🧵 1/15 - "THERE ARE ONLY TWO SEXES AND THERE CAN NEVER BE MORE" - my publication in Archives of Sexual Behavior.
Thread with link and very brief summaries of the main points I cover in the paper...
Thread with link and very brief summaries of the main points I cover in the paper...
link.springer.com/article/10.100…
2/15 - Biologically, sex is determined by gamete type: females produce large gametes (eggs), males produce small gametes (sperm). This definition applies at the level of phenotype. Infertile, prepubescent, or post-reproductive individuals are still male or female, because they belong to a sexed phenotype evolved to produce one gamete morph. They do not contradict the gamete-based definition of sex.
I do not focus on 'gender' or 'identity' in this paper.
2/15 - Biologically, sex is determined by gamete type: females produce large gametes (eggs), males produce small gametes (sperm). This definition applies at the level of phenotype. Infertile, prepubescent, or post-reproductive individuals are still male or female, because they belong to a sexed phenotype evolved to produce one gamete morph. They do not contradict the gamete-based definition of sex.
I do not focus on 'gender' or 'identity' in this paper.
3/15 - Outside of biology, sex and gender are often blurred, leading to confusion in science, medicine, education, law, and policy. Clarity matters, because mistaking categories of sex for categories of identity has real world costs, e.g. legal ambiguity creating problems in policy, and defunding of important sex-related research.
4/15 - My focus is strictly evolutionary biology. I aim to communicate simply Geoff Parker's (and other's) game-theoretic and mathematical models of the evolution of two sexes to researchers beyond biology, in psychology, anthropology, sociology, and related fields grappling with questions of sex, identity, and human variation.
5/15 - The two-sex system originates in anisogamy: the condition where reproduction involves two distinct gamete morphs, large and small. This evolutionary split is the foundation of “female” and “male.” Claims about “third sexes” misunderstand or misapply this biology.
6/15 -What Sex is Not
Many misconceptions muddy the picture. Hermaphrodites are not a third sex; they embody both sexes, not a new one. No human (or any other mammal) has ever been both sexes. Disorders of Sexual Development (DSDs) are atypical variants of male or female, not new sex categories. And "mating types" in microbes are not sexes at all, just compatibility systems.
Many misconceptions muddy the picture. Hermaphrodites are not a third sex; they embody both sexes, not a new one. No human (or any other mammal) has ever been both sexes. Disorders of Sexual Development (DSDs) are atypical variants of male or female, not new sex categories. And "mating types" in microbes are not sexes at all, just compatibility systems.
7/15 -Before Two Sexes
Originally, there were no sexes. Early eukaryotes were isogamous: all gametes were the same size. Mutations introduced size variation, creating a trade-off: larger gametes produced fewer in number but made zygotes more viable, while smaller gametes were numerous but less provisioning. Crucially, zygote survival scaled with the combined gamete size—but with diminishing returns, setting an upper limit on how big gametes could get.
Originally, there were no sexes. Early eukaryotes were isogamous: all gametes were the same size. Mutations introduced size variation, creating a trade-off: larger gametes produced fewer in number but made zygotes more viable, while smaller gametes were numerous but less provisioning. Crucially, zygote survival scaled with the combined gamete size—but with diminishing returns, setting an upper limit on how big gametes could get.
8/15 - Fusion Dynamics Led to Two Sexes
Once large gametes emerged, a new strategy could invade: producing very small gametes. Small–small fusions were nonviable, but large-small fusions still produced viable zygotes. Large gametes benefited from these unions too, even if less well-provisioned than large–large fusions. Because small gametes were produced in huge numbers, this strategy persisted through sheer frequency.
Once large gametes emerged, a new strategy could invade: producing very small gametes. Small–small fusions were nonviable, but large-small fusions still produced viable zygotes. Large gametes benefited from these unions too, even if less well-provisioned than large–large fusions. Because small gametes were produced in huge numbers, this strategy persisted through sheer frequency.
9/15 - Large gametes could in principle fuse with each other, but the size–number trade-off meant small gametes were produced in vastly greater numbers. They saturated the environment and tended to reach large gametes first. As a result, large–large fusions became vanishingly rare, swamped by the frequency of large–small pairings.
10/15 - What about intermediate-sized gametes? One might think that this "third gamete" strategy could succeed. But intermediates were stuck in an evolutionary dead end on three levels: too few in number to compete with the abundance of small gametes, too under-provisioned to rival the viability of large ones, and too rare to reliably fuse with large gametes. Disruptive selection eliminated them, leading to a stable divergence into two gamete morphs: large (eggs) and small (sperm). It is mathematically impossible for intermediates to persist.
11/15 - Once anisogamy evolved, the two-sex system became an Evolutionarily Stable Strategy (ESS). Any deviation - slightly smaller eggs or slightly larger sperm - reduced reproductive success. Models consistently show that across conditions, no alternative gamete strategy can invade. Once anisogamy evolves, two sexes are locked in.
12/15 - Fusion dynamics reinforced this stability. Small gametes evolved to specialise in targeting large ones, while large gametes benefited from avoiding each other; large–large fusions were rare and less efficient than abundant large–small pairings. This disassortative fusion increased efficiency and left no room for the evolution of a third gamete morph, which would have no viable partner.
13/15 - Inevitable Cascade of Change
The divergence into two gamete morphs set off an evolutionary cascade. Large gametes were costly and few, so selection favoured choosiness and investment in the organisms that produced them. Small gametes were cheap and numerous, so selection favoured competition and mating effort in their producers.
Over time, these asymmetric selection pressures shaped the evolution of mating behaviours, reproductive strategies, and widespread sex-differences in physiology and behaviour.
The divergence into two gamete morphs set off an evolutionary cascade. Large gametes were costly and few, so selection favoured choosiness and investment in the organisms that produced them. Small gametes were cheap and numerous, so selection favoured competition and mating effort in their producers.
Over time, these asymmetric selection pressures shaped the evolution of mating behaviours, reproductive strategies, and widespread sex-differences in physiology and behaviour.
14/15 From this divergence a vast landscape of biological complexity, and in Darwin’s words, “endless forms most beautiful”, have followed.
15/15 - Conclusion
The two-sex system is not arbitrary or fragile. It is an evolutionarily stable outcome of natural selection acting on gamete size, fusion patterns, and reproductive efficiency. Decades of game-theoretic models and comparative studies show the same result: once anisogamy evolves, no alternative strategy can persist.
Anisogamy has evolved independently in many lineages, yet all have converges on the same binary outcome: two sexes.
As an anisogamous species, humans are governed by the same evolutionary forces that shape sex across the rest of the living world. The existence of only two sexes is not a cultural construct, but a biologically constrained inevitability of anisogamous reproduction. As such, there are only two sexes, and there can never be more.
The two-sex system is not arbitrary or fragile. It is an evolutionarily stable outcome of natural selection acting on gamete size, fusion patterns, and reproductive efficiency. Decades of game-theoretic models and comparative studies show the same result: once anisogamy evolves, no alternative strategy can persist.
Anisogamy has evolved independently in many lineages, yet all have converges on the same binary outcome: two sexes.
As an anisogamous species, humans are governed by the same evolutionary forces that shape sex across the rest of the living world. The existence of only two sexes is not a cultural construct, but a biologically constrained inevitability of anisogamous reproduction. As such, there are only two sexes, and there can never be more.
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