Why Sex? And why only in Pairs?

Understanding the purpose of sex is a fundamental unresolved problem in evolutionary biology. The difficulty is not that there are too few theories of sex, the difficulty is that there are too many and none stand out. To distinguish between theories, we ask: Why are there no triparental species with offspring composed of the genetic material of three individuals? A successful theory should confer an advantage to biparental sex over asexual reproduction without conferring an even greater advantage to triparental sex. Of two leading theories (red queen and mutational), we show that only one is successful in this sense.

That is a new Economic Journal paper by Motty PerryPhilip J. Reny, and Arthur J. Robson.  Of course the core question is a classic example of thinking at the margin.  The core conclusion is that mutations continue to rise with the number of sex-participating partners, but in simple Red Queen models the limiting features of the genotypes is the same whether there are two, three, or more partners.  The argument on pp.2739-2741 is not readily blog-summarizable, and I do not grasp it fully, but at the moment I have the following intuition.  If a parasite attack comes, the species needs only move away from the targeted genome to continue reproducing, due to some all-or-nothing assumptions about the nature of the attack.  This differs from the mutational game, where there is always some marginal (expected value) gain from moving yet further away from the initial nature of the species.  Playing a game against an identified opponent brings a better-specified and more stable and less varying response strategy than playing a game against an as-yet-unidentified opponent.  That isn’t how the authors put things, but…

Since we don’t observe much three-party reproduction (hardly any in fact), that suggests the Red Queen model is more likely to apply.

For the pointer I thank TEKL.


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