We’ve discussed the fact know that most biologists define “sex” in animals by the type of gamete they produce: males produce small mobile gametes (“sperm”) while females produce large, immobile gametes (“eggs”). Of course not all biologists use that definition, but it’s the most wide-ranging one, covering virtually all animals and plants save some protists, algae or fungi, as well as a very fruitful definition, which helps resolve previously mysterious aspects of evolution, like “why, in many species of animals, is the ornamented or colorful (or ‘armed’ sex) nearly always male, while the less colorful, less ornamented, or less armed sex nearly always female?This is the issue of sexual selection, first discussed by Darwin.
Definitions using chromosome type, genital configuration, and so on, don’t cover the world of most eukaryotes, nor do they give answers to evolutionary questions. And methods for “determining” sex may be the worst definer of all. As Luana Maroja and I wrote a few months ago:
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 just the past two days I’ve tried to track down the earliest formal gamete-based definition of sex, and, thanks to MIT philosophy professor Alex Byrne, I’ve got one, which happens to turn exactly 100 years old this year.
First, though, it’s likely that sexes have been either implicitly defined or recognized for a lot longer than a century. One colleague wrote me this:
I can see how this was always kind of known, since antiquity – as birds lay eggs and mammals give birth, while males seems to pass only liquids, but not produce anything. But scientists studying algae, yeast or protists try to force a more familiar “sex” into their favorite critters and thereby muddy the waters.
Two other colleagues said they couldn’t lay hands on a formal definition, but the gamete-based definition had been used implicitly for about fifty years. And it’s hard to find formal definitions of “the sexes” as opposed to “sex” itself. Consulting my go-to college text, the third edition of Evolution by Doug Futuyma (1998), sex is implicitly defined as gamete type since, as he writes, males and females appear only after the evolution of anisogamy (two different types of gametes), which, he says, “explains the origin of distinct sexes” (p. 613).
As for the number of sexes, the only people who claim that there are more than two in humans, other animals, and vascular plants are ideologues, and we need not discuss these malefactors here.
But back to the main question: when do we find an early definition of sex based on gamete size? Alex Byrne dug one up from 1924, and it comes from a University of Chicago biologist, the once-famous zoologist and geneticist Horatio Hackett Newman. As Alex says, “This is from his 1924 Outlines of General Zoology. [p. 330],” adding that “You can download the book from Google Books, or read the second edition here.”
So, here is a copy of the text. There may be earlier ones, and if you find one, email me. There may be a prize for the earliest one. I’ve put a red box around Newman’s definition:
Note that in the last sentence in the box he emphasizes the binary nature of the sexes. We’ll have a bit more to say about this tomorrow, assuming I’m able to dissect about the muttonheaded paper I’ve seen in Science this year.
This is a fascinating post that partly answers a question that I am sure many of us have had. The meaning of “species” in asexual organisms has always been puzzling. Thanks very much for the hard work involved in summarizing and analyzing this paper!
Oops, this was for the previous science post about bacterial species!
I have no citation, but I suspect it is much older than this, and probably dates at least to to the 19th century with the discovery that pollen grains produce sperm (and thus should be described as the ‘male’ gametophyte).
And in the next paragraph we see a description of secondary sexual characters – which are the principle ways to identify sex. There might not be mention of sex chromosomes, given the date.
There is this article from Mathew Cobb, which cites the discovery of egg and sperm in the 17th century: Reprod Dom Anim 47 (Suppl. 4), 2–6 (2012); doi: 10.1111/j.1439-0531.2012.02105.x ISSN 0936-6768
This is great.
Only thing that comes to mind is symmetry – a bit of brainstorming :
is sex not just binary, but symmetrical – was there a physical origin for the symmetrical relationships like, perhaps, sex – as it is situated by body symmetry, for instance… and so on …
Fun to think…
Thanks this is super helpful. Great discovery!
PCC(E), I’m wondering whether the phrase “produces gametes” in the definition of a sex requires that the organism is currently producing full-fledged, operational gametes. Do definitions of sex based on gametes including menopausal women, for instance? Castrated men? Children?
Do biologists consider that the phenotype of an individual *changes* based on whether they are currently producing gametes?
Am in conversation argument with someone who claims this definition – “we DEFINE females to be the sex that PRODUCES [present tense] the larger gametes and vice-versa for males (Parker et al. 1972)”, from the “Encyclopedia of Evolutionary Psychological Science”, https://link.springer.com/search?facet-eisbn=978-3-319-16999-6&facet-content-type=ReferenceWorkEntry&query=Reproductive%20cell%20size
Colin Wright’s definition of sex is along the lines: “one of two body plans built around the production of large versus small gametes”.
That gets round your objection. The body doesn’t have to be actually producing gametes.
Yes, I think “sex” has to be expanded to a definition of something like “on a developmental pathway that would normally result in the production of a particular gamete”. That way you can still speak of a male or female zygote.
Very good!
Correct, but the person I’m arguing with is tied to definitions he sees in journal articles and textbooks, which did not, perhaps, consider the full implications of the definition given today’s issues.
Emma Hilton’s twitter feed @FondOfBeetles is a treasure trove of nuance on this topic. Highly recommended.
The rebuttal about pre-pubescent and post-menopausal individuals, etc., are not really effective points any more than is pointing out that not everyone has 4 limbs or bilateral symmetry or is able to see in color. As a species, we are still bilaterians, quadrupeds, and evolved to have trichromatic vision.
Thank you! That gets to an issue I was trying to raise about the definition applying not to an individual, but to a phenotype. But the dividing line between variations that are considered to be a different phenotype and those that aren’t wasn’t clear (I actually didn’t research anything that gave me any answer to that issue, one way or the other).
A nice discovery raising hope that today’s regression from science, logic and common sense may be short-lived.
Another great science article. Thanks so much!
Evolution is difficult, creationism is easy. One creeps along, gathering knowledge, the other has been “known” for centuries.
Thanks for this post.
I accidentally posted this and then the power went out. Wanted to add after edit disappeared: evolution is very difficult. I mean, it takes years of serious study to really master the subject to a point you can get a masters, phd, teach it, write a book, etc. To grok creationism takes a matter of seconds. Humans like easy explanations. But for a laymen like me, the gamete description of sex is one of easiest definitions there is in biology. Biology is messy, but the sex binary based on gamete size and locomotion is pretty damn simple, not to mention really cool. I find it head-banging-wall frustrating that intelligent people twist in pretzels to somehow codify a 3rd sex. Stop, just stop! And guess what, if those 2 outta 10,000 were somehow an actual 3rd sex, the human species would be an entire different animal- in every sense of the word.
Once we have a clear definition of the sex binary and what it means, can we ask WHY?
It would help understand why natural selection settled on the binary basis of propagation and not some other means.
I’m pretty sure it wasn’t because some like to apply lipstick and another form likes to have f**k off antlers.
Spectrum gender adherents would be lost to the world without the ‘binary and it seems ludicrous to deny it. After that they can be whatever they like.
Various contributors here, particularly Mike and PCC(E) himself, have expounded on this from their own work as to why non-binary sex systems would be unstable and die out very early in phylogeny.
Sorry I wasn’t here much yesterday. This is the standard reference to the theory explaining why early sexual lineages probably evolved a stable sex binary: a tradeoff between making big gametes (that could contain provisions to fuel the early life of the zygote) and making lots of gametes (in the competition to fertilize the other mating type).
The origin and evolution of gamete dimorphism and the male-female phenomenon
G.A. Parker, R.R. Baker, V.G.F. Smith
https://doi.org/10.1016/0022-5193(72)90007-0
Such a clear definition and well-verified by date. Nice to have.
I just find it poignant for Prof. Newman to be referred to as “once-famous”. Happens to the best of us, I suppose. My paternal grandmother is remembered by no one alive now.
There is a fun little historical novel, Dreamers of the Day by Maria Doria Russell set during and after the Spanish flu pandemic where this idea becomes a plot device.
Species have never been defined by gamete type. You are confusing species with sexes, which pretty much tells us everything we need to know to ignore you.
To whom are you speaking?
Oh, never mind. I see you’re responding to an obvious mistake which has been corrected.
Run along, then. Don’t let the screen door hit ya.
Fascinating! I wonder what John Maynard Smith might have said about it in his 1978 book, The Evolution of Sex.* I no longer have it, sadly. It’s possible that he didn’t cover the gamete matter, as the main question he was asking is why sexual reproduction exists at all, given that parthenogenesis would be so much easier.
Maynard Smith, J. (1978) The Evolution of Sex. Cambridge University Press. ISBN 0-521-29302-2
Off the top of my head, a proved way to counter parasites? Red Queen book iirc.
Very interesting, thanks! I look forward to someone finding an earlier reference.
I fail to understand all of the stress and strain about defining sex. Set aside some messy issues about “mating type” since that is not the same as sex. There is well understood theory and an abundance of evidence that there are (and can be) only two sexes, females that produce large gametes in modest numbers and males that produce small gametes in large numbers. A hypothetical third sex that produced gametes of intermediate size in modest numbers could not compete: if fertilized by normal sperm, it would not provide adequate resources, and their gametes would not be numerous enough to compete with normal males.
Thanks for this
Similar, in-depth interview about sex and gender (audio and transcript) with Alex Byrne on Persuasion:
https://www.persuasion.community/p/byrne