A strong, immediate impact of the demographic transition on human evolution?

by Alan Cohen

This blog post is different than most others. I had a Eureka moment in the shower, and I just need to write the idea down so I don’t forget it, and to share it with potential collaborators so we can maybe develop it into a paper if it hasn’t been done already by someone else (a quick search in Google Scholar suggests no).

Over the last 150 years, many human societies have undergone what is known as the demographic transition, first dramatically reducing mortality rates and subsequently reducing fertility rates. In simple terms, your great-great-grandma had 15 kids and 3 lived. You will have one or two, and they will live. What is the impact of this change on human evolution? I think it could be profound, and here is why:

In almost every species, there are a certain number of offspring that do less well, probably due to either a bad mix of genes or developmental bad luck. And in most species – including pre-industrial humans – this really doesn’t matter so much because most offspring do not survive and reproduce. All else equal, natural selection would surely select for reducing the probability of producing such aberrant offspring, but all else is rarely equal. Allowing genes to mix freely may also result in unusually good combinations, for example, combinations which would not be found if there were strong selection against reassortment. Producing a perfectly stable developmental environment may be hard to do, and may involve costs that are not compensated for by the reduced production of aberrant offspring. Because, for most species, life is so random, having a certain percentage of offspring that are doomed to fail from the get go is hardly the end of the world.

The only species I can think of for which this is not true is humans in industrialized, post-demographic transition societies. For us, we’ve got about 2 chances to get it right, end of story. This should put a sudden, strong selective pressure on genetic assortment mechanisms to avoid possibly unsuccessful configurations (even if it also means reducing the chance of highly successful combinations). Similarly, it should increase selective pressure for a predictable, stable developmental environment, whatever the costs.

This tendency will be furthered by a little-mentioned aspect of the demographic transition (I’m guessing here, educatedly): a reduction in the variance in the number of surviving, successful children. For most people, it’s between 0 and 3. For our ancestors, some women had many many more surviving children, and some men more still. The lack of much of an upper tail on the distribution of surviving offspring means weaker selection for aberrantly good genetic combinations.

In other words, the demographic transition should result in natural selection that increases the probability that every modern couple can have about 2 kids, with less risk than today of congenital abnormalities, developmental disorders (autism, trisomy 21, etc.), and other traits that will mean they don’t integrate well into society. But as a consequence, we should also see a reduction in the frequency of unusually good aberrations – geniuses, prodigies, and so forth. And it could eventually involve genetic and developmental changes that bring us in entirely new evolutionary directions.

This idea rests on there being enough of these aberrant offspring today for selection against them to be strong. I’m not sure if there are yet sufficient data to test the idea, particularly given that we have also altered our environments dramatically in ways that reinforce this evolutionary trajectory (less smoking resulting in fewer birth defects, for example). Perhaps a mathematical model would be sufficient to show this.

(Apologies for the politically incorrect term “aberrant offspring.” It’s not meant as a moral judgment, just in terms of the cold eye of natural selection. And I’m writing quickly so I can attend to my own crying offspring…)

Interested in collaborating? Let me know…