If Sex is so Great, Why Don't We Have More Sexes?

I’m reading Matt Ridley’s “The Red Queen” about the evolution of sex and its utility in reshuffling genes so as to thwart parasites. The “Red Queen” is an allusion to a character in a Lewis Carroll story. The Red Queen was always moving just to stay in the same place. Sexual species are always reshuffling just to stay abreast of corresponding evolutionary changes in parasites.

I was astonished to learn that some fungi have hundreds of sexes, and I am looking forward to finding out why we humans have only the two and are not all hermaphrodites. I can see why asexual reproduction would not be advantageous in an environment of predation and parasitism, but I don’t yet know why sexes don’t proliferate so that we can reproduce with more people. As it stands now, I can only reproduce with the just over half the population that is female. If there were three sexes, that would mean that two thirds of the population could reproduce with me, and I would be fitter, or so I think now not having read far enough in the book to find out why I’m wrong.

A three sex society could be interesting. Suppose that you had sexes A, B and C where A could impregnate B, B could impregnate C, and C could impregnate A. What would religious fundamentalists reckon would be the divinely sanctioned household organization? If everyone is potentially both a pitcher and a catcher, what would that do to notions of power in sexuality?

Everyone would have a slot and a tab, of course. If a B inserted his/her slot in an A’s tab, which would not contribute to reproduction, would that be wrong or deviant? Would folks go about complaining that it was “unnatural”? Presumably, the three sex species would be driven both to pitch and catch, so it is likely that threesomes would be common and that reverse slot to tab sex would be a substitute for a proper third party. There would still be same sex partners and threesomes, I reckon. Auto-erotica would doubtless still be practiced.

Porn would be way different. I imagine that orgies with chains of slots in tabs would be a common theme, and folks would fantasize about “making the beast with twelve backs”, “doing the horizontal conga”, or “making a benzene ring”. Strap-ons would not be in much demand.

Families would be different. An adult in a family might be mother to one and father to another child and genetically unrelated to a third child. Children could be full siblings or half siblings. Come to think of it, that’s not so different from the families that emerge from serial monogamy. I have a full sibling, a half sibling and several unrelated step-siblings, and I am a member of a two sex species. Extended kin would get weird and complicated.

In developed societies, ideal family size would be one child of each sex.

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