Gardner A & Hardy ICW (in press) Adjustment of sex allocation to co-foundress number and kinship under local mate competition: an inclusive-fitness analysis. Journal of Evolutionary Biology.

Hamilton’s theory of local mate competition (LMC) describes how competition between male relatives for mating opportunities favours a female‐biased parental investment. LMC theory has been extended in many ways to explore a range of genetic and life‐history influences on sex allocation strategies, including showing that increasing genetic homogeneity within mating groups should favour greater female bias. However, there has been no quantitative theoretical prediction as to how females should facultatively adjust their sex allocation in response to co-foundress number and kinship. This shortfall has been highlighted recently by the finding that sex ratios produced by sub‐social parasitoid wasps in the family Bethylidae are affected by the number of co‐foundresses and by whether these are sisters or unrelated females. Here we close this gap in LMC theory by taking an inclusive‐fitness approach to derive explicit theoretical predictions for this scenario. We find that, in line with the recent empirical results, females should adopt a more female‐biased sex allocation when their co‐foundresses are less numerous and are their sisters. Our model appears to predict somewhat more female bias than is observed empirically; we discuss a number of possible model extensions that would improve realism and that would be expected to result in a closer quantitative fit with experimental data.

Hitchcock TJ & Gardner A (2020) A gene’s-eye view of sexual antagonism. Proceedings of the Royal Society of London Series B — Biological Sciences 287, 20201633.

Females and males may face different selection pressures. Accordingly, alleles that confer a benefit for one sex often incur a cost for the other. Classic evolutionary theory holds that the X chromosome, whose sex-biased transmission sees it spending more time in females, should value females more than males, whereas autosomes, whose transmission is unbiased, should value both sexes equally. However, recent mathematical and empirical studies indicate that male-beneficial alleles may be more favoured by the X chromosome than by autosomes. Here we develop a gene’s-eye-view approach that reconciles the classic view with these recent discordant results, by separating a gene’s valuation of female versus male fitness from its ability to induce fitness effects in either sex. We use this framework to generate new comparative predictions for sexually antagonistic evolution in relation to dosage compensation, sex-specific mortality and assortative mating, revealing how molecular mechanisms, ecology and demography drive variation in masculinization versus feminization across the genome.

Faria GS, Gardner A & Carazo P (in press) Kin discrimination and demography modulate patterns of sexual conflict. Nature Ecology & Evolution. doi: 10.1038/s41559-020-1214-6

Faria GS & Gardner A (2020) Does kin discrimination promote cooperation? Biology Letters 16, 20190742.

Genetic relatedness is a key driver of the evolution of cooperation. One mechanism that may ensure social partners are genetically related is kin discrimination, in which individuals are able to distinguish kin from non-kin and adjust their behaviour accordingly. However, the impact of kin discrimination upon the overall level of cooperation remains obscure. Specifically, while kin discrimination allows an individual to help more-related social partners over less-related social partners, it is unclear whether and how the population average level of cooperation that is evolutionarily favoured should differ under kin discrimination versus indiscriminate social behaviour. Here, we perform a general mathematical analysis in order to assess whether, when and in which direction kin discrimination changes the average level of cooperation in an evolving population. We find that kin discrimination may increase, decrease or leave unchanged the average level of cooperation, depending upon whether the optimal level of cooperation is a convex, concave or linear function of genetic relatedness. We develop an extension of the classic ‘tragedy of the commons’ model of cooperation in order to provide an illustration of these results. Our analysis provides a method to guide future research on the evolutionary consequences of kin discrimination.

Gardner A (2020) Price’s equation made clear. Philosophical Transactions of the Royal Society of London Series B — Biological Sciences 375, 20190361.Price’s equation provides a very simple—and very general—encapsulation of evolutionary change. It forms the mathematical foundations of several topics in evolutionary biology, and has also been applied outwith evolutionary biology to a wide range of other scientific disciplines. However, the equation’s combination of simplicity and generality has led to a number of misapprehensions as to what it is saying and how it is supposed to be used. Here, I give a simple account of what Price’s equation is, how it is derived, what it is saying and why this is useful. In particular, I suggest that Price’s equation is useful not primarily as a predictor of evolutionary change but because it provides a general theory of selection. As an illustration, I discuss some of the insights Price’s equation has brought to the study of social evolution.

Micheletti A, Ruxton GD & Gardner A (in press) The demography of human warfare can drive sex differences in altruism. Evolutionary Human Sciences.

Recent years have seen great interest in the suggestion that between-group aggression and within-group altruism have coevolved. However, these efforts have neglected the possibility that warfare – via its impact on demography – might influence human social behaviours more widely, not just those directly connected to success in war. Moreover, the potential for sex differences in the demography of warfare to translate into sex differences in social behaviour more generally has remained unexplored. Here, we develop a kin-selection model of altruism performed by men and women for the benefit of their groupmates in a population experiencing intergroup conflict. We find that warfare can promote altruistic, helping behaviours as the additional reproductive opportunities winners obtain in defeated groups decrease harmful competition between kin. Furthermore, we find that sex can be a crucial modulator of altruism, with there being a tendency for the sex that competes more intensely with relatives to behave more altruistically and for the sex that competes more intensely with non-relatives in defeated groups to receive more altruism. In addition, there is also a tendency for the less-dispersing sex to both give and receive more altruism. We discuss implications for our understanding of observed sex differences in cooperation in human societies.

Stilwell P, O’Brien S, Hesse E, Lowe C, Gardner A & Buckling A (2020) Resource heterogeneity and the evolution of public-goods cooperation. Evolution Letters doi: 10.1002/ev13.158Heterogeneity in resources is a ubiquitous feature of natural landscapes affecting many aspects of biology. However, the effect of environmental heterogeneity on the evolution of cooperation has been less well studied. Here, using a mixture of theory and experiments measuring siderophore production by the bacterium Pseudomonas aeruginosa as a model for public goods based cooperation, we explore the effect of heterogeneity in resource availability. We show that cooperation in metapopulations that were spatially heterogeneous in terms of resources can be maintained at a higher level than in homogeneous metapopulations of the same average resource value. The results can be explained by a positive covariance between fitness of cooperators, population size, and local resource availability, which allowed cooperators to have a disproportionate advantage within the heterogeneous metapopulations. These results suggest that natural environmental variation may help to maintain cooperation.

I’m currently offering a fully-funded (research costs + tuition fees + stipend) 3.5 year PhD studentship, for uptake in Sep 2020 (though the start date is flexible).

Theory of social evolution: adaptation of genes, individuals and groups.

Natural selection explains the appearance of design in the living world. But at what level is this design expected to manifest – gene, individual or group – and what is its function? Social evolution provides a window on this problem, because it is in the context of social interaction that the interests of genes, individuals and groups come into conflict with each other.

I invite applications for a PhD studentship in my research group at the School of Biology, University of St Andrews, Scotland, to develop new theory on the topic of social evolution. The project will suit a Biology graduate with a strong interest in social evolution, but applications from graduates with other backgrounds are also encouraged, and although prior experience in mathematical modelling would be helpful this is certainly not required as the requisite training will be provided.

Current research in my lab involves development of general theory – using kin selection, multilevel selection, game theory and theoretical population genetics approaches – as well as more specific mathematical and computer simulation models that are tailored to the biology of particular organisms, from microbes to insects to humans. Much of our ongoing work is focused on intragenomic conflicts and associated clinical pathologies, plus the role of sex and gender in social evolution. See the rest of this website for details.

If evolutionary biology really fascinates you, and you are a careful thinker, then you will flourish in the kind of project that I enjoy supervising.

This studentship is funded by the European Research Council and the School of Biology at the University of St Andrews. There are no nationality restrictions on who can apply, and the studentship will cover both Home and Overseas tuition fees.

Applications are to be made through the School of Biology website by 1 Dec 2019. Please direct informal enquiries to me at [email protected].

Gardner A (2019) The greenbeard effect. Current Biology 29, R430-R431.

What is the greenbeard effect? The greenbeard effect is a driver of social evolution, and one of the three basic mechanisms of kin selection. It was first described in the 1960s, in the context of W.D. Hamilton’s work on the evolution of altruism and other social behaviours. Hamilton noted that a gene encoding altruistic behaviour, and hence reducing the fitness of its carrier, can nevertheless be favoured by kin selection if the individuals who benefit from the altruism also carry copies of the same gene. The first two mechanisms of kin selection (kin discrimination and population viscosity) involve the beneficiaries of altruism being genealogically close kin of the altruist, whereas the third mechanism (the greenbeard effect) can operate even when the altruist and her beneficiaries are not genealogical kin.

Kerstin Stucky has begun her PhD with us this week; she’ll be working on social and cultural evolution in humans. Welcome to the lab, Kerstin!