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[New Paper] Restricting mutualistic partners to enforce trade reliance

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Wyatt GAK, Kiers ET, Gardner A & West SA (2016) Restricting mutualistic partners to enforce trade reliance. Nature Communications 7, 10322 doi: 10.1038/ncomms10322

mycorrhizae

Mutualisms are cooperative interactions between members of different species, often involving the trade of resources. Here, we suggest that otherwise-cooperative mutualists might be able to gain a benefit from actively restricting their partners’ ability to obtain resources directly, hampering the ability of the restricted partner to survive and/or reproduce without the help of the restricting mutualist. We show that (i) restriction can be favoured when it makes the resources of the restricting individual more valuable to their partner, and thus allows them to receive more favourable terms of trade; (ii) restriction maintains cooperation in conditions where cooperative behaviour would otherwise collapse; and (iii) restriction can lead to either an increase or decrease in a restricted individual’s fitness. We discuss the applicability of this scenario to mutualisms such as those between plants and mycorrhizal fungi. These results identify a novel conflict in mutualisms as well as several public goods dilemmas, but also demonstrate how conflict can help maintain cooperation.

 

[New Paper] Presence of a loner strain maintains cooperation & diversity in bacteria

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Inglis RF, Biernaskie JM, Gardner A & Kümmerli R (2016) Presence of a loner strain maintains cooperation and diversity in well-mixed bacterial communities. Proceedings of the Royal Society of London Series B — Biological Sciences 283, 20152682.

Cooperation and diversity abound in nature despite cooperators risking exploitation from defectors and superior competitors displacing weaker ones. Understanding the persistence of cooperation and diversity is therefore a major problem for evolutionary ecology, especially in the context of well-mixed populations, where the potential for exploitation and displacement is greatest. Here, we demonstrate that a ‘loner effect’, described by economic game theorists, can maintain cooperation and diversity in real-world biological settings. We use mathematical models of public-good-producing bacteria to show that the presence of a loner strain, which produces an independent but relatively inefficient good, can lead to rock–paper–scissor dynamics, whereby cooperators outcompete loners, defectors outcompete cooperators and loners outcompete defectors. These model predictions are supported by our observations of evolutionary dynamics in well-mixed experimental communities of the bacterium Pseudomonas aeruginosa. We find that the coexistence of cooperators and defectors that produce and exploit, respectively, the iron-scavenging siderophore pyoverdine, is stabilized by the presence of loners with an independent iron-uptake mechanism. Our results establish the loner effect as a simple and general driver of cooperation and diversity in environments that would otherwise favour defection and the erosion of diversity.

[PhD opportunities] 30 studentships for Chinese nationals

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The University of St Andrews, in conjunction with the China Scholarship Council, is offering up to 30 PhD studentships for Chinese nationals. If you’re interested in pursuing PhD research in evolutionary theory, and meet the eligibility criteria, then do get in touch. More details can be found here. Note that the deadline is 30 Nov 2015.

[New Paper] The constant philopater hypothesis

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Rodrigues AMM & Gardner A (in press) The constant philopater hypothesis: a new life history invariant for dispersal evolution. Journal of Evolutionary Biology doi: 10.1111/jeb.12771

Surprising invariance relationships have emerged from the study of social interaction, whereby a cancelling-out of multiple partial effects of genetic, ecological or demographic parameters means that they have no net impact upon the evolution of a social behaviour. Such invariants play a pivotal role in the study of social adaptation: on the one hand, they provide theoretical hypotheses that can be empirically tested; and, on the other hand, they provide benchmark frameworks against which new theoretical developments can be understood. Here we derive a novel invariant for dispersal evolution: the ‘constant philopater hypothesis’ (CPH). Specifically, we find that, irrespective of variation in maternal fecundity, all mothers are favoured to produce exactly the same number of philopatric offspring, with high-fecundity mothers investing proportionally more, and low-fecundity mothers investing proportionally less, into dispersing offspring. This result holds for female and male dispersal, under haploid, diploid and haplodiploid modes of inheritance, irrespective of the sex ratio, local resource availability and whether mother or offspring controls the latter’s dispersal propensity. We explore the implications of this result for evolutionary conflict of interests – and the exchange and withholding of contextual information – both within and between families, and we show that the CPH is the fundamental invariant that underpins and explains a wider family of invariance relationships that emerge from the study of social evolution.

 

[New Paper] Group selection versus group adaptation

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Gardner A (2015) Group selection versus group adaptation. Nature 524, E3-E4.

Pruitt and Goodnight describe how the ratio of aggressive versus docile females varies among naturally occurring colonies of the social spider Anelosimus studiosus, with larger colonies exhibiting more aggression in high-resource environments and the reverse in low-resource environments. They experimentally manipulate this ratio to show that it influences a colony’s reproductive success. Pruitt and Goodnight conclude that this work demonstrates group-level adaptation and contradicts an earlier theoretical analysis. Here, I show that this conclusion is unfounded and arises from a conceptual misunderstanding.

 

[New Paper] Sex-biased dispersal, kin selection and the evolution of sexual conflict

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Faria GS, Varela SAM & Gardner A (in press) Sex-biased dispersal, kin selection and the evolution of sexual conflict. Journal of Evolutionary Biology. doi: 10.1111/jeb.12697

There is growing interest in resolving the curious disconnect between the fields of kin selection and sexual selection. Rankin’s (2011, J. Evol. Biol. 24, 71–81) theoretical study of the impact of kin selection on the evolution of sexual conflict in viscous populations has been particularly valuable in stimulating empirical research in this area. An important goal of that study was to understand the impact of sex-specific rates of dispersal upon the coevolution of male-harm and female-resistance behaviours. But the fitness functions derived in Rankin’s study do not flow from his model’s assumptions and, in particular, are not consistent with sex-biased dispersal. Here, we develop new fitness functions that do logically flow from the model’s assumptions, to determine the impact of sex-specific patterns of dispersal on the evolution of sexual conflict. Although Rankin’s study suggested that increasing male dispersal always promotes the evolution of male harm and that increasing female dispersal always inhibits the evolution of male harm, we find that the opposite can also be true, depending upon parameter values.

[New Paper] More on the genetical theory of multilevel selection

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Gardner A (in press) More on the genetical theory of multilevel selection. Journal of Evolutionary Biology. doi: 10.1111/jeb.12684

In my article The genetical theory of multilevel selection, I provided a synthesis of the theory of multilevel selection (MLS) and the theory of natural selection in class-structured populations. I framed this synthesis within Fisher’s genetical paradigm, taking a strictly genetical approach to traits and fitness. I showed that this resolves a number of long-standing conceptual problems that have plagued the MLS literature, including the issues of ‘aggregate’ vs. ‘emergent’ group traits, ‘collective fitness1’ vs. ‘collective fitness2’ and ‘MLS1’ vs. ‘MLS2 ‘. In his commentary, Goodnight suggests this theoretical and conceptual synthesis is flawed in several respects. Here, I show this is incorrect, by: reiterating the theoretical and conceptual goals of my synthesis; clarifying that my genetical approach to traits is necessary for a proper analysis of the action of MLS independently of non-Darwinian factors; emphasizing that the Price–Hamilton approach to MLS provides a consistent, useful and conceptually superior theoretical framework; and explaining the role of reproductive value in the study of natural selection in class-structured populations. I also show that Goodnight’s contextual analysis treatment of MLS in a class-structured population is mathematically, biologically and conceptually inadequate.

 

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