Physics has fundamental forces as its core. Chemistry has its elements. And in biology we have species. Like chemists studying elements, once we know the different species we can work out how they interact with each other and how they work. And in so doing we can learn about our natural world. But what actually is a species?
What is a species?
You probably learned the “biological species” definition at school. This definition states that:
species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups (Mayr, 1942)
This is an often-used definition, which is why you learn it in school, but there are problems with it. For one thing, there are species that are parthenogenetic, meaning they don’t breed but instead reproduce asexually, with the egg developing an embryo without the need for sperm. If you don’t breed you can’t be in a group of interbreeding individuals, so under this definition you cannot be a species. Oops. Bye-bye New Mexico whiptail lizard, bdelloid rotifers and brahminy blind snakes among many others.
Another problem is that “potentially interbreeding” line. If you have two groups of similar animals separated by, say, a river or a mountain, can you be sure they couldn’t interbreed given the chance? Polar/grizzly bear matings result in fertile offspring, as do American buffalo/domestic cow matings, so what does this mean for them as species? Can we stop worrying about the loss of polar bears because they’re all just grizzlies anyway? (No, in case you’re wondering.)
Even with just that short examination I think it’s fair to say the definition we learned at school isn’t great. And let’s be honest, when we’re trying to identify a species we’re not sitting around waiting for it to make babies. What most of us use in our everyday lives is the morphospecies definition. This definition states:
a species is recognised based on similarity of morphology to other members of that species and on dissimilarity of morphology to members of other species.
You see a bird in the garden. It’s got black feathers and a bright yellow bill. You instantly recognise it as a blackbird. But then you see it hop towards another bird that’s begging at it for food. This bird is of a similar shape, though slightly smaller and it’s brown with a speckled breast. What could it be? It’s a juvenile blackbird. Same species, but a different age. Another bird approaches, same size and shape, also brown like the juvenile but instead of a speckled breast it’s faintly striped. What on earth could this be? It’s the female. Same species, but three distinct forms (more if you count the chicks).
One of my favourite papers has an extreme example of this – three deep sea fish, so different in appearance that they were classed as different families, were found through genetic analysis to be the male, female, and juvenile of a single species. The lesson from this is that appearance isn’t the precise guide we think it is.
So that’s two commonly used definitions of species, neither of which are great. But it’s ok, there are other definitions. The “legal” definition (at least for animals) is defined in the International Code of Zoological Nomenclature as the taxonomic species, which states:
The fixation of the name-bearing type of a nominal taxon provides the objective standard of reference for the application of the name it bears.
This is rather legalistic but basically means that there is a specimen, the “type”, against which all other examples are compared. These types are often stored in museums where they can be accessed by researchers. Somewhere in a museum collection there is “The” blackbird, against which all other blackbirds are theoretically compared and judged to be close enough to it to also be called a blackbird. But there are, as you’ve probably guessed, problems with this definition too. You may have even guessed at least one of those problems already – if there’s only one type, what do you do for species where there are different forms? What about the females and juveniles?
Another problem is that when you have a single type specimen, if that specimen is damaged, lost or destroyed then comparing against it becomes difficult or impossible. The type specimen is also often one of the first specimens collected, and as such may not be a particularly good example. This is a common problem with fossils, where quite often specimens are incomplete and species can be described based on a few bones before other, more complete specimens are found. Another problem, particularly for specimens gathered early in the history of modern science, is that collectors often went for the biggest and most attractive specimens, rather than the most typical. I’ve written elsewhere about the problems this can cause in relation to the Lord Howe Island stick insects.
So that’s three definitions down, and none are ideal. What to do? Well, you do what biologists do and realise that actually there is no one-size fits all definition of species but rather you choose the one that suits your work the best. Are you a taxonomist describing a species new to science? If so, you’d better follow the taxonomic species concept if you want your work to be published. Are you out doing a spot of nature-watching? Probably best to use the morphospecies concept. Are you trying to learn more about the evolutionary history of a species? Use either the evolutionary or the phylogenetic species concept. Are you trying to work out how best to conserve plants and animals? You might well be better off abandoning the species concept entirely and work with evolutionarily significant units, which can apply to species, subspecies, races or populations, and basically means “group of organisms whose long-term survival we want to ensure”.
So, we can see that there’s no all-encompassing definition for a species. It’s context dependent and complicated (and I haven’t even got into plants, viruses and bacteria!). You really need to know why you want to define a species before you decide how to define it.
But that’s species. Of course they’re a bit complicated, they’re always evolving, always changing. Of course it’s going to be a bit difficult to find the precise boundaries between closely-related ones. But the rest of biology is easier to define, right?
Let’s try and another one.
What is an individual?
You may think this should be easy – we all know what an individual is, surely? After all, we are all individuals. But let’s actually try to define it. Common definitions are basically versions of “a person separate from other people and possessing their own needs or goals, rights and responsibilities” which is pretty uncontroversial (at least when talking about vertebrates). But let’s break it down. A person separate from other people brings us to an immediate sticking point when we consider conjoined twins: two distinct personalities but a single body.
Looking at it from a more biological position, we might define an individual as someone with a unique set of genes. But then we are confronted with genetically identical twins and even triplets. And the rarer case of chimeras, where a person has two sets of DNA in a single body. We could go more metaphysical and say that an individual is someone who possesses a unique consciousness, but what about when you’re unconscious, in a coma for example? Do you stop being an individual then?
Very interesting, but so what?
From these two examples I hope I’ve shown that just because a concept is one we use a lot, that doesn’t mean it is straightforward. There is no obviously correct definition of a species, or an individual. There are definitions that are right enough (a bit like Newton’s Laws of Motion which work great, as long as you’re not working at relativistic speeds or at very small scales) but no definition that works all the time under all circumstances. We have what you might call an “operational” understanding of these terms, an understanding that works well enough in enough circumstances that we don’t really consider there are limitations. But it’s important to remember that those limitations still exist and are still important.
Having taken the time to look at these basic biological terms, I’d like to look at another: gender. Gender is a topic which has been the subject of extraordinarily intense discussion of late, at least some of which has been based on very faulty understandings of science. A lot of these misunderstandings fall outside my field of expertise – genetics, biochemistry, and psychology – but some of these misunderstandings are precisely in my wheelhouse, as they revolve around how we define biological concepts.
Is gender really a special case?
Some people who are very keen to define what a woman is have popularised the definition: “adult human female“. It seems pretty straightforward on the face of it. But to test its usefulness, we can dissect it, just as we can do with “species” and “individual” (and any other biological concept).
“Adult”. How do we define that? Biologically, an adult is an organism that has reached sexual maturity and is technically capable of producing offspring. Legally, for humans, (at least in the UK since 1970) adults are people aged 18 or older. So “adult” could cover people from as young as young as 8, or no less than 18, depending on what definition we are using and who we are applying it to. I think you’ll agree that’s quite vague for something so universal.
We are fortunate that we have no close relatives so I don’t have to spend any time on “Human”. If I was writing 50,000 years ago this would be very different.
Finally, female. Biologically, “female” is used to refer to organisms whose gametes are “usually immotile”. These gametes are usually referred to as ova or eggs. Unless you are a fertility doctor, it’s unlikely you will encounter too many ova, so we must be using other definitions in everyday life. Another biological definition is that, for humans, men have XY sex chromosomes and women have XX. But again, unless you have reason to analyse someone’s genetic make-up, you’re unlikely to know what combination of sex chromosomes they have (and the XX/XY dichotomy is massively oversimplifying the wide range of combinations that are found in humans).
So, if we’re not examining people’s gametes, and we’re not analysing their genetic composition, how are we telling who is male and who is female? Who is a man and who is a woman? The answer is that we are using what are termed ‘primary and secondary sexual characteristics’. For humans the ones we think of most often are the breasts, vulva and vagina in females, and the penis and testes in males. In most modern societies, these characteristics are rarely visible to other people, except in intimate circumstances. Women may accentuate their breasts using tight-fitting tops and bras, and men may emphasise their penises with tight-fitting trousers or underwear, but in most situations most people at best hint at their presence. So how are we generally fairly good at telling who is male and who is female?
Let’s talk about jizz
In birdwatching there is a term that often raises a chuckle when used around non-birdwatchers: “jizz”. Jizz is the “the overall impression or appearance” of a bird. It’s a formal term for all the incredible processing our brain does without us realising, allowing us to recognise something without needing to study it in detail. You may recognise its similarity in practice to the morphospecies concept discussed above. We use “jizz” in many situations without realising: when you see a friend in the distance and can recognise them even though you can’t properly see their face, you’re recognising their jizz – the way they walk, the clothes they wear, the shape of their body, the way they’ve styled their hair. You just know it’s your friend though if asked to explain how you recognised them you’d probably struggle.
So, when we are identifying men and women, we aren’t looking at their specific sexual characteristics but the gestalt that they produce. The amount and distribution of muscle and fat, the length and distribution of hair, the height, and so on. However, as none of these characteristics are unique to one sex, what we are really looking at is the combination of these characteristics, and from there we unconsciously make an educated guess. Tall, muscular, no breasts, short hair, and a beard? They’re probably a man. Short, thin, long hair, no visible facial hair? They’re likely a woman.
Most of the time this sort of educated guesswork is right. After all, we’ve been doing it all our lives – and our ancestors have been doing it for a really long time, too – and practice makes perfect. But there are times when the jizz is indeterminate, that’s when our innate (or, equally likely, socially-derived) curiosity to know if someone is male or female kicks in, and we find it a source of great consternation when we can’t immediately tell.
Sex and gender
So how does this relate to trans people? Some of the more hurtful accusations levelled at trans people is that they are merely “pretending” to be a different sex. This is not only an upsetting accusation, but it’s also based on a flawed assumption, given that (as I’ve hopefully made clear) it is rare that we actually know someone’s sex. We can assume, and most of the time that assumption will turn out to be correct, but we don’t actually really definitely know. What we are actually looking at is someone’s gender. Gender refers to the “socially constructed roles, behaviours, expressions and identities”. It is how we present ourselves to the world. And as such it is reliant on understanding and following (or subverting, if you’re in the mood) cultural norms.
It is easy to find people making the claim that trans women are not “real” women, because they do not have a particular set of characteristics ‘definitive’ of being a woman. The problem is that this relies on there being a robust definition of ‘woman’ – which, as we’ve seen, doesn’t exist. For some, the definition involves menstruation, as getting your period is what indicates a transition from a juvenile human female to an adult human female. But not all females have periods. Primary amenorrhea is rare but does happen. And of course, women don’t have to have periods. Secondary amenorrhea stops the menstrual cycle, and women who undergo hysterectomies and menopause also stop having periods. Some hormonal contraceptives prevent periods.
If having periods is key to being a woman then what does that say about womanhood when you no longer have periods? Are you less of a woman? I haven’t had a period in over a decade thanks to my hormonal contraceptive implant. Periods, for me, are a vague memory and honestly, most of the time I completely forget they were ever a part of my life. I feel no bond of womanhood because I used to discharge my uterine lining for a few days each month.
What about those other primary and secondary sexual characteristics: breasts, vagina, hairlessness etc? These characteristics are not diagnostic of womanhood – there are many cis women who lament their lack of a cleavage; it is possible to be born without a vagina despite being assigned female at birth; and many women are hairy, some even having pronounced facial hair. Sometimes this is a sign of polycystic ovarian syndrome, but other times it’s just the luck of the genetic draw. I don’t think anyone would argue that those women are not women. Trans women can also have breasts, with just as much a “luck of the draw” as cis women experience in terms of their shape and size.
Looking from the other direction, breasts are not unique to women. Gynaecomastia (“man boobs”) is surprisingly common. Trans men have vaginas if they do not – or have not yet – undergone gender reassignment surgery. Hairlessness, even ignoring male pattern baldness, is common in men and has a wide range of causes. Ectodermal dysplasia is a genetic condition that, among other things, prevents hair growth anywhere on the body. It runs in my family. I was embarrassingly old before I realised that hairless chests was the result of manscaping and not just the way men were.
Put simply, there are no characteristics that are unique to women that are not found in trans women without excluding a lot of cis women in the process, and the same is true for cis and trans men. Similarly, a lot of the characteristics we claim are definitive of womanhood, either by their presence or their absence, are not as definitive as they first appear. Just as with “species” and “individual”, our everyday understanding of these terms are usually sufficient in most cases, but if you’re trying to be precise and, dare I say it, scientific, it becomes clear that nature abhors clean divisions. The closer you get towards the boundaries the blurrier they get, and the harder it becomes to decide whether something is on this side of the line or that side.
So, what can we do?
One thing nobody is disputing is that recognising women as a group is important. Women face problems that men do not, and men face problems that women do not. Identifying these problems, identifying their causes, and fixing them is key to making the world a better place.
But we should also bear in mind that women aren’t discriminated against because they have vaginas, or breasts, or even because they have babies. Having babies makes it easier to discriminate against us, but the pay gap still exists for childfree women. It goes back to gender – the “socially constructed roles, behaviours, expressions and identities” that have led women to be less valued than men in society.
Those social constructions may have had biological roots long ago, but that’s no reason to continue perpetuating them unquestioningly. If someone says they are a woman and are seen by society as a woman then they experience the same socially constructed barriers and stigmas that all women experience to varying degrees.
Not all women face all the same barriers. A woman living in a favela in Rio de Janeiro has a very different life to me. A woman who married at 18 and has had 5 kids has a very different life to me. A trans woman who went to an all-boys school has a very different life to me. Our differences are what makes womanhood so rich and diverse. And what binds us is that we all face barriers and stigma as a result of being women. Cis and trans, we are all women.
This article was updated on 22nd March 2021 at 8pm, as it previously omitted a reference to primary sexual characteristics.