A good article on the meaning of biological sex

September 23, 2020 • 1:30 pm

There’s lot of confusion among laypeople, and even among scentists, about what “sex” is, in the sense of “what do we mean by a biological sex?” This goes along with questions like “Is sex binary?”, “How many sexes are there?” and so on.

For a scientific but accessible discussion of how biologists construe sex, the article below from Aeon (click on screenshot) is quite good.

There’s not much to quibble with in the piece, as it’s a straightforward discussion about how biologists regard sex, so I won’t do anything except list some of the questions it takes up:

  • What do biologists mean by “the sexes” of an animal or plant?
  • Why do we have sexes in the first place? Why doesn’t everything just reproduce asexually by budding or parthenogenesis (production of an offspring from an unfertilized egg)?
  • Why are there only two sexes in the vast majority of animal species? Why can’t there be three or more sexes?
  • Are hermaphrodites or developmental anomalies members of other sexes?
  • Why do biologists define the sexes by gamete size rather than by chromosomal constitution or characters like sexual organs?
  • Why doesn’t the existence of individuals with combinations of male and female traits prove that sex is a continuum?
  • Why we can’t necessarily extend biologists’ views of sex to questions like “who participates in women’s sports?” or “who goes to a women’s prison?”

I have a few minor quibbles with the piece, but they’re so trivial that they’re not worth mentioning. What especially interested me was the evolutionary question. Biologists have long wondered “Why did sexual reproduction evolve in the first place?”, and, truth be told, we don’t have an answer everyone agrees on. There is more agreement on why there are just two sexes in the vast majority of animals, although some organisms like protists have dozens of “mating types” that might be seen as sexes (Griffiths doesn’t).

Have a read of the piece if you want to be informed about biological sex before you wade into the gender wars.

42 thoughts on “A good article on the meaning of biological sex

  1. An interesting article despite the Templeton connections. I hadn’t thought of anisogamy in terms of the problems of combining motility and nourishment in a single gamete. Surprising that it leads to such complete specialization.

    And though I found the malleability of sexual strategies in nature fascinating, I doubt that much of it casts light on understanding human sexual conventions. The male pipefish may incubate eggs, but never in the history of our species as a human male incubated a fetus. (I think, anyway. Maybe I should not be so sure.) So the relevance of the pipefish to humans is none.

    In any case, thanks for posting.

      1. From what I’ve seen on CNN the police in Louisville is behaving in a quite offensive manner and clearly trying to provoke the protesters by arresting women and forcing them to sit on the street surface with their hands tied on their backs. These policemen are behaving like Nazis.

  2. Often when the question is asked why we have two sexes, I think about “The Mote in God’s Eye” by Larry Niven and Jerry Pournelle. The book describes the Moties, an alien species with a complex arrangement with way more than just two sexes. Perhaps they are subspecies but I imagine this is not an important distinction when talking about aliens. Warning: The Wikipedia page contains spoilers.


  3. I think there is relevance of pipefish (and the many other bizarre examples mentioned) to humans, though not in the way you might think. The point I took from it is that even in animals that have nothing resembling XX and XY chromosomes, in animals that switch between sexes over their lifetime, in animals where temperature of the nest determines sex, and animals where the male incubates eggs, they all have one thing in common: without exception, gamete size determines biological sex, and there are never more than 2 sexes.

  4. The mystery to me about sex is that a species has half its population contributing nothing but sperm. Sorry, tiny gametes. Uteruses are the thing in short supply reproduction-wise, but half the resources available to a sexual species are wasted on uterus-less males. Yeah, in some species, like our own, males contribute something to raising the offspring, but that is more the exception than the rule. You could argue that useless males are the price of the advantages of recombination. But is it? Why are not hermaphrodite species more common? You can have all members of the species with uteruses or egg-bearing ability and still have the advantages of sex. The danger of self-fertilization perhaps?

    1. One explanation I read suggests that sexual reproduction improves the average fitness of one’s grandchildren, because it greatly increases variance. The variance of your children’s fitness will be large with sexual reproduction, and the fittest children will produce the most grandchildren.

      Another (compatible) explanation might involve species extinction. For comparison, see Emma Goldberg et al, Species Selection Maintains Self-Incompatibility and discussion by Phil Goetz. Sexually reproducing species might be harder to kill off.

      1. Ronald Fisher posited that the 50/50 sex ratio in most animals maintains itself because departures from that ratio are only stable for a short time before natural selection pushes it back. Whenever the sex ratio departs from 50-50, the members of the rarer sex will average more children than the members of the more common sex. Therefore the sex ratio favored by natural selection is 50-50. Here’s a nice summary of the idea: https://www.blackwellpublishing.com/ridley/tutorials/Adaptations_in_sexual_reproduction26.asp

        1. Thanks. Yes, I understand that. If there are males and females they must each comprise about half the population. My puzzlement is why there are males and females in the first place, even given anisogamy. Mike has helped dispel some of my puzzlement.

    2. It’s helpful to leave out uteruses and other organs, and ignore things like caring for offspring after the egg and sperm fuse, and just focus on the gametes. In many marine animals (including many fish) neither males nor females have reproductive organs other than gametes; sperm and eggs are spawned into seawater; fertilization happens outside the body of either parent; and neither parent cares for the offspring. So no uteruses or pouches or nests etc. But even in those species, there are two sexes, one with relatively fewer larger immotile gametes (eggs) and one with many smaller motile gametes (sperm). That’s where the puzzle of anisogamy is clearest.

      One explanation for the evolution of tiny sperm versus larger eggs is that, in a species with isogamy (two mating types, where the two types have similar cell size and properties), the population can be invaded by a mutant that makes one of its two gamete types much smaller (call them sperm) so that more of them can be made for any given investment in reproduction. That mutant and it’s many small motile cells of one mating type could get to many more of the gametes of the other mating type (the larger ones, call them eggs) and have much greater reproductive success than other genotypes where both mating types are the same size and made in the same numbers. In the limit, that mutant replaces all of the other genotypes, and all of its descendants make either the male mating type (sperm) or the female mating type (eggs).

      Then the question is why not make both many tiny sperm and smaller numbers of larger eggs (that is, why not be a hermaphrodite)? Again hermaphrodites evolve in many animal groups where there is no maternal care, no uteruses or nests, but just spawning of eggs and sperm into seawater. There is lots of quantitative sex allocation theory that predicts the conditions under which hermaphrodites should evolve, and have greater reproductive success than a population of gonochoric individuals (with separate sexes), and how much energy or tissue or time individual hermaphrodites should devote to male function (and sperm) or to female function (eggs, care of young, etc.). The predictions are complex, and a lot depends on variables like whether individuals are more likely to be successful as females later in life (at large size, when more eggs can be made) or more successful as males later in life (at larger size, when large body size allows better competition for mates and access to eggs).

      Sorry not to explain further – this comment is already far too long – just to say this is not a mystery it’s just complex but there are good ideas to account for why and under what circumstances these different trait combinations are expected to evolve.

      1. Hi Mike. Your comment is most enlightening and helpful to me. Thank you for posting.

        I suspect you are right that starting with uteruses is starting at the wrong end of the problem. It is best to start with gamete competition, or as close as possible, since species with uteruses evolved from a primitive form of sex. I do not have a problem with anisogamy. Your comment and Griffiths’ article are fairly convincing on that. My problem, as my question about hermaphrodites suggest, is why the production of the larger and smaller gametes becomes specialized in separate males and females rather than a single version of the organism. In fact, it is this specialization that leads, eventually, to the wasteful uterus-less males that puzzles me. (And I understand if there are two separate sexes, selection will ensure they each will comprise roughly half of the population, whether males are wasteful or not.) Your comments on the difficulties of coordinating the production of both gametes in a single organism were most helpful in understanding the difficulties of the hermaphrodite solution. Do you have any thoughts on the self-fertilization problem?

        On the subject of the spawning fish. I agree that the disparity between the contributions of males and females is much less pronounced, although the female still bears the extra burden of producing the larger gamete. I have read, however, that in this case, since the females can leave once her eggs are laid, the male who then fertilizes is coerced by circumstance to guard the eggs against predators until the fry hatch and disperse. Thus, the benefit of the females moving first shifts some of the burden to the males making them “less wasteful.”🙂

        1. Hi Darwin,

          I think it’s helpful to step back from the idea that males are wasteful. The two gamete types are just alternative ways to package a genome and send it off into the world in the hope of merging it with another genome copy to make an offspring. Males aren’t wasteful, and hermaphrodites aren’t more efficient. Males produce offspring just like females produce offspring, it’s just the size and shape and number of gametes made by the two sexes that differ. But a female doesn’t make offspring in any way that a male does not, and a female is not more efficient or less wasteful than a male. We tend to think of offspring as being somehow more descended from the female because eggs are larger, and females typically provide more parental care (at least in many species), etc. But that doesn’t mean that population with a 1:1 sex ratio is wasting effort on males, or that the population would do better if it had fewer males and more females.

          Here’s a second way to think about it. A hermaphrodite could produce just a little bit of sperm and a lot of eggs, fertilize those eggs itself, and be done. But by sacrificing just a few of those eggs, it could make a lot more sperm, and maybe fertilize someone else’s eggs as well. It’s a win-win. In a population where that’s a winning strategy, in the limit the best strategy wrt male function will be to make no eggs, make only sperm, and compete to fertilize everyone else’s eggs. And then you’re back to separate sexes.

          I hope that’s helpful.

          1. It is helpful, thanks for your patience. I still think males are wasteful, the same way Dawkins argued that tall trees are wasteful, but you cannot stop the free riding interloper from upsetting the efficient apple cart. A population of efficient hermaphrodites is no more possible than a population of efficient short trees. I have been abused by the hermaphrodite solution for a while, so thanks for disabusing me.

    3. The mystery to me about sex is that a species has half its population contributing nothing but sperm.

      Because the species doesn’t plan how it is going to evolve. The situation where females and males are produced in a ratio of (say) ten to one is unstable because a gene that produces twice as many males per female (five to one) will have a selection advantage because it will have twice as many males competing for females as the gene that produces in the ratio of ten to one.

      The argument is symmetrical with respect to the sexes which is why you end up at 50:50.

      1. Hi Jeremy. Yes, thanks, I understand the 50:50 result. If you have males and females they will be 50:50 in the population. My question is why are there males and females in the first place? Why are not both gametes produced by a “single sex” to avoid the waste associated with having males produce the tiny gametes? (If we could get around the 50:50 problem and simply have males comprise a small part of the population the problem would be solved.)

        Mike makes several arguments against the difficulties of hermaphrodites for complex species which answers my question. I think. Gamete competition leads to anisogamy, physics dictates that two different sexes are needed to produce the different gametes, and everything follows from that.

  5. I like the Griffiths article. But some of the details are not correct. He writes “In some species, these two gametes are identical; many species of yeast, for example, make new individuals from two, identical gametes. They reproduce sexually, but they have no sexes, or, if you prefer, they have only one sex.”

    This is not correct. In yeast and other fungi, there are two (or more) mating types, and they are genetically distinct from each other, and two gametes of the same mating type cannot fuse with each other to form a zygote. They do not have one sex; they have two sexes, but the sexes make gametes that are not obviously morphologically different in the way that the gametes of animal mating types (sperm and eggs of males and females) are morphologically different. Instead the two sexes make gametes that are genetically different and can recognize each other via that genetic difference. The Wikipedia article on yeast mating types is clear and readily understandable even to a philosopher of science…

    1. Griffiths distinguishes between mating types and sexes later in the article, but this is an artificial distinction. He essentially defines sexes as different gamete morphology; if the gamete types are morphologically the same, he calls them mating types. It seems to be a wholly semantic distinction and I don’t understand it.

    2. I particularly liked Griffith’s explanation from a Kurt Vonnegut novel for why there are not many sexes, with a gamete from each one required to make a new individual: it makes the scheduling too complicated. This is similar to the advice one gives to graduate students making up their supervisory committees: add as many committee members as will be useful, but not so many that they can never arrange to meet at the same time in the same place.

  6. The thing is … you can’t compare mammals to fish. Or birds. Or any other kind of animals. Mammals are a specific kind of animal & humans are MAMMALS. So when people start saying that some kind of fish are “non-binary” or that the male “is the mother”, it’s totally immaterial to the argument. Humans are mammals & we come male & female for a reason. Sex is real & sex matters for obvious reasons.

Leave a Reply