Daniel Bolnick wrote a thread on Bluesky about the biology of sex that I thought would make a great blog post. So I saved him the trouble and compiled it.
This has been lightly edited from the original thread.
Well, no surprise that (Jerry) Coyne used his blog (whyevolutionistrue) to argue with the SSE/SSB/ASN letter about definitions of sex. I’m not going to engage deeply with the letter here. Like any committee-drafted text it reflects compromises; I agree with some phrasing but some could have been clearer.
Instead, I want to respond specifically about his comments regarding my work on stickleback. He points out, quite correctly, that I have personally used the terms male and female, and sex, extensively in my own research on stickleback. True.
But it misses the point.
Yes, stickleback can generally be categorized as male and female. And they have a XX female / XY male sex determination system (in some species, not in others). And yes, when I run stats I often check for effects of sex, and often find them. I’ve had whole papers devoted to sexual dimorphism. But Jerry might want to talk to me (or another sticklebacker) before making claims about stickleback sex differences. Because its not as clear cut as he likes.
Yes, the large majority of stickleback I’ve caught are phenotypically male or female, in the sense that they have ovaries or testes. But I have caught quite a few stickleback (hundreds at least) with neither identifiable ovaries nor testes. I have twice seen stickleback with both (yeah, we dissect a lot of fish). These are rare, but they exist. In statistical analyses, these fish get coded as NA, and excluded.
So when I publish on sexual dimorphism, or use male/female as a factor in my stats (which I do a lot), I am simplifying, setting aside a small minority to focus on overall tendencies of the majority.
That’s okay for stickleback, but its not ethical when applied to humans.
I could readily have published my papers instead with three levels of “sex”: male, female, and “unknown,” with the third group being comparatively small and under-powered for statistical purposes given my sample sizes.
Then there’s the issue of trait differences between the sexes, which are clear-cut for most individuals in some environments, but less so in other environments. Stickleback are famous for males having bright red chins and blue eyes as a sexual signal... except when they don’t. There are plenty of lakes where I work in BC, in which males are dull brownish-green, and indistinguishable visually from females. Indeed, in some lakes we have a hard time generating in vitro crosses because we’ll grab a fish we think is male, only to find after we euthanize and dissect it to obtain sperm, that it had ovaries.
For many traits that we have measured (diet, isotopes, morphology, size, color, immune traits, gene expression, metabolomics), males and females are on average different. But for most of these traits the sexes distributions overlap extensively. The magnitude of that dimorphism varies. In some lakes it can be easy to distinguish the sexes (for most individuals), other lakes it can be very hard, even in multivariate trait space. We can measure this dimorphism using things like linear discriminant function analysis (LDA). This LDA can score individuals as to how distinctively male or female they are and it varies numerically in ways we can measure precisely. Some populations are more dimorphic, others less. These differences persist in the lab, meaning there is genetic variation in between-sex differences.
To conclude: yes, I use the terms male and female in my work, reflecting real features of the fish I study. But I also set aside individuals who don’t fit neatly into the categories; again, that's a fair simplification when working with fish, but not morally defensible with people.
I also emphasize that even after 25 years studying stickleback, I can mis-identify an individual’s sex, even when the gonads are unambiguous. In these cases the gametic sex is a real distinction, but its relation to other traits is complex.
I'm not personally a deep expert on the biology of sex, that's not my specialty. But, if Coyne wants to have a conversation about sex and sexual traits in stickleback, I'd be happy to point out the subtleties. Most of all, the subtlety that seems to elude him is we can recognize male/female differences that characterize many individuals, and use this distinction, yet still acknowledge that these differences do not apply completely to fully 100% of a population.
Anisogamy (Differences in size of gametes - ZF) will surely come up in comments. I don’t have a problem with this criterion as a broad rule, but again there exist exceptions that make it less than universal. Individuals exist that produce no gametes: genetic sterility, environmentally induced sterility, parasitic castration. For instance, the parasite I study, Schistocephalus, can completely sterilize its host in some cases: we find males with no functional testes, females with non-functional ovaries. Do they cease to be males/females because they don’t have any gametes (whether large or small)?
To wrap up at last: my impression as an empiricist specializing on a particular fish species: stickleback sex is clearly delineated most of the time (hence, I use the term a lot), but not in every single instance.
The latter is where Jerry misses the point: Once we pivot into the world of humans, the biology of sex in stickleback (and how I simplify it slightly in my work) is irrelevant to the ethical standards by which we treat each other.
