I spent most of my scientific career working on speciation, and my thoughts on the issue were published in a book with my student Allen Orr, Speciation (this collaboration is still the bit of science that I’m proudest of). The first chapter and the appendix of the book deal with species concepts, and we argue that the Biological Species Concept, in which species are defined as groups of organisms which, when they co-occur in nature (i.e., are “sympatric”), do not exchange genes, is the most useful one. There are various traits that impede gene flow between species—the variety of so-called reproductive isolating barriers include mate discrimination, hybrid inviability or sterility, differences in niches in the same area, differences in timing of reproduction, and so on. (We have chapters on all of these barriers in the book.) The conceptualization of species as groups of organisms separated by barriers to gene flow was called the Biological Species Concept (“BSC”) by the great evolutionist Ernst Mayr.
Now there are some gray areas about some groups, as gene exchange between what we may think are “good” species actually does occur. What do we call these forms? We talk about that in the book, too. After all, as one reader noted, speciation is an evolutionary process, and as populations diverge genetically when they’re geographically isolated (the way we think most speciation begins), there will be a longish period in which they will exchange genes if they became sympatric, and may even fuse back into one species. We are increasingly recognizing that more species than we thought exchange some genes in nature. We call these populations “species-like” as opposed to “good species”, which exchange no genes. (Again, this is in the book.) Still, there are many species that don’t exchange genes with any other species. Homo sapiens is one of these, but there are many others like koalas, American robins, lions, and so on.
And I’ll add that we freely admit that the BSC can’t be used with some groups, including asexual species, populations that never contact each other in nature (although if they produce sterile or no hybrids when forcibly mated in zoos, they are good biological species), and species that are extinct. We discuss these problems and some possible solutions in Speciation,.
But the BSC has been the species concept adopted, explicitly or implicitly by most biologists. And that’s for two reasons. First, the BSC appears to explain why nature really is “lumpy” rather than a spectrum of plants and animals with no boundaries between any groups. The lumps are kept apart by the evolved reproductive barriers.
Second, and importantly, the BSC gives us a research program to understand the origin of the “lumps” that are species. If you can understand how the reproductive barriers arise between populations, then you’ve explained the problem of why nature—at least the moiety of nature that reproduces sexually—is lumpy in a given area. That is, you’ve explained the origin of species. And that is something that Darwin didn’t do, despite the title of his great book.
The implicit recognition that the BSC explains the lumpiness of nature is buttressed by my observation that no matter what other species concept people adhere to, when they are discussing in the scientific literature the origin of species, they invariably address the origin of reproductive isolation. This is a tacit admission that they adhere to a form of the BSC. It’s the near-universality as well as the utility of the BSC that has led to its adoption.
And that also holds for the “gametic concept” of biological sex that we’ve discussed before.
There are four parallels between using the BSC and using the gametic concept of sex, or GCS (the GCS holds that sex is binary in animals and vascular plants, with males having small mobile gametes and females large, immobile ones). Here are the parallels between the two concepts:
1.) If you use the BSC, the way you recognize species may differ from the way you “define” (or “conceptualize”) them. This difference also holds for biological sex. Sex can be defined gametically, but newborns are usually recognized as males and females by their genitalia. That does not efface the GCS because the correlation between genitalia and gamete-formation system is nearly perfect. There are some exceptions, but they are very rare. Sex remains binary, though, because the exceptions to this correlation do not constitute a third sex. There remain only two types of gametes.
2.) The BSC and GCS are both the most useful concepts for understanding how evolution works. Only the BSC is capable of explaining why sexually-reproducing species comes in ‘lumps,” and only the GCS can explain sexual selection, a form of selection that is ubiquitous and almost universal in explaining the differences between males and females.
3.) Biological species can diverge through a number of processes. These include, among others, natural selection, sexual selection, meiotic drive, and differential infection with bacteria that can lead to reproductive isolation in hybrids (e.g., Wolbachia). Likewise, the two sexes can diverge through a number of processes, though the evolutionary advantage of such divergence via disruptive natural selection.
4.) Like the BSC, there are concepts or definitions of sex that don’t rest on gamete type but on other things: what genes are present, chromosome configuration, even “lived experience” or self-image. But no other concept of sex is as universal or utilitarian as the gametic concept.
This post will demonstrate the fourth point.
As I’ve said before, the sexes evolved in “isogamous” species (ones whose gametes were originally identical) almost a dozen times, yet in all cases we wind up with two sexes, not three or five. That’s because, at least according to theory, the two-sex situation is evolutionarily stable and resists going back to isogamous species or producing species with more than two sexes. Again, the recognition of the sex binary was important in helping understand the evolution of why sex is binary. It was the great evolutionist R. A. Fisher who pointed this out.
Here I’ll show that, as with species, there are many ways that sex can evolve and be developmentally determined, doubtlessly resting on the independent evolution of the sexes. Yet despite this diversity of determination, we always wind up with just two sexes that produce either large, immobile gametes or small mobile ones. This paper by Doris Bachtrog et al. in PLoS Biology demonstrates that (click below or find the pdf here):
The paper shows, as summarized in the figure below, the diverse ways sex is determined in animals. The caption for the figure is below (click figure to enlarge it) and gives examples of each kind of sex determination.
Figure 4. Schematic overview of some sex determination (SD) mechanisms. M refers to meiosis, F to fertilization. Haploid stages (n) are indicated as shaded areas and diploid stages (nn) are unshaded. Hermaphrodites: Most flowering plants (and gastropods and earthworms) simultaneously contain both male and female sexual organs (simultaneous hermaphrodites). Many fish and some gastropods and plants are sequential hermaphrodites; clownfish, for example, are born males and change into females, while many wrasses or gobies begin life as females and then change to males. Environmental Sex Determination: In turtles and some other reptiles, sex is determined by incubation temperature of the eggs (temperature-dependent sex determination). Social factors can act as primary sex-determining cues: sexually undifferentiated larvae of the marine green spoonworm that land on unoccupied sea floor develop into females (and grow up to 15 cm long), while larvae that come into contact with females develop into tiny males (1–3 mm long) that live inside the female. Genotypic Sex Determination: Almost all mammals and beetles, many flies and some fish have male heterogamety (XY sex chromosomes), while female heterogamety (ZW sex chromosomes) occurs in birds, snakes, butterflies, and some fish. In mosses or liverworts, separate sexes are only found in the haploid phase of the life cycle of an individual (UV sex chromosomes). In some flowering plants and fish, such as zebrafish, sex is determined by multiple genes (polygenic sex determination). In bees, ants, and wasps, males develop from unfertilized haploid eggs, and females from fertilized diploid eggs (haplodiploidy), while males of many scale insects inactivate or lose their paternal chromosomes (paternal genome elimination). In some species, sex is under the control of cytoplasmic elements, such as intracellular parasites (e.g., Wolbachia) in many insects or mitochondria in many flowering plants (cytoplasmic sex determination). In some flies and crustaceans, all offspring of a particular individual female are either exclusively male or exclusively female (monogeny). doi:10.1371/journal.pbio.1001899.g004
I’ll repeat here some of the ways that sex is determined.
Single genes
Multiple genes working together
Temporal effects (either sequential or environmentally based sex change, with clownfish an example of the latter). Note that there are still only two sexes, but they appear at different times of an individual’s life
Temperature dependence, as in many reptiles. Whether you turn out male or female depends on the temperature at which the fetus is incubated
Environmental effects (as in clownfish when the alpha female dies)
Whether or not you have a half set or a full set of chromosomes (e.g., bees)
Whether you have like or unlike sex chromosomes
There are others, too, as you see. The point is the despite these many ways that sex is determined, all of them result in just two sexes—males and females. So it’s a canard to point out this diversity as a way to dismiss the sex binary. Yet people still use this canard to argue that sex is a spectrum, just as they use the canard that the recognition of sex can be based on different traits as a way to dismiss the sex binary. As Luana and I wrote in our paper:
We can see the stability of the two-sex condition by realizing that what triggers the development of males versus females varies widely across species. Different sexes can be based on different chromosomes and their genes (e.g., XX vs. XY in humans, ZW vs. ZZ in birds, individuals with like chromosomes being female in mammals and male in birds); different rearing temperatures (crocodiles and turtles); whether you have a full or half set of chromosomes (bees); whether you encounter a female (marine worms); and a host of other social, genetic, and environmental factors. Natural selection has independently produced diverse pathways to generate the sexes, but at the end there are just two destinations: males and females. And so we have an evolved and objectively recognized dichotomy—not an arbitrary spectrum of sexes.
In the end, the whole kerfuffle about whether sex is binary or a spectrum results from only one thing in one species: some members of H. sapiens do not accept their biological sex because they feel as if they are members of their non-natal sex—or no sex at all. We would not be having this argument if we were foxes or robins, And nobody (except the three evolution societies, who have now retracted their claim) argues that sex is a spectrum in all species.
The argument against the binary rests on good motives—the desire to not oppress those who don’t conform to their natal sex. But the argument is still misguided. It assumes that we can impose our ideas of social justice onto nature. But nature is recalcitrant, and throws up just two sexes in all animals and vascular plants.
It is the failure of those who argue for a sex spectrum to admit their motivations that bothers me the most. Instead, they pretend that they are suddenly recognizing a truth about our species (or about nature in general) that has not been realized for over a century. The lesson: never impose your ideology onto nature, a tactic that I call the “reverse naturalistic fallacy.”
And I’ll say for the umpteenth time that the fact that sex is binary in humans should not affect the way we treat those who don’t accept or conform to their natal sex. The binary is an observation, but the rights and treatment of “nonbinary” folks is an ethical issue.
Exactly. All those different modes of sex determination lead to the same result: two types of gametes, reinforcing the validity of the sex binary.
I never exactly understood why the difference between “small, mobile” and “large, immobile” gametes was the key factor. To a layman such as myself, I would think the act of developing the baby in the womb was the key factor.
Think of all the organisms that lay eggs or otherwise produce babies without a womb. Birds, for example, are a prime example of sexual dimorphism and oft-used examples of sexual selection, but they produce eggs, and in many species both parents take care of the chicks and/or sit on the eggs.
It’s the key factor because it’s shared across all animal lineages. Humans have two sexes in the same way all other animals have two sexes. Only some animals have internal gestation and development of embryos; lots of others have embryos that don’t develop inside the body of a parent. But both groups have the same two gamete types. Because of this continuity across species, it’s possible to think about the evolution of sex differences within species (like the evolution of larger body size in males than in females) or the evolution of differences in the traits of one sex between species (like the larger eggs produced by female birds than by female mammals) using the same models and processes. One doesn’t need to resort to some special explanation for human sex differences, or differences between female humans compared to other female animals.
[woops I started to reply then posted a few minutes after our host’s more succinct reply 🙁 ]
This difference between males and females in humans and other mammalian species is seemingly very obvious, and very significant. Remarkable, therefore, that it is so little noted in the word salads of “gender theory”. But looking away from the obvious is the hallmark of every variety of wokeliness. All of the griping to “decolonialize white empiricism” looked away from the fact that everything that wokies do—from their phones to their visits to the dentist to their lifespans —works on the basis of “Western empiricism”.
And don’t forget the seahorses! In these fish, the babies develop in the males. Wikipedia says:
“The male seahorse is equipped with a brood pouch on the ventral, or front-facing, side of the tail. When mating, the female seahorse deposits up to 1,500 eggs in the male’s pouch. The male carries the eggs for 9 to 45 days until the seahorses emerge fully developed, but very small. The young are then released into the water, and the male often mates again within hours or days during the breeding season.”
How do we know which seahorse is the male? He’s the one whose gametes are small and mobile.
Thanks for this. I didn’t need convincing that sex is binary but further clarity on the issue is always welcomed.
PCC(E) : “.. they pretend that they are suddenly recognizing a truth about our species (or about nature in general) that has not been realized for over a century.”
🎯🎯🎯
As if a framework, hidden from ordinary perception, is poised to fundamentally transform all life as we know it, out of its carceral mire to its next stage…
Thanks for this, Jerry. I need to go back and refresh myself on a few of the biological terms.
Frankly, though, I think the “sex is a spectrum” argument is just sophistry. Why is it that sex is a spectrum, but something the Progressives don’t like, racism, isn’t? For their political purposes everything from asking a black person about their hairstyle to advocating or practicing genocide is all the same.
Whiteness is property, and a source of power, hoarded by the occupants of the empty shell at the center.
Likewise, “sexuality is linked to power” (UNESCO document on Comprehensive Sexual Education (CSE)).
Who has power, who doesn’t, power is hidden in properties like sex, race… transformation of the empty shell at the center with the scattered shards at the margins requires power.
… hastily put…
I don’t know how you do it.
So here on this side of things we have torrents and torrents of data that do show a great diversity of ways that biological sex is determined, and similarly diverse ways in which it is identified. These evolved independently along different lines, and yet all share one common denominator across the eukaryotes, that being that isogametic sex cells (egg and sperm) reliably evolved once multicellularity is explored in the different eukaryote groups.
It seems simplest to say that the isogametic state is a relatively primitive state, and that the differences in sex determination and identification are derived states, having been installed later in different divergent lines. Hence, the universal factor for biological sex is gamet-o-centric.
On the other side of this Great Debate, you have inaccuracies abounding, conflations around every corner (clown fishes switch sexes, so human sexes are a spectrum), and emotion emotion emotion.
Even if one were to use more conciliatory language that the varieties of sex determination and sex identification across the eukaryote kingdom means that sex is a spectrum across life on earth, that does not grant an opening to say that sex is a spectrum in any one species. In a given species, sex is determined and identified in a particular way and these result in two biological sexes.
Notice that the famous example of the clownfish requires the sex binary anyways. If we say that a male clownfish under certain circumstances changes to female, we must mean SOMETHING clear when we say that the fish was male prior to the transformation and female afterwards.
Reflects trends in academia developing over the past generation: innumeracy, cherry picking and the triumph of qualitative over quantitative. A few may challenge idea of sex binary. Some slave traders made a lot of money. Some profits from slavery helped to finance the Industrial Revolution. Some students might perform better if taught by somebody who “looks like them.” Some pirates freed slaves to be egalitarian goodie goodies. A few, very tiny few, women became pirates, challenging various social this or that, etc.
I wasn’t smart enough or motivated enough to finish an engineering degree but I am smart enough to spend a lifetime in academia churning out postmodernist claptrap. In a world of no universal truths anyone can build a career on assertions alone.
Have a gander at this Xitter thread where the inimitable JK Rowling repeatedly asks the author of a book called Is gender fluid? what “gender identity” is.
https://x.com/jk_rowling/status/1864696142195511654
The author’s Wikipedia page describes her as having a “scientific career” and is Professor of Sociology at the University of Sheffield. She completed her PhD in 2004 so I’m guessing her academic career spans nearly 30 years.
In another Xeet, Rowling lists a few inconsistencies from Hines’ book and then lets us know:
So there we have it. In the 21st century there’s a luxury living to be made out of brainfarts. Bookstores are a great reminder of this. The science section is only a few meters wide while the self-help/spirituality/wellness section is a few walls wide.
I am not on X. But I had recently doom-scrolled past a post on FB where someone suggested that the Harry Potter books are now author-less, given the wide agreement to not even say her name. Comments skewed strongly toward how all these young adults are furious with her for being trans-phobic (she isn’t), mainly bc she was [sob] so important to her when they were growing up. Therefore she should somehow have the same opinions that they do. Imagine that. An older white woman of mid-liberal background (I assume. She might lean conservative for all I know) who is supposed to suddenly not have the opinions of the majority of people, let alone the majority of people of her demographic, just bc a lot of young people grew up with her books. So those majority opinions that trans-identified people are to still have some restrictions are THE FAULT OF ONE PERSON. She is …. THE ONE.
What I found conspicuously absent from my school curricula (1980s) was epistemic literacy. We were taught what to think, not how to think. We were handed conclusions but never shown how to light the path that led to them. Only in my late twenties did I learn that logical fallacies were a thing and wondered why we weren’t gifted this magical toolkit as kids.
Instead we studied Greek Mythology (at a Greek school in South Africa) but they somehow forgot to mention how Epicurus made a mockery of their omnipotent sky guy. The reason for this is obvious.
Fast forward a few decades and things seem to have gotten worse. I’d hoped the internet would serve humanity well by bringing better ideas to more people but how naïeve I was… It just breeds more sacred cows.
“Mass movements can rise and spread without belief in a God, but never without belief in a devil”
(Eric Hoffer, The True Believer)
I want to counter this general idea of sociology as nonsense, as from what I’ve seen, social science education in the UK isn’t the ideological disaster some people make it out to be.
Both my daughters are doing politics and sociology degrees at top-tier UK universities (one’s going into her third year, the other into her second). I’ll be honest, based on everything I’d read online, I was bracing myself for their courses to be full of uncritical ideology and identity-first dogma, but I’ve been very pleasantly surprised.
I take a genuine interest in what they’re studying. I help them pick modules, and often read through lecture slides and course materials just out of curiosity. And so far, I’ve seen almost nothing I’d call “woke” or even ideological. In fact, the material has been solid, evidence-driven, and intellectually challenging. If I hadn’t seen it with my own eyes, I probably wouldn’t believe it, given the reputation these fields often have online.
A good example: last semester, my younger daughter took a psychology module aimed at social science students. I was sure this would be the moment the cracks showed, with blank-slate thinking and social construct explanations for everything. But no. A few weeks in, she was explaining the biological basis of status-seeking and the genetic components of Big Five personality traits. Before the final exam, we were chatting about polygenic traits and specific alleles. I was genuinely impressed.
Yes, this is just my experience, and only two courses at two universities, but from what I’ve seen, the standard is high, the content is rigorous. The assessments demand proper scholarship and critical thinking. It’s nothing like the caricature we so often hear. Interestingly, while in the US it’s often the elite universities that seem to have the worst reputations for ideological excess, in the UK the opposite might be true—the more prestigious institutions seem, if anything, more intellectually grounded.
Sure, there may be issues elsewhere (and from what I understand, some US institutions really are as bad as people say), but the blanket dismissal of social sciences as unserious or indoctrinating doesn’t line up with what I’ve seen up close.
The concept of truth in different disciplines has me thinking. In the hard sciences, it seems to be based on evidence that can be consistently replicated and that is accepted by credentialed peers. In the humanities, truth seems mostly subjective, based primarily on either the experience an observer has or on a belief about the thoughts or emotions the authors or artists had while creating the works in question. In the social sciences, truth seems also to be mostly subjective. Observers or researchers observe and interpret behavior or the reports of prior behavior. The interpretation is usually subjective and may not be replicable. If I’m within shouting distance of accurate, the hard and social sciences are different creatures entirely and the efforts to use the biological science approach to support trans philosophy seems to be doomed. Its adherents should stick with disciplines that cater to philosophical and emotional truths and celebrate their successes in those domains.
I don’t think activists can give up trying to use science to legitimize trans philosophy without giving up the idea of trans rights. “Let’s make allowances” is different than “they belong.”
The hard sciences are, well, hard. Very little is a matter of personal wants or beliefs; there are many definitely-wrong answers; you need actual facts when making an argument. “Math class is hard” (Mattel’s Barbie, obsoleted version).
I think the social sciences COULD be part of the hard sciences if they did more empirical based research. And in fairness, some members of those groups actually do (e.g., there are anthropologists who use genetics to explore the origin and dispersal of humans across the planet). Many of their questions seem suited to the scientific method. However, there just doesn’t seem to be the emphasis on answering questions and finding truth, but instead a focus on social justice.
The classic issue was presented by Max Weber as the distinction between “explanation” and “understanding” in which the humanities and much of the social sciences are hoping to achieve better understanding of cultural and social phenomena…..
I enjoyed reading this, though it was demanding for this non-scientist. I appreciate that Professor Coyne continues to point out the fact that sex is binary does not conflict with every person’s ethical right to live in a non-conformist way. Unfortunately, for some percentage of “sex is a continuum” believers, this view is hollow. Everyone else must believe that a natal man is literally a woman because the person believes it about themselves. If we state that sex is binary, then they believe we are “denying their existence”.
I don’t think I’ve brought this up before, but it seems appropriate here.
Where the friction often exists is whether a trans woman is a woman. Or a trans man is a man. Acquiescing to that apparently means we get these significant problems where biological males are allowed into women sports, or bio-males, intact and with a history of rape, are housed in woman’s prisons. The former problem is well known. The latter problem has happened multiple times but gets less press for some reason.
But would it be possible to split some hairs? Could there be a kind of compromise where we agree that no, trans women are not literally women, so no to women’s sports and prisons. But could we still call them women? Informally? So one could say “a trans woman is a woman”, but not really mean that in the literal sense. “a trans woman is a *woman” – with an asterisk.
I believe in civil rights for trans people. But your hair splitting- which sounds reasonable to me- is denying their existence and we are TERFs to a very powerful minority. That is why we keep having this conversation over and over, and why biologists need to keep explaining the facts of nature over and over!
No. It is the slippery slope which got us into this mess in the first place.
Nothing stops you from calling a man a woman if he asks you to, Mark, and you can use the pronouns he requests. You can even sincerely believe it yourself if you like. You might well do this for a loved one with whom you were hoping to keep channels open. The problem comes when he compels me, or you (or enlists the state to compel us) to do this, on pain of some kind of punishment. Professional licensing regulators enabled by the state will discipline their members if a trans person complains that he was “misgendered”, such as being instructed not to use the women’s washroom that the female staff in the clinic use. (This is the heart of the NHS Fife kerfuffle in the UK and there has been at least one case here in Ontario.) To avoid state-enabled Human Rights complaints or lawsuits brought by the trans employee, an employer must discipline his employees if they decline or forget to use his “preferred” (read “demanded”) pronoun. This means the employer must privilege one trouble-making kook over an entire complement of steady, reliable, collegial workers, some of whom will quit rather than take this crap. And you can go to jail in Canada if you “promote” conversion therapy which could mean you support allowing “trans” kids to just grow up gay or autistic instead of “affirming” them.
All that ought to be unacceptable in a free society. No asterisks if there is state compulsion attached to them. And civil-rights-creep being what it is, a voluntary kindness asterisk today means a mandatory asterisk tomorrow. After all, that’s exactly what happened.
👍
Hitchens said it this way: “One must have the nerve to assert that, while people are entitled to their illusions, they are not entitled to a limitless enjoyment of them and they are not entitled to impose them upon others.“
But why compromise on something so fundamental? To be kind? The kindness only extends to the trans women then, as women’s category is expanded and looses its natural and social meaning. This kind of compromise is what has led to men in women’s spaces – if someone is called something he is not, it seems inevitable that that someone will eventually make demands in accordance with his epithet.
Extending the use of a category denotation to individuals outside that category because they feel/wish/demand such use is an infringement on those who are actually part of that group. If this were any other group that was asked to extend such “kindness” to what are in essence imposters to that group, it seems unlikely society would agree. But this discussion ultimately always ends up being about women and girls as a group making accommodations for men. Imagine any other group being asked to make the same allowances? Would you ask for a compromise then too?
Re meiotic drive, I’m impressed that genes are not only selfish¹, but can actually cheat.
. . . .
¹ In Dawkins’ sense, not the anthropocentric one.
Even the motivation of the activists is muddle headed.
How does it help to deny the existence of the sex binary if the intention is to help individuals who don’t conform to the social norms that their society associates with each sex ?
Surely the solution would be to address the social norms rather than deny biology ?
Thank you! This was so clearly explained!
http://georges.biomatix.org/storage/app/uploads/public/60c/e7c/320/60ce7c3200e13162292219.pdf
There is a large literature on the evolution of alternative forms of sex determination. See the attached for one fine overview of the question….’why environmental sex determination?’. By evolution of I mean both historical description and the force of Natural Selection on sex determining mechanisms, as well as the physiology, development and genetics…after all the ‘wet machinery is always part of the story.
Wow, you are a good teacher