[drabiega]

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.

[Symmetry]

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.

[hx87]

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.

[Symmetry]

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:

  dN/dt = rN(1 - N/K)

[dzdt]

Religion is a much stronger predictor of number of children than any genetic factor. Religion is typically passed on to children, but not genetically!