Biological Sex Is Binary and Rooted in Anisogamy

Abstract

The definition of biological sex has become a renewed focus of societal debate, fuelled by the conflation of biological principles with discussions of human gender diversity. Here, we argue that conceptual clarity critically depends on separating these domains. Drawing on evolutionary theory and empirical evidence, we maintain that biological sex is best defined as a binary classification of male and female reproductive strategies rooted in anisogamy, characterised by the production of two discrete gamete types of different sizes. We stress that gamete size constitutes the ultimate criterion for biological sex and that this definition applies consistently across sexual systems, from separate-sexed species to hermaphrodites, irrespective of variation in karyotype, hormonal profile, somatic phenotype, or behaviour. Further, we emphasise that evolutionary insights offer a coherent explanation for recurring, though not universal, associations between biological sex and patterns of sex-specific selection, sexual dimorphism and parental care. We conclude that the definition of biological sex as a binary classification based on gamete size is a powerful scientific framework compatible with the diversity of sexual phenotypes found in anisogamous organisms and distinct from the concept of human gender.

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BibTeX RIS

BibTeX

@article{janicke2026biological,
  title = {{Biological Sex Is Binary and Rooted in Anisogamy}},
  author = {Tim Janicke and Göran Arnqvist and Pierre‐andré Crochet and Patrice David and Philippe Jarne and Jussi Lehtonen and Thomas Lenormand and Edward H Morrow and Geoff A Parker and Paul A Saunders and Jeanne Tonnabel and Frédéric Veyrunes and Christoph R Haag},
  journal = {Ecology Letters},
  year = {2026},
  month = jul,
  doi = {10.1111/ele.70426},
  url = {https://hal.science/hal-05756266v1},
  note = {CC BY 4.0},
}

RIS

TY  - JOUR
TI  - Biological Sex Is Binary and Rooted in Anisogamy
AU  - Tim Janicke
AU  - Göran Arnqvist
AU  - Pierre‐andré Crochet
AU  - Patrice David
AU  - Philippe Jarne
AU  - Jussi Lehtonen
AU  - Thomas Lenormand
AU  - Edward H Morrow
AU  - Geoff A Parker
AU  - Paul A Saunders
AU  - Jeanne Tonnabel
AU  - Frédéric Veyrunes
AU  - Christoph R Haag
PY  - 2026
DA  - 2026/07/10
JO  - Ecology Letters
DO  - 10.1111/ele.70426
UR  - https://hal.science/hal-05756266v1
AB  - The definition of biological sex has become a renewed focus of societal debate, fuelled by the conflation of biological principles with discussions of human gender diversity. Here, we argue that conceptual clarity critically depends on separating these domains. Drawing on evolutionary theory and empirical evidence, we maintain that biological sex is best defined as a binary classification of male and female reproductive strategies rooted in anisogamy, characterised by the production of two discrete gamete types of different sizes. We stress that gamete size constitutes the ultimate criterion for biological sex and that this definition applies consistently across sexual systems, from separate-sexed species to hermaphrodites, irrespective of variation in karyotype, hormonal profile, somatic phenotype, or behaviour. Further, we emphasise that evolutionary insights offer a coherent explanation for recurring, though not universal, associations between biological sex and patterns of sex-specific selection, sexual dimorphism and parental care. We conclude that the definition of biological sex as a binary classification based on gamete size is a powerful scientific framework compatible with the diversity of sexual phenotypes found in anisogamous organisms and distinct from the concept of human gender.
SN  - 1461-023X
SN  - 1461-0248
ER  -

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