The Fuzziness of Biological Definitions

Physics has fundamental forces as its core. Chemistry has its elements. And the fundamental unit of biology is the species. In this regard it's been likened to stamp collecting. A more modern analogy might be to Pokemon - gotta catch em all! 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. Fun stuff! 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 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 the brahminy blind snake 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 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 that:

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. This states that,

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. It’s also not practical to compare every specimen against this type. It would take far too long for one thing. 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 briefly about the problems this can cause in relation to the awesome 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? 

 

Ok, let's try and another one.

What is an Individual?
Well, 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 take a look at this. 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.

Now, hopefully you think this is all very interesting, but why have I decided to take the time to give a basic biology lesson? Well, it's because I keep seeing stuff in the press and online about transgender people, and a lot of the opposition is based on very faulty understandings of science. A lot of these misunderstandings are outside my field of expertise - genetics, biochemistry, psychology etc. But some of these misunderstandings are in my wheelhouse, as they revolve around how we define biological concepts. And this is what I want to address in the rest of this piece.

What is a Woman?
How do we define a woman? One definition that is gaining particular prominence is "adult human female". It seems pretty straightforward on the face of it. But let's do the dissection that we've done with definitions of "species" and "individual". Adult. How do we define that? Biologically, an adult is an organism that has reached sexual maturity and is 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 8 or at least 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. Now, unless you are a fertility doctor, it's unlikely you will see that 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 male from female? Man from woman? The answer is that we are using what are termed '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. But 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 so good at telling who is male and who is female?

In birdwatching there is a term that often raises a chuckle when used around non-birdwatchers. It's the term "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 that allows us to recognise something without needing to study it in detail. We use "jizz" in many situations, without realising (you may recognise its similarity in practice to the morphospecies concept discussed above). 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 telling apart 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. But none of these characteristics are unique to one sex. What we are really looking at the combination of these characteristics and making an educated guess. Tall, muscular, no breasts, short hair, beard - it's a man. Short, thin, long hair, no facial hair - it's a woman. Most of the time this sort of educated guesswork is right. After all, we've been doing it all our lives and practice makes perfect. But there are times when the jizz is indeterminate. There's a very dated episode of Jonathan Creek where Jonathan and Maddie spend an inordinate amount of time and mental energy trying to decide whether a police officer is male or female. Bones has a similar episode. It seems that we have this innate (or maybe socially-derived) need to know if someone is male or female and we find it a source of great consternation when we can't immediately tell. (I don't have the knowledge to explore why, though I have some uneducated guesses, which I won't divulge as they're just that.)
 

Transgender Women
So how does this relate to transgender people? Well, transphobes like to accuse them of "pretending" to be the other sex. As I've hopefully made clear, it is very rare that we know someone's actual sex. In fact, we rarely know our own unless we've had cause to undergo genetic testing. We can assume, and most of the time that assumption will turn out to be correct, but we don't 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. 

Transphobes often claim that transgender women (and the focus is almost always on trans-women, for reasons  I don’t have time to go into here) are not “real” women because they do not have a particular set of characteristics. The problem is no-one can decide on what constitutes the correct set of characteristics for a woman. It usually starts with periods. Getting your period is, after all, what turns a female from a girl to a woman. But not all females have periods. Primary amenorrhea is rare but does happen. And of course, women can stop having periods. Secondary amenorrhea stops the menstrual cycle, and women who undergo hysterectomies and menopause also stop having periods. As do some women on hormonal contraceptives. 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 the hormonal contraceptive implant. They’re 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. 

Claims that it is motherhood that makes “woman” a distinctive class that trans-women cannot join also fail under scrutiny. What about those who never have children, never even want children? Does that lack of a maternal instinct render us un-woman? 

What about those other secondary sexual characteristics - breasts, vagina, hairlessness etc? Trans-women, particularly those who take hormones, can develop breasts and change their hair coverage. But let’s take a moment to realise that these characteristics are not diagnostic of womanhood, even in cis women. There are many 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. Are we staying that these hairy women are not women? 

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. My dad and uncle both had ectodermal dysplasia which meant they had hardly any hair anywhere on their body.

I could go on but hopefully I’ve made my point - 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. A lot of the characteristics we claim to be definitive of womanhood, either by their presence or their absence, are not as definitive as we first thought. Like “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 Do We Do?
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 how do we recognise women? If we can’t tell who is a woman just by looking at them then what are we to do? Well, I have an answer. It’s radical and it may not work but I think it’s worth a try. 

We could ask people how they identify. 

I know it sounds crazy but hear me out.

How do you know you’re male/female [delete as applicable]? Is it because you have the secondary sexual characteristics of that gender? Is it because people refer to you as that gender? Or is it because you just know you’re that gender? I’m sure the secondary sexual characteristics help, as do the references by other people, but I’ve known I was a girl for as long as I can remember and I’m sure that you’ve known your gender for as long as you can remember too, if you’ve even given it that much thought. That is, unless you’re transgender or non-binary. If that’s the case you’ll have given this question a lot of thought. You’ll have questioned yourself, doubted yourself, tried to go along with what people said you were, what your secondary sexual characteristics told you you were, but you knew that they were all wrong. 

So if someone says they're a woman, or a man, why not believe them? What’s the risk?

Well, a common one is the scare stories about men pretending to be women to get access to bathrooms to attack women. But that’s all they are. Scare stories. It’s not like entrance to women’s bathrooms is carefully controlled, or that men would need to go to such lengths to hurt us. Think of how many places say “this bathroom has male and female attendants”. Wouldn’t it be easier to dress as the cleaner to get in? Or even just say they got confused? Or just walk in and hope that no-one stops them? What policing of bathrooms actually does it make them less safe, for both cis- and trans-gender women. 

The thing is, women aren’t discriminated against because they have vaginas, or breasts, or even because they have babies. Sure, 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. Sure, they 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. True, 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 had a very different life to me. A trans-woman who went to an all-boys school had a very different life to me. But none of those experiences make them any less of a woman and all they do is show how diverse and rich womanhood is. What binds us in womanhood s that we all face barriers and stigma as a result of being women. Cis- and trans-, we are all women.