Sunday, May 3, 2020

The power of Darwin compels you to doubt him about sex differences

Evolution is true but our evolutionary explanations are always evolving and expanding in their complexity, or at least they should be encouraged to, when evidence supports it.

I've found, however, that compelling stories about human evolution, especially those penned by Darwin, are too often preferred by scientists over others or over complexity.  I think scientists are especially protective of evolutionary stories that preserve certain macho conceptions of masculinity and that are porn-adjacent. So it's no surprise that these stories dominate pop culture's understanding of human evolution.

Everyone knows all about sexual selection.  (Now before I go through it, briefly, you need to promise you'll read the rest of this blog post. I know from actual experience with readers and listeners in the classroom, that lots of folks will mistake this upfront explanation of the idea that I'm about to critique for my endorsement of this stuff. They'll read this part then tune out by the time I get around to critiquing it. Please don't fall for that.)

Okay, right. Everyone knows all about sexual selection. Males compete with other males for sex with females. Winners get sex. This ratchets up, in their descendants, whatever physical attributes got them to win that sex that sent those attributes into the future. And, as arbiters of what's sexy and what's worthy of sex, females decide which male traits get shot into the future, ratcheting up over time what they thought was so sexy in those winners that they had sex with. Didn't you know? Where have you been? Evolution is a game in which the winners get sex and the losers get nothing. It's like Survivor and The Bachelor had a baby that got in a time machine, went back to England in the 1860s, and buddied up with Darwin.

Sexual selection is the dominant evolutionary explanation for why men are taller than women. Men are tall because their tall winning ancestors won the sex. Over time this pulled men's height above the average height of women. That men are taller on average than women is taken to be legitimate evidence for this explanation.

And we're all supposed to be totally cool with that automatic explanatory power of sexual selection. After all, it's Darwin's "second great idea" and he was so forward thinking by giving agency to females!

So many of us who don't research  sexual selection, directly, but who teach human evolution courses feel like we really have no choice but to carry on like this. It's canon. And we're supposed to be grateful that females got the power of choice! See? It's not *all* about males bashing each other apart for opportunities to have sex. It's okay! And sometimes it's about males being beautiful and doing beautiful things to attract females! So get on board, people... human evolution is evolution, too! We're not special. We're just like birds and fruitflies. Humans need to embrace sexual selection as a "force" in our understanding. Get with the program.

I sure did. Sexual selection for big competitive males is what I've been teaching countless students for years and years. But, over those years it became an increasingly bizarre thing.  I stopped growing when I got my period and that's when all the boys, who didn't get periods, kept growing. My period seemed to be the evolutionary reason I'm shorter than the average man. When you think about menstruation, ovaries, puberty, never growing tall enough to touch the rim, not that you have a chip on your shoulder or anything about that, then Darwin's ideas about male competition seem, suddenly, ridiculous.

This is a great time to quote Sarah Hrdy:

“Compared with Darwin’s exquisitely detailed observations of barnacles, coral reefs and orchids—even the emotional development of  his own children—this consummate naturalist’s observations of women and other female primates were curiously cursory.”

And then in late 2016, after years of my teaching and being and increasingly doubting, Jerry Coyne got mad at PZ Myers for his understanding of evolutionary complexity and bias in evolutionary stories, specifically about sexual selection explaining  human height differences. So, I piped up and it's all documented here.

Then, I got to work, because the Coyne thing suddenly made it clear to me how powerful and influential this story is. It was suddenly very important that I do something about it.

The first thing I did was rant about it in the Washington Post, who gave it a clickbait title, which probably drove away the people I was primarily speaking to.

And then I did a lot of reading about how skeletons grow. I sent a paper about it to Evolutionary Anthropology in the summer of 2018. Four reviewers saw the first version. Two of the four disliked it greatly. The other two disliked it less, and so it miraculously got "major revisions" rather than a rejection. Some of the major blowback was against my focus on "proximate" processes (like skeletal growth and hormone effects) which are not "evolutionary" to the minds of some reviewers and many people out there. One reviewer actually wrote that "the estrogen explanation is not evolutionary" to which I can only cry out to the universe: then what is it, magical?

The two mad reviewers dropped off in the second round and three new ones were added, totalling the anonymous reviewers to seven. By this time I had found a paper by some heavyweights and in a heavyweight journal to back up my approach that eschews the "proximate" versus "evolutionary" convention, passed down from Ernst Mayr.

link to paper

Without Laland et al. to cite in revisions, I think my paper would have been rejected. That's how entrenched Mayr's convention is, to my mind. I wouldn't have been permitted to work outside of it, not yet at least. I'm grateful to the reviewers and the editor-in-chief who pushed me to improve the paper, like that and in countless other ways. It was published yesterday.

link to paper; Open access (free) link to paper

By this point, if you haven't heard me talk about this (which has been great fun where they've invited me into their lovely groups to do so), then you're wondering what the paper says. It says a lot about estrogen, ovaries, and periods. (It also says a lot about pelvic differences... so it says a lot about vaginas, uteruses, and clitorides too, which is a whole other ball of wax.) But I'm too pooped from all the activity around the Twitter thread I posted yesterday to write too much more here on the Mermaid's Tale today. So, for now, I'll link to Twitter and to where someone unrolled that thread.

If you hit a paywall at the journal where the paper lives, just download the pre-print for free here:

I think my beloved former Professor Jeffrey Kurland from those glory days at Penn State would have hated this paper at first! But, if he weren't up in heaven now, we'd be yelling and yelling together, and it would be so much fun yelling together, and he'd at least entertain these ideas, and he'd think up brilliant ways to take the work further. And I so wish he were here with us now, but he will always be.


finaslegendarystories said...

ty Holly for the splendid telling of ur idea why im taller than my sisters...ur style of writing is straight forward and convincing...keep fighting the Good Fight! Be well HD.

Ross Vodt said...

Ernst Mayr was wrong about many things but because he long outlived the other contributors to the Modern Synthesis his influence was outsized. Both proximate (including mechanism) and distal causes are, of course, evolutionary. Without mechanism biology would be wholly theoretical and evolutionary models could be as adaptationist as desired, unencumbered by actual physical entities and processes. Like designing airplanes without considering gravity or friction.

Prum argues that aesthetic-based as opposed to fitness-related mate choice should be considered the null hypothesis. But that's wrong; the null is not one kind of sexual selection as opposed to another, but rather that the sexually dimorphic trait was not selected at all.

This leads to what I think of as an explanatory hierarchy (not particularly original): which explanation for any trait, including sexually dimorphic traits, has to be exhausted before others can be considered. Phylogenetic inertia > developmental constraint > drift > plasticity > natural selection > sexual selection.

So I agree, analysis of sexually dimorphic traits cannot begin and end with sexual selection. Even with socioecological correlations, since it's likely not solely a matter of socioecology driving dimorphism but also the other way around. And in the case of humans, cultural and technological evolution makes our socioecology open-ended regardless of its relationship with dimorphism in other hominoids*.

*Aside: Including hylobatids, the underrated apes.