Wednesday, January 23, 2019

Data: No, evolution has not come to an end because all women have two kids

I read a tweet quoting Richard Lewontin the other day (can't find the article -- I'd love a copy if someone has one) that suggested that perhaps evolution was coming to an end in modern societies because few people die until they're old, and a model was emerging where everyone has two kids, effectively creating a new generation just like the last.

I hope I read that wrong because it's simply stupid.  It's been more than 40 years since Lewontin presented the idea, so the Two-Child Model should have become even more solidified in US society. Look at this graph to see the current distribution of family size for women ages 40-55:
















Does everyone have two kids? Hardly.  The mean is 2.12, while the standard deviation (SD) is 1.49.  One way to interpret SD is to say that if we randomly grabbed two American women in the 40-55 age range, we would expect them to differ in number of children by 1 1/2 kids. (No jokes about half a kid.)

With a normal distribution (known as the "bell curve"), the standard deviation is about 1/6 of the mean, but with this family size variable, the SD is 70% of the mean.  In plain English, women are all over the map in terms of how many babies they have.

Some women (and their partners) are contributing much more genetically to the next generation, and since most traits are heritable, they are shifting the distribution of traits.  That's called evolution, and it's alive and well.

1 comment:

  1. Lewontin measured in Charles Murray's "Human Accomplishment" metric for Jews would, if negative numbers obtained in the tally, substantially lower their score.

    ReplyDelete

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