So, Saturday afternoon, while I really should have been working on other things, this happened:
What the heck is going on here? Well, it might be obvious if you’re connected to the biology-oriented end of Science Twitter, but there’s a lot of context, and science, behind that tweet. And it’s occurred to me that it might be useful to put it all in one place, and provide some additional information that can’t be packed into even a series of 140-character snippets.
Believe it or not, it all started last Friday, when astrophysicist and popular science demigod Neil deGrasse Tyson tweeted:
If there were ever a species for whom sex hurt, it surely went extinct long ago.— Neil deGrasse Tyson (@neiltyson) March 11, 2016
Anyone with much background in biology knows this is false on its face, since there are lots of species for whom sex is painful, or at least it looks like it must be. Tyson’s responses and @-mentions filled up with biologists and science communicators pointing out spiny penises, traumatic insemination, and other ways that animal mating systems get kinky. A spoof hashtag, #BiologistSpaceFacts, encouraged biologists to turn the tables on Tyson by tweeting (deliberately, hilariously) uninformed descriptions of astronomical phenomena. All of which is to say, it was a pretty fun Friday on Science Twitter.
Then, Saturday, Tyson teed up for another go:
I think I’ll try out one more on the biologists…— Neil deGrasse Tyson (@neiltyson) March 12, 2016
And he tweeted thusly:
If you have a gene for celibacy, you didn’t inherit it.— Neil deGrasse Tyson (@neiltyson) March 12, 2016
And there was much renewed face-palming. Some folks objected to this usage of the word “gene”, which is more usually used to refer to a single location in the genetic code—the more precise wording would be “an allele for celibacy,” or “a gene variant for celibacy.” Real hard-core pedants pointed out that it’s not strictly right to say a gene is “for” anything. These objections are really not such a big deal, though, compared to the confusion inherent in the rest of the tweet. Because there are, of course, social species with non-reproductive workers, whose non-reproductive status is ultimately determined by, yes, their genes. And gene variants that prevent reproduction can be recessive, so that they can, indeed, be inherited, and only have their deleterious effect manifest in individuals who inherit them from both parents. (In fact, basic evolutionary theory predicts that most gene variants with this kind of effect will be recessive.) It may be that there are multiple genes involved in “celibacy,” in which case an individual will only be celibate if he or she inherits the right variant for most or all of them. And if we’re talking about humans, it might be that a gene variant which causes celibacy in one cultural context doesn’t do so in others.
All of these factors come together nicely in a topic which is near and dear to my heart, and about which I’ve written a fair bit: the evolutionary context of human sexual orientation.
As the article I linked to explains, we have reasonably good indirect evidence that attraction to members of the same sex has a genetic basis—identical twins, who carry identical genetic codes, are (somewhat) more likely to have the same sexual orientation than fraternal twins, who do not. The search for specific genes responsible for being gay has been relatively unsuccessful, but that’s consistent with a complex, hard-to-measure psychological trait, which might well be created by many genes with small individual effects—or if those effects are mediated by some environmental factor. (There’s also pretty good indirect evidence that the prenatal developmental environment is involved in sexual orientation.)
Gene variants that, for instance, make men more likely to be gay, can persist in the population if they’re recessive, or if they have small individual effects, or if they don’t have similar effects when carried by women. It’s also possible that the cultural context in which they’re carried has a big impact—in some societies, men who might identify as gay in the U.S. or other “western” cultures stay at home and help their close relatives raise children, which could offset the next selective effect of their own disinterest in reproductive sex. Or, in less tolerant cultures, many men who would otherwise identify as gay may marry and raise families because of social pressure.
Good detail on key points: cultural context is VITAL for human “fitness,” and that “fitness” ≠ “children I father”. https://t.co/MOou1oIhQ1— Jeremy Yoder (@JBYoder) March 12, 2016
I also clarified that, for the purpose of my rebuttal example, “celibacy” really means abstention from reproductive sex:
(Also it occurs to me that I should clarify that I’m talking about “celibacy” in a VERY SPECIFIC sense.) https://t.co/sGMybRBe0k— Jeremy Yoder (@JBYoder) March 12, 2016
That said, Tyson’s “celibacy gene” tweet does express a very specific population genetic hypothesis.
(There is actually a FASCINATING hypothesis implied here, but I very much doubt this is what NDT was going for.) https://t.co/0WlS8s9i6w— Jeremy Yoder (@JBYoder) March 12, 2016
What’s that hypothesis? Well, a gene variant that has sufficiently strong deleterious effects, one that is not recessive, can only exist in the population as a brand new mutation. That is, if the variant would completely prevent the person carrying it from reproducing, or surviving to reproductive age, a living person could only acquire that variant through a mutation that occurred in the cells that produced their parents’ eggs and sperm, or a mutation that occurred early in embryonic development. Animal bodies are composed of populations of cells with their own pedigrees tracing all the way back to the union of an egg cell and a sperm cell, and mutations can and do occur in the many mitosis events involved in building an independent organism from a fertilized egg.
Geneticists can identify gene variants with strong deleterious effects by comparing human genome sequences to find regions where everyone has the same sequence — those are places where any mutation would be immediately lethal. A non-recessive gene variant for “celibacy,” which wouldn’t be lethal but would (one way or another) prevent reproduction, wouldn’t be completely absent from a big sample of sequenced human genomes, but it should be very rare, and mostly present in the population because it has been continually reintroduced by mutation. In the subset of cases when those mutations occurred after fertilization, it’d be fair to say that, indeed, the “gene” for celibacy wasn’t inherited.
These are things we can look for, given the right kind of data—which is getting cheaper to obtain all the time. But all of this is the kind of nuance lost in a flippant tweet about a cartoonish understanding of natural selection.
Some folks have suggested, as the dust settles on this second round of “Neil deGrasse Tyson trolls all of biology”, that the response to Tyson has been disproportionate and not very helpful. Given Tyson’s enormous audience, and the fact that he’s specifically known for his pickiness about astronomical accuracy—remember that time he complained about the constellations in the sky for the climactic scene of Titanic?—I think he, of all people, could stand to be a little more self-aware about stepping outside his expertise. The tone of most of the responses I’ve seen—certainly the tone I tried to aim for—remained on the spectrum from jovial to disappointed. (Another weird tweet he broadcast the same day, on the subject of the Republican presidential primary, and responses to it, are in another class beyond the scope of this blog post.) I don’t dislike Tyson as a result of all this—quite the contrary, I think he’s been one of our better science-popularizers, particularly in recent years. But precisely because I respect him, I expect him to do better.
The living world is not only queerer than you imagine, @neiltyson, it is queerer than you can imagine. Evidently.— J.B.S. Haldane (@JBS_Haldane) March 13, 2016
As an endnote, for more on why an astrophysicist, particularly, might be prone to disregard details outside his own field of expertise, Cheryl Rofer has posted some some enlightening and even-handed thoughts.