There’s no need to reject how biologists define the sexes to defend the view that trans women are women. When we look across the diversity of life, sex takes stranger forms than anyone has dreamt of for humans. The biological definition of sex takes all this in its stride. It does so despite the fact that there are no more than two biological sexes in any species you’re likely to have heard of. To many people, that might seem to have ‘conservative’ implications, or to fly in the face of the diversity we see in actual human beings. I will make clear why it does not. [...]
Many people assume that if there are only two sexes, that means everyone must fall into one of them. But the biological definition of sex doesn’t imply that at all. As well as simultaneous hermaphrodites, which are both male and female, sequential hermaphrodites are first one sex and then the other. There are also individual organisms that are neither male nor female. The biological definition of sex is not based on an essential quality that every organism is born with, but on two distinct strategies that organisms use to propagate their genes. They are not born with the ability to use these strategies – they acquire that ability as they grow up, a process which produces endless variation between individuals. The biology of sex tries to classify and explain these many systems for combining DNA to make new organisms. That can be done without assigning every individual to a sex, and we will see that trying to do so quickly leads to asking questions that have no biological meaning.
While the biological definition of sex is needed to understand the diversity of life, that doesn’t mean it’s the best definition for ensuring fair competition in sport or adequate access to healthcare. We can’t expect sporting codes, medical systems and family law to adopt a definition simply because biologists find it useful. Conversely, most institutional definitions of sex break down immediately in biology, because other species contradict human assumptions about sex. The United States’ National Institutes of Health (NIH) uses a chromosomal definition of sex – XY for males and XX for females. Many reptiles, such as the terrifying saltwater crocodiles of northern Australia, don’t have any sex chromosomes, but a male saltie has no trouble telling if the crocodile that has entered his territory is a male. Even among mammals, at least five species are known that don’t have male sex chromosomes, but they develop into males just fine. Gender theorists have extensively criticised the chromosomal definition of human sexes. But however well or badly that definition works for humans, it’s an abject failure when you look at sex across the diversity of life. [...]
Nothing in the biological definition of sex requires that every organism be a member of one sex or the other. That might seem surprising, but it follows naturally from defining each sex by the ability to do one thing: to make eggs or to make sperm. Some organisms can do both, while some can’t do either. Consider the sex-switching species described above: what sex are they when they’re halfway through switching? What sex are they if something goes wrong, perhaps due to hormone-mimicking chemicals from decaying plastic waste? Once we see the development of sex as a process – and one that can be disrupted – it is inevitable that there will be many individual organisms that aren’t clearly of either sex. But that doesn’t mean that there are many biological sexes, or that biological sex is a continuum. There remain just two, distinct ways in which organisms contribute genetic material to their offspring.
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