Why doesn’t Heather have two daddies and one mommy?

What happens when economists start thinking about sex?  One paper at the recent American Economics Association conference (pdf here) which I’ll summarize looked at the question of why sex is biparental.  Or, more precisely, what does the fact that sexual reproduction involves just two parents tell us about why sex evolved?

A population that reproduces asexually can grow twice as fast as one that reproduces sexually, so why have sex in the first place?  One possible answer is that sex lets a species get rid of harmful mutations by genetic mixing.  But if mixing from two parents is good, shouldn’t three be even better?  One possibility would be to have a mother contributing half the genes, and two fathers contributing one quarter each.  This lets the three-parent population grow as fast as the two-parent one.

You might argue that it’s just too biologically hard to evolve triparental reproduction.  While I’m blissfully ignorant of the genetics, the authors note that bacterial viruses can get their DNA from more than two parents.  So how hard could it be to evolve this in a sexual context?  You might say that the coordination costs would be too high, but they point out there are a lot of species where litter mates have different fathers.

To jump to the punchline, the authors look at one of the leading explanations for the existence of sex – that it stops harmful mutations from adding up, and show that three parents would be better than two in the case of one particular model.  This leads them to favor the alternative leading class of explanations – that it allows a species to adapt to environmental changes, though they haven’t investigated these models in detail.

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2 Responses to “Why doesn’t Heather have two daddies and one mommy?”

  1. PeaceLily Says:

    Interesting. I read a book once called Woman at the Edge of Time (or something like that) by Marge Piercy. In it, people did parent in groups of threes, although this was more of a social arrangement. You should check it out. A great read.

    • erehweb Says:

      Thanks for the pointer. The book “The Player of Games” has an alien species with three sexes (as opposed to the two considered in this paper), but the trisexuality plays a very minor role there.

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