It was great to be out Marching for Science yesterday. However, looking across various social media responses to the marches across the globe, it was clear some people think that science has shown that there are only two sexes (male and female), and that if scientists want to talk about the science of climate change for instance, we should also talk about the science of the sexes. But what is that science?
To start with, one can spill a lot of ink discussing the differences or otherwise between sex and gender (I take what I think is the reasonably common view that sex is a biological entity, whilst gender – at least in humans – is a concept bound up with both biological and socio-cultural associations and meanings, and so sex and gender are two different, if related, concepts). Instead, I will just briefly think about sexes, and make three points.
First, across organisms, sex is remarkably dynamic in evolutionary terms, in the sense that the machinery that determines what sex an organism develops as can evolve (i.e. change) very quickly. Perhaps the most famous example is the humble housefly Musca domestica, that has a veritable melange of sex determining systems, even co-occurring in the same population, so that what genetic factor makes a male or female can differ within and among populations.
Another fabulous example is the snow skink Niveoscincus ocellatus, a live-bearing lizard that lives across a wide altitudinal range in Tasmania. At lower altitude, this species using temperature-dependent sex determination (i.e. the ambient temperature whilst females bask influences whether an offspring will be male or female). However, at high altitude they use genetically determined sex determination, so that the genetic make-up of an offspring determines its sex. This means that the method of sex determination is plastic, depending on location, environment and genes.
These examples – plus the widespread evidence of rapid evolutionary changes and shifts in sex chromosomes and sex determination mechanisms more generally – challenge the idea that a binary sex system of males and females has an unchanging mechanistic foundation – it generally doesn’t.
Second, whilst one might agree that the underlying mechanisms may be dynamic, surely the result is a binary male or female. Well, again, not necessarily. Just looking at animals, there is clearly an extraordinary diversity of ways of being. Yes, we have plenty of organisms with separate males and females, but we also have hermaphrodites, including sex-changing sequential hermaphrodites that might change male to female, or female to male, and species where you may have a mix of hermaphrodites and single-sex animals, or even a mix of sexual and asexual individuals. In ants, bees, wasps and sawflies (the Hymenoptera), we typically see both males and females, but males are produced asexually by their mothers, and so males have no father. And don’t get me started on the little fire ant Wasmannia auropunctata – both males and females can be produced clonally, with males producing sons, and females producing daughters. So here, sons do have a Dad but not a Mum, whilst daughters have a Mum, but not a Dad. Indeed, the two sexes have almost become two asexual species.
Where does all this diversity come from? Evolution by natural selection is all about increasing your genetic contribution to the next generation relative to others in the population. Getting control of the sex determination system is one good way to do that, leading to evolutionary conflicts over the control of sex determination. Mutations that arise allowing you to gain that control will be favoured, as will mutations that prevent others gaining control. Over the last two or three decades, our discovery of so much diversity in how organisms determine sex and how rapidly it evolves strongly suggests that this conflict is on-going, pretty much everywhere.
Which brings me to the third point: what about humans? Well, in humans the empirical fact is that sex is not binary either. (Remember, we are leaving gender and gender-identification out of this, not because it is not extremely important, but rather to focus on what might be called “the science” by some.) Yes we have males and females, and yes the SRY gene on the human Y chromosome is the key starting point of our sex determination cascade. However, there is diversity here, and the frequency of intersex individuals – though small – is not negligible. Whilst there is much we don’t understand about how intersex individuals arise – and there are a number of known mechanisms – they are real and they show without question that humans also have the potential for the evolution of new sex-determining systems and indeed new ways of being. You might not like it, but the science says that sex is not straightforward, and the science does not say that sex is strictly binary, in us, or indeed in many other organisms.
Finally, for an inspiring intersex story, please go here: