In light of last week’s posts about why human evolution continues, I think it is critical to make concrete the reality of reproductive variance. It seems highly likely now that Jamie Lynn Spears is pregnant again. This might be a moot point if she has an abortion, though now that the word is out the public relations fall out might reduce the likelihood of that choice. The behaviors and outcomes of the lives of the Spears sisters are in the public spotlight, so let’s leverage this into an illustration of evolutionary theory. Last year I wrote Jamie Lynn Spears: it runs in the family?:
“I heard about it on the radio, they were talking about it. It’s real popular down there. Everybody knows about them,” Raynard Norman laughed. “It’s embarrassing, kind of. If it’s not her, it’s Britney, so at least it’s not Britney this time. But I’m not surprised, not really. … Nobody’s surprised because it’s not uncommon with her family. Next time, use a condom.”
Not uncommon in her family? Look, here is what we know: out of the 4 known pregnancies of the Spears daughters only 1 seems to have been planned. Britney has reported that it was not her wish to have another child with Kevin Federline, that it was a “mistake.” Unlike Britney Jamie Lynn’s first pregnancy was not expected, and now it looks it she’s pregnant again! What’s going on here?
It may be that the Spears’ daughters are very fecund for physiological reasons. Perhaps the likelihood of insemination and implantation during any given instance of sexual intercourse when they’re ovulating is just very high compared to the mean human female. If this tendency is heritable, then the Spears’ exhibit a high reproductive fitness in a society where the average woman is not even replacing themselves. That is, the proportion of genes which can be traced by descent to the Spears’ family in future generations is more likely than not to be higher than for those who do not exhibit such innate physiological fertility. This change in gene frequencies is evolution.
Of course there’s a simpler explanation for what’s going on here: the Spears’ are not good long term decision makers. These sorts of personality traits are often heritable, that is, some of the population level variation can be accounted for by gene level variation. This is obviously the Idiocracy thesis; anecdotally I’m sure you know from your own family experiences that the irresponsible siblings are more likely to breed while the responsible ones are more likely to put off becoming parents. I wouldn’t be surprised if the Spears’ case there’s relatively high time preference, and right now those with high time preference have a much higher fertility than those with low time preference, all things controlled. If Jamie Lynn’s daughter, Maddie Briann Aldridge, becomes pregnant before the birth of her second child we’ll know that this is an exceptional family….