The number of children you have is selection (in an evolutionary sense) and because it is not a trait being selected for directly by anyone (we are not trying to breed people that want or don't children), the selection is being done by nature.
The environment of humans has radically changed and we are undergoing very strong selection and adaption to this new environment. In the past the number of children you had survive to adulthood was tied to the amount of food you could bring in (as with most animals). In the new human environment the number of children you have is a consequence of how resistant you are to the cultural meme that children are undesirable.
I think he meant more that natural selection is essentially summarized as how many children various groups have. And for some weird reason, natural selection is biasing away from "smart people".
I don’t think it’s too hard to figure out. Natural selection disfavors a strategy that requires significant investment in non-reproductive activity (education, career ladder-climbing) during prime reproductive years.
The selection part works on individuals, what changes from selection is the frequency of alleles in the population. By not having children the alleles you carry will become rarer in the population of the next generation (all things being equal).
The environment of humans has radically changed and we are undergoing very strong selection and adaption to this new environment. In the past the number of children you had survive to adulthood was tied to the amount of food you could bring in (as with most animals). In the new human environment the number of children you have is a consequence of how resistant you are to the cultural meme that children are undesirable.