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Chokechaipaisarn C & Gardner A (in press) The consequences of constrained sex allocation in diploids and haplodiploids under local mate competition. Journal of Evolutionary Biology https://doi.org/10.1093/jeb/voaf088

Unmated females in haplodiploid populations may enjoy reproductive success but with the constraint that all their offspring—developing from unfertilised eggs—are male. The presence of such females, constrained to produce only male offspring, is expected to lead to a corresponding female bias being favoured among the offspring of unconstrained females. Godfray (J Evol Biol 3, 3–17) derived a mathematical expression for the unbeatable sex allocation strategy for unconstrained females in the context of local mate competition in two-foundress patches, and concluded that there is negligible impact of the presence of constrained females on the unbeatable sex allocation of unconstrained females. However, Godfray’s result assumes diploid—rather than haplodiploid—genetics and his derivation contains a mathematical error. We correct Godfray’s error and extend his model to incorporate haplodiploid genetics. This results in a more substantial impact of constrained females on the sex allocation behaviour of unconstrained females under local mate competition.

Baird RB, Hitchcock TJ, Ševčik J, Monteith KM, Gardner A, Ross L & Mongue AJ (2025) Faster adaptation but slower divergence of X chromosomes under paternal genome elimination. Nature Communications 16, 5288.

Differences in transmission and ploidy between sex chromosomes and autosomes drive divergent evolutionary trajectories, with sex chromosomes generally evolving faster. Because sex-linked genes are transmitted less frequently, they are under less efficient selection. Conversely, exposure of recessive mutations on haploid sex chromosomes creates more efficient selection. In most systems, these effects occur simultaneously and are confounded. The fly families Sciaridae (fungus gnats) and Cecidomyiidae (gall midges) have X0 sex determination, but males transmit only maternally inherited chromosomes. This phenomenon results in equal transmission of the X and autosomes, allowing the effect of haploid selection to be studied in isolation. We discover that, unlike well-studied systems, X chromosomes diverge more slowly than autosomes in these flies. Using population genomic and expression data, we show that despite the X evolving more adaptively, stronger purifying selection explains slower divergence. Our findings demonstrate the utility of non-Mendelian inheritance systems for understanding fundamental evolutionary processes.

Twyman KZ & Gardner A (in press) The clonality window: relatedness and the group covariance effect in the evolution of division of labour. Evolution. https://doi.org/10.1093/evolut/qpaf093

 

 

Cellular division of labour is closely associated with the emergence of organismality in the evolution of obligate multicellularity. Michod has suggested that a trade-off between viability and fecundity may—through a ‘group covariance effect’—lead to a group’s fitness being augmented above the average of its constituents’ fitnesses, offering a first step towards division of labour and obligate multicellularity. However, it is difficult to see how a group’s fitness could be different from the aggregate of its constituents. Here, we investigate the same fitness trade-off and its consequences for division of labour. We recover the covariance effect, revealing that it is a consequence of cells sharing the products of their labours and clarifying that the group’s fitness remains equal to the aggregate of the fitnesses of its constituent cells. We show that the covariance effect imparts an inclusive-fitness benefit for cells that share, but that—all else being equal—natural selection favours sharing only when groupmates are genetically identical, yielding a ‘clonality window’. Lastly, we find that sharing is a critical determinant as to whether division of labour is favoured by natural selection, such that the ‘clonality window’ is also a prerequisite for division of labour in Michod’s trade-off scenario. (Image: ChatGPT)

Scott TW, Wild G & Gardner A (in press) Kin-discriminating partner choice promotes the evolution of helping. Evolution.

Kin selection theory predicts that individuals should evolve to help relatives, either by helping indiscriminately in a population where they do not move very far from their relatives, or by discriminating kin and conditionally helping them. It has been argued that, because kin discrimination enables individuals to reduce how helpful they are with some social partners as well increase how helpful they are with others, this could lead to an increase or a decrease in the overall level of helping. Specifically, it was argued that kin discrimination would increase the overall level of helping if the function relating the optimal level of help and genetic relatedness is convex, but kin discrimination would decrease the overall level of helping if the function relating the optimal level of help and genetic relatedness is concave. However, this prediction was based on a model in which individuals were not able to choose their social partners but only adjust how helpful they should be towards those social partners they have been allocated. Here, we perform a mathematical analysis showing that being able to choose social partners increases the overall level of helping. Consequently, if kin discriminators are allowed to choose whom they help, kin discrimination is more likely to increase the overall level of helping than previously anticipated. We obtained these results in two complementary theoretical settings: one more general, which makes few demographic assumptions, and the other more specific and concrete, which assumes a patch-structured population with complete dispersal. (Image: ChatGPT)

 

Dong B & Gardner A (2025) Kin competition drives the evolution of earlier metamorphosis. Ecology and Evolution 15, e70806.

Metamorphosis, the discrete morphological change between postembryonic life stages, is widespread across the animal kingdom. The suggested advantages of metamorphosis have usually been framed in terms of population benefits, i.e., ecological explanations. In contrast, evolutionary explanations concern whether and how metamorphosis spreads through a population owing to individual-fitness benefits. However, how kin selection modulates evolution of metamorphosis remains to be investigated formally. Here we develop a mathematical model to investigate how kin selection shapes the optimal timing of metamorphosis from foraging, non-reproductive larva to reproductive adult, when larvae tend to cluster with their genetic relatives. We consider the full range of larval competition intensities—from no competition to full competition—and the full range of relatedness coefficients—from unrelated to clonality. We provide testable predictions as to how kin selection modulates the timing of metamorphosis, as well as a conceptual framework within which empirical observations may be understood.

 

Chokechaipaisarn C & Gardner A (2024) Density-dependent dispersal reduces conflict over the sex ratio. Biology Letters 20, 20240378.

Haplodiploids—in particular, wasps—are the workhorses of sex-allocation research. This is owing to their unusual system of sex determination, which provides a ready means of sex ratio adjustment. Notably, their sexually asymmetrical mode of genetic inheritance leads mothers and fathers to come into conflict over the sex ratio of their offspring. In the simplest outbreeding scenario, a mother is favoured to employ an even sex ratio while a father prefers that all his mate’s offspring are female. An important modulator of evolutionary conflict between mating partners is genetic relatedness, raising the possibility that this sex ratio conflict is reduced in low-dispersal settings with mating occurring between relatives. However, the impact of population viscosity on sex ratio conflict in haplodiploids remains unknown. Here, we develop and analyse a kin-selection model to investigate how the rate of dispersal modulates sex ratio conflict in a haplodiploid, viscous population setting. We find that population viscosity is associated with a reduction in the extent of sex ratio conflict—the effect being very weak under density-independent dispersal and much stronger under density-dependent dispersal.

Dong B, Paracchini S & Gardner A (in press) Kin selection as a modulator of human handedness: sex-specific, parental and parent-of-origin effects. Evolutionary Human Sciences.

Image: Wikimedia Commons

The frequency of left-handedness in humans is ∼10% worldwide and slightly higher in males than females. Twin and family studies estimate the heritability of human handedness at around 25%. The low but substantial frequency of left-handedness has been suggested to imply negative frequency-dependent selection, e.g. owing to a “surprise” advantage of left-handers in combat against opponents more used to fighting right-handers. Because such game-theoretic hypotheses involve social interaction, here, we perform an analysis of the evolution of handedness based on kin-selection, which is understood to play a major role in the evolution of social behaviour generally. We show that: (1) relatedness modulates the balance of right-handedness versus left-handedness, according to whether left-handedness is marginally selfish versus marginally altruistic; (2) sex differences in relatedness to social partners may drive sex differences in handedness; (3) differential relatedness of parents and offspring may generate parent-offspring conflict and sexual conflict leading to the evolution of maternal and paternal genetic effects in relation to handedness; and (4) differential relatedness of maternal-origin versus paternal-origin genes may generate intragenomic conflict leading to the evolution of parent-of-origin-specific gene effects—such as “genomic imprinting”—and associated maladaptation.

Hitchcock TJ & Gardner A (2023) Sexual antagonism in sequential hermaphrodites. Proceedings of the Royal Society of London Series B – Biological Sciences 290, 20232222.

Image: Wikimedia Commons

Females and males may have distinct phenotypic optima, but share essentially the same complement of genes, potentially leading to trade-offs between attaining high fitness through female versus male reproductive success. Such sexual antagonism may be particularly acute in hermaphrodites, where both reproductive strategies are housed within a single individual. While previous models have focused on simultaneous hermaphroditism, we lack theory for how sexual antagonism may play out under sequential hermaphroditism, which has the additional complexities of age-structure. Here, we develop a formal theory of sexual antagonism in sequential hermaphrodites. First, we construct a general theoretical overview of the problem, then consider different types of sexually antagonistic and life-history trade-offs, under different modes of genetic inheritance (autosomal or cytoplasmic), and different forms of sequential hermaphroditism (protogynous, protoandrous or bidirectional). Finally, we provide a concrete illustration of these general patterns by developing a two-stage two-sex model, which yields conditions for both invasion of sexually antagonistic alleles and maintenance of sexually antagonistic polymorphisms.

Gardner A (in press) A geometric approach to the evolution of altruism. Journal of Theoretical Biology https://doi.org/10.1016/j.jtbi.2023.111653

Fisher’s geometric model provides a powerful tool for making predictions about key properties of Darwinian adaptation. Here, I apply the geometric model to predict differences between the evolution of altruistic versus nonsocial phenotypes. I recover Kimura’s prediction that probability of fixation is greater for mutations of intermediate size, but I find that the effect size that maximises probability of fixation is relatively small in the context of altruism and relatively large in the context of nonsocial phenotypes, and that the overall probability of fixation is lower for altruism and is higher for nonsocial phenotypes. Accordingly, the first selective substitution is expected to be smaller, and to take longer, in the context of the evolution of altruism. These results strengthen the justification for employing streamlined social evolutionary methodologies that assume adaptations are underpinned by many genes of small effect.

Gardner A (2023) R. A. Fisher on J. A. Cobb’s The problem of the sex-ratio. Notes and Records http://doi.org/10.1098/rsnr.2023.0067.

The logic of the rarer-sex effect, concerning how natural selection acts to balance the sex ratio among newborns, was long supposed to have originated with Ronald Aylmer Fisher in his 1930 book The genetical theory of natural selection. However, the principle is now understood to have originated with John Austin Cobb in his 1914 paper ‘The problem of the sex-ratio’. Fisher did not provide a citation of Cobb’s sex-ratio paper, and it has been unclear whether he was aware of its existence. Here, I show that Fisher was indeed aware of Cobb’s paper in 1930, as revealed by him citing it elsewhere that same year. Fisher’s willingness to highlight Cobb’s sex-ratio work lends support to the view that his failure to mention it in his book reflects the less stringent citation standards of the time rather than an attempt to deceive readers as to the provenance of the rarer-sex effect.