[harpastum]

Yes, but it seems like the grandparent was trying to fit in every polysyllabic word they could find.

They're saying that some groups tend to have more children and some have fewer. The groups that have more will become a larger and larger percentage of the populace (because they have more kids), and the groups that have few children will disappear. Therefore the global birth rate will go back up to the levels of a couple hundred years ago.

(I don't agree with this idea, just trying to present their argument with more straightforward vocabulary)