reproduction even without as much desire for children. With the advent of birth control the environment changed and there are new selective pressures. Also the modern world has a lot of new, competing, sources of joy.
R strategies certainly don't always beat K strategies! But humans are currently in a very rare situation where our population seems to be very far below K and most people in wealthy countries aren't living anywhere near subsistence level. So, until Malthus rears his head again, evolution is going to be paying a lot more attention to r.
For those following along at home, for N as the number of individuals:
Yes. The only way to long run sustainability is central control of population with mandatory birth limits or unlimited expansion of land area to extraterrestrial territory.
Otherwise evolution will always exceed its limits and produce a crash. Malthus explained it all mathematically 200 years ago, even before Darwin documented the mechanisms.
> The only way to long run sustainability is central control of population with mandatory birth limits
This is demonstrably not true. Look at Japan and western Europe: declining population. Its a function of economic situation (no need for more than 1-3 children) and female reproductive choice. If we give that to the world, the population problem goes away. The mathematics of population dynamics work for animals, roughly, but the assumptions don't hold for humans because of the changes in behavior.
There are people, even in Japan, that want a whole mess of children. That preference is heritable. Each generation has more of these people because they're the ones that reproduce.
Eventually those fertile and natal enthusiasts will dominate the population and exponential growth resumes. The current situation is a temporary response to an external shock, specifically to reliable contraception. But Malthusian conditions will return; Darwinism requires it.
Statistics suggest your view of Darwinism is incorrect.
http://www.infoplease.com/ipa/A0884238.html shows avg household size has been declining since before contraception, and it doesn't look like contraception changed the slope.
Couldn't it not be a gene but a meme that is inherited, instead?
> Each generation has more of these people because they're the ones that reproduce.
I.e. each generation has more of these people since people who see having children as something positive reproduce and teach these values to their children.
Being a memetic instead of genetic factor would have the advantage that it is easier to "unprogram" it, i.e. by having rules/incentives in the society that discourage reproduction or by education.
The appearance of exponential growth is always temporary.
As the population increases, negative feedbacks reduce the population growth rate. Examples of these feedbacks include cost to raise children, reduced dependence on large family for security in old age, etc.
Estimates of the "replacement fertility rate" are about 2.1 per woman in a developed society (higher in less-developed societies). Many first world countries are already below this rate, and without immigration, will have declining populations as their native populations age.
Malthus and his disciples have been wrong for 200 years.
Paul Ehrlich in particular, because he used to like giving short-term dates for his predictions, so we could watch them slide by. Then he decided time was different to him than to an "average person".
“How many years do you have to not have the world end” to reach a conclusion that “maybe it didn’t end because that reason was wrong?” -- Steward Brand, former disciple of Ehrlich
Well, Paul Ehrlich is an entomologist, so it's not too surprising that he should consider human populations to be essentially automatic, mindless swarms.