Why we worship thee, O great Sex!
A response to “Reframing the evolutionary benefit of sex”, which explains the evolutionary role of sex as something something E[dx * (1 + dX)] = E[dX] + E[dX²] something something variance! Which I’m sure would have been very interesting and enlightening if I had been paying closer attention. I’ve been thinking of sex quite differently: not in terms of an abstract concept like “variance”, but in terms of concrete ideas like “improvements” and “suckiness”.
I used to be a Christian, and as a software developer I had a novel intuition against evolution that I never heard anyone else express. I observed that the DNA code was like a computer program, and that if you start randomly changing bits in a (human-built) program, the chance of “improving” the program any way is infinitesimal, while the chance of breaking it, or making it function less well, is probably thousands, tens of thousands, or hundreds of thousands of times higher. Despite a natural selection step, it didn’t seem plausible to overcome such an immense force of entropy.
(I also figured that “intelligent design” was a perfectly reasonable concept, because if natural evolution of intelligent beings was possible, surely those intelligent beings could design more intelligent beings, and thus the concept of evolution implied the possibility of intelligent design, and thus I thought myself clever. I digress, but I like digressing. Against this, the standard argument against intelligent design was unpersuasive. This argument said that if it could be shown that it was possible for a thing to have evolved, this was proof that it had indeed evolved and intelligent design didn’t happen. It wasn’t until much later that I heard a better argument from Yudkowsky, namely that “a human would be out of place in a community of actually intelligently designed life forms” — the whole argument is less than a paragraph, as Yudkowsky seems to feel that this is so obvious that there is no need to write a whole essay on it. My church has an counterargument, mind you, that we’re poorly designed so that life will suck more, because cancer, arthritis, diseases and cognitive biases are all part of God’s mysterious plan, which He will explain to us after we die and you just need to be patient. Then I found out that my church was false. So now I say, damn, God is an incredibly bizarre guy, and isn’t it weird how the “7 days” in Genesis was code for “4 billion years”, just the sort of time frame that makes evolution plausible.)
Anyway, So! Sex? What is it good for? You see, it takes good advantage of the ultimate motor of evolution: death.
If you have 20 mutations in your genes, maybe 9 are harmful, 1 is beneficial, and 10 have a negligible effect we can ignore. I’m pulling these numbers out of my ass, but bear with me: the argument should work across a range of different values.
In asexual reproduction, you’re stuck with those 9 harmful mutations, and if you’re allowed to reproduce freely, you tend to suffer from accumulation: 18 bad + 2 good, then 27 bad + 3 good, and so on until your genome sucks enough ass to kill you outright or at least let the other local microbes eat you, or at least beat you to the food source so that you starve. In bacteria-land, this still somehow works out, apparently because there are billions of bacteria (and because Muller broke his ratchet, I guess). If the ratio of good:bad mutations is 1:9, then 1 bacteria in every billion is extremely lucky and receives 9 good mutations and 0 bad mutations over a certain period of time, allowing it to compete really well for food, so it makes zillions of copies of itself and spreads throughout the local microbiome. (Most of these copies will get harmful mutations, and so the cycle begins again; even so, the surviving bacteria can be more fit than before.)
Sexual reproduction offers something far less dystopic. If a bacterium gets 9 harmful mutations, it is shit out of luck unless the bad mutations spontaneously un-mutate, and the chance of that is negligible. If a human gets 9 harmful mutations, it’s a different story. Each offspring — which, by the way, we get by having sex — isn’t life grand? — each offspring has a 50% chance of not having one of the bad mutations, so if you have 8 kids, you can expect each one to have 4.5 bad mutations from you, plus, who knows, maybe 4.5 from your partner for a total of 9. But wait, you can have multiple kids! So you have 8 children, because that was once very popular, and probably one of those children has only 6 bad mutations instead of 9.
Also, what about the 1 good mutation? About 2 of your kids were lucky enough to have both good mutations, and 25% of the time, the kid with only 6 mutations also gets the 2 good mutations! So each generation of humans in a given community is likely to produce a few exceptional kids with unusually few bad mutations plus a few good mutations, and if the other humans like what they see (in a sexual way, wink wink), those exceptional kids will spread their genes around copulously.
So, assuming the mutation rate is slow enough, and assuming that the ratio of bad to good mutations is not extremely high, sexual reproduction should eventually allow bad mutations to be eliminated, by killing off the people who hold them and destroying their souls — you see why people stay religious? Evolution is fukking horrible!
Meanwhile good mutations not only keep the species alive, but improve it. This allows sexually-reproducing species to improve in fewer generations, with much fewer malfunctioning incarnations, than bacteria suffer. Which allows them to survive in giant colonies of cells called “animals” and reproduce at a ridiculously slow rate (“once every 20 years? that’s absurd. liar.” is what the bacteria would say to the first bacterium scientist to come along and claim such a ridiculous interval between generations). Animals can be slow because they only have to compete with dumbass bacteria who still haven’t quite worked out how to efficiently eliminate bad mutations after billions of years, and with other large animals who also reproduce at slow rates.
(okay, actually they also have to compete with small and fast-reproducing eukyrotes so I’m a bit confused why those little buggers haven’t evolved to kill us all like the scarabs in The Mummy…)
Now, what about my original argument, that the ratio of bad-to-good mutations should be much, much greater than 9:1? Meh. Exercise for the reader. I can’t write all day, you know.
