Jussi Lehtonen

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.

Mathematical models linking zygote size to survival until maturity are central to evolutionary ecology, yet many existing formulations remain fragmented and conceptually disconnected. These models fall broadly into two categories: phenomenological models, which lack explicit biological justification, and those grounded in survival analysis, which we, in contrast to phenomenological models, call mechanistic models. Although the latter are theoretically preferable, many commonly used mechanistic functions lack a common basis despite being better integrated than phenomenological models, and some are known to contain inconsistent dimensional assumptions. Here, we use a general framework that links zygote survival functions to growth dynamics and size-dependent mortality to show that many existing zygote survival models are corollaries of allometric growth models and generalize readily to account for allometric mortality, revealing previously hidden connections among models. The unification presented here provides principled grounds for re-evaluating the parameters and predictions of existing models.

Reproductive value (RV) and evolutionarily stable primary sex ratios are fundamentally linked, centrally important topics in evolutionary biology. Recent theoretical studies have advanced our understanding of (i) the relationship between the RV of juveniles of the two sexes under arbitrary transmission genetics, age structures, and fecundity schedules, and (ii) sex ratio evolution under facultative asexual reproduction. Here, we unite and build upon these two strands of work. We derive the relative RVs of daughters and sons in an age-structured population with arbitrary transmission genetics when both sexes can reproduce asexually and can produce offspring of either sex. We then apply this result to sex ratio evolution and derive a general expression for the primary sex ratio of the sexual pathway. A key finding is that the primary sex ratio of the sexual pathway is unaffected by asexual reproduction by either sex when asexually produced offspring are of the same sex as their parents. If asexual reproduction produces offspring of the opposite sex, the sex ratio of the sexual pathway is altered. The post-hatching population sex ratio results derived in earlier studies are recovered as special cases.

Contributorship refers to indicating who did what in a project, going beyond a simple list ofauthors. In scholarly journal articles about a project, the Contributor Roles Taxonomy (CRediT)has become a popular way to provide individual contribution information, often withaccompanying machine-readable metadata. While CRediT is used by hundreds of scientificjournals, the official version of CRediT exists only in English. To support scientific publishersand researchers writing in other languages, we have created translations of the fourteen CRediTroles and their descriptions into thirty-six languages. To ensure high quality, at least one speakerfluent in the target language drafted the translation, with additional involvement of a secondfluent person. Because hundreds of scientific journals publish non-English work, the use of ourtranslations could improve the recognition of the associated researchers’ contributions. We havecontacted relevant publishers and academic organizations to make them more aware of CRediT,of our translations, and of contributorship generally. To conclude, we discuss the potential forCRediT and other ontologies to be applied more broadly, for example to facilitate greater recognition of people who are not co-authors but are named in Acknowledgments sections.

Gamete dynamics (GD) theory for the evolutionary transition from isogamy to anisogamy relies on the biophysics of fertilization, combining the dynamics of gamete limitation and gamete competition with the provisioning requirements of gametes and zygotes. A recent development by Siljestam & Martinossi-Allibert, which incorporates competitive gamete traits (motility, target size and chemoattractants) into anisogamy evolution reaches very different conclusions from previous GD predictions, and challenges current views on sexual selection. We develop models incorporating evolution of motility and target size traits under conventional GD theory assumptions showing that (i) unless gamete limitation is strong and the trait is more efficient in the larger gamete, such traits tend to arise in the male gamete, complying with previous analyses predicting that sexually selected expenditures are most likely to arise in males, (ii) gamete trait evolution does not alter the conditions under which anisogamy evolves from isogamy, (iii) the differences between our results and those of Siljestam & Martinossi-Allibert arise from their specific function for zygote survival, which is arguably less plausible than those used in previous GD theory, and (iv) as a novel finding, we show that the coevolution of gamete size with gamete traits (here, motility) can result in the evolution of slight anisogamy.

Phenotypic traits rarely evolve in isolation. Instead, multiple traits typically interact to influence fitness, resulting in complex coevolutionary dynamics. Such dynamics can be predicted using mathematical frameworks, such as adaptive dynamics and quantitative genetics. Selection gradients play a crucial role in these frameworks, describing the direction and strength of selection and thus predicting evolutionary trajectories and potential endpoints. Current theory focuses mainly on analysing how traits change in response to selection, which changes over time as traits evolve. However, the extent to which changes in each trait contribute to changes in the selection environment remains unquantified, leaving much of our understanding of trait coevolution reliant on verbal reasoning. To advance a more comprehensive and quantitative understanding of coevolutionary dynamics, we develop a general framework that examines how trait changes feedback to influence the selection environment. This framework enables a fine-grained and systematic investigation of coevolutionary feedback between traits and selection gradients by quantifying the pathways through which they influence one another. Our framework can be applied both to adaptive-dynamic models and to quantitative-genetic models under the weak selection limit. We illustrate our approach with three examples that showcase its potential to deepen our understanding of established models.

The life cycle of a sexually reproducing species starts from the formation of a zygote by fusion of two gametes. In external fertilisers, the survival of an individual organism from a zygote to an adult is precarious and depends largely on the gametic provisioning of resources by its parents (via the female gamete in species where anisogamy has evolved). In evolutionary ecology, this early survival time has been modelled using a variety of zygote survival functions. In evolutionary modelling applications, the fitness of an individual is measured by the number of its offspring that survive to reproductive maturity, making the survival probability of the offspring a key component in these models. Despite their significance, the existing zygote survival functions remain scattered, with little consensus on their theoretical principles and mathematical tools. Here, our aim was to provide clarification and coherence for future theoretical and empirical research. We achieve this by reconstructing the history of zygote survival modelling, analysing existing zygote survival functions and discussing their theoretical details and problems. Systematising existing survival functions will facilitate future modelling work and will decrease duplication of efforts by concentrating theoretical focus into a unified modelling discourse.

Reproductive value (RV) is the expected contribution of genes by an individual or class of individuals to the gene pool in the distant future, and it plays a crucial role in understanding adaptation in long-term evolution. Class RVs are linked to the genetic system and to life history: for diploid genetics, Grafen derived expressions for relative RVs of female and male juveniles and for the absolute RVs of all females and of all males. Subsequently, Gardner presented a derivation for relative RVs of juvenile females and males under haplodiploidy. Here, we generalize these results to any genetic system for biparental sexual reproduction, such that RV is explicitly linked to parameters of the genetic system and life history. The earlier results by Grafen and Gardner arise as special cases. Finally, we derive expressions for absolute juvenile and total RVs of both sexes under any genetic system.

The Bateman gradient is a central concept in sexual selection theory that relates reproductive success to mate number, with important consequences for sex-specific selection. The conventional expectation is that Bateman gradients are steeper in males than females, implying that males benefit more from multiple mating than females do. This claim is supported by much empirical evidence as well as mathematical modelling. However, under some reproductive systems, reversed Bateman gradients are observed, perhaps most notably in syngnathid fishes with male pregnancy. Unlike conventional Bateman gradients, the causal basis of such reversed Bateman gradients has never been modelled mathematically. Here, we present a sex-neutral mathematical model demonstrating how restrictions in capacity for carrying or incubating gametes and embryos (brooding) interact with anisogamy, generating both conventional and reversed Bateman gradients from a single mathematical model. The results clearly demonstrate how anisogamy tends to cause conventional Bateman gradients, but diminishing male brooding capacity under male pregnancy or nesting causes a gradual reversal from conventional to fully ‘reversed’ Bateman gradients.