Your Grandchildren Are Your Future
Generation time is the elephant in the room
A lot of popular writing treats evolutionary success as a single metric: how many children you have. Articles about evolution routinely frame “fitness” as simply “having more offspring,” and headlines often imply that whatever trait is advantageous will show up as higher fertility.
Even in the scientific literature, number of children (lifetime reproductive success, LRS) is often used as a convenient proxy for fitness, mostly because it’s measurable. But researchers have also noted the limitation: when there’s a quality–quantity tradeoff, children can be a noisy proxy for long-run genetic contribution, and grandchildren (or later descendants) can track fitness better (Bolund & Lummaa, 2017).
There is also an elephant in the room that most of these discussions miss. Fitness is not only about how many children you have, but how quickly lineages turn over. In a low-fertility world, differences in generation time, meaning the age at which people have their first and last child, can compound across centuries. That compounding reshapes family trees. However, if it persists across groups, it can slowly reshape the demographic and eventually genetic future of populations.
In the rest of this post I run two simulations that make this visible: first, a “modern” case where timing is mostly independent of survival, and second, a case where later reproduction is safer. The results are counterintuitive and have implications that go beyond individual families.
