The seminar is getting away from areas I’m already sorta familiar with — public choice, voting paradoxes, development economics — and into areas where I’m more ignorant. Which is great, because the lectures become much more information-dense for me. The lecture/discussion by Leda Cosmides sparked a lot of thought still in process… but I thought the evolutionary explanation for why we have sex was so cool I have to share it.

Now, of course, I know why I have sex — it’s just kinda fun — but when you think about it, it is a little odd that animals don’t just reproduce asexually. After all, there are all sorts of costs associated with sexual reproduction… aside from the human-specific ones we talk about with our buddies over coffee, there’s the whole “throwing-away-half-your-genes-every-generation” thing. The standard answer is that the function of sexual reproduction is to introduce genetic variation… but why should that kind of variation be worth the huge cost involved?

As it turns out, it’s all about pathogens. Y’see, germs have a certain advantage over us: they get probably hundreds of generations of evolution to each one of ours. If our descendants had the same precise immunological characteristics we do, the buggers would adapt and wipe us out in a few centuries. There are, for example, certain bacteria that can emulate A+ blood type signals, and avoid triggering the immune system. For the game theorists in the room, it turns out that we’re in a strictly-competitive game with the pathogens, so we need a mixed strategy that (partially) randomizes over immunological phenotypes. The problem, of course, is that fast-reproducing virii and bacteria have the whole random-change-through-mutation thing down in a way we can’t compete with. And just having a more mutation-prone system of asexual reproduction (one with lower general copying fidelity, say) would screw us up hardcore, since something vital might mutate in a bad way. What we really need is a way of randomizing combinations that keep the basic stuff running. Hence, sex.

We worry a fair amount these days about STDs — the pathogens’ clever way of getting back at us for our tricky immunological strategy. But next time you’re enjoying a fantastic lay, maybe you should give a moment’s thanks to the diseases that made it all possible in the first place.