First let's not anthropomorphize nature, or natural selection. But secondly, in 1st world countries, the more money you make, the less likely you are to have kids! We've done a terrible job at incentivizing couples to have kids since women have entered and made up a good part of the work force, and it's hard to blame someone in a good career, married to someone in a good career, to take off 10-15 prime years of their lives to have children.

I think that waiting to have children until later (late 20's to early 30's) is a big problem in terms of fertility and successful child bearing. Unfortunately the human clock doesn't really jive with the "4 years of college, work a bit and then think about marriage and kids".

I don't have a conclusion except we might want to think about increasing the birth rates among high and medium earners in our populations where they are struggling, lest we become like Japan or other countries (some in Europe which depend on importing labor in order to satisfy demand)