[New paper] Density-dependent dispersal promotes female-biased sex allocation in viscous populations

Chokechaipaisarn C & Gardner A (2022) Density-dependent dispersal promotes female-biased sex allocation in viscous populations. Biology Letters 18, 20220205.

A surprising result emerging from the theory of sex allocation is that the optimal sex ratio is predicted to be completely independent of the rate of dispersal. This striking invariance result has stimulated a huge amount of theoretical and empirical attention in the social evolution literature. However, this sex-allocation invariant has been derived under the assumption that an individual’s dispersal behaviour is not modulated by population density. Here, we investigate how density-dependent dispersal shapes patterns of sex allocation in a viscous-population setting. Specifically, we find that if individuals are able to adjust their dispersal behaviour according to local population density, then they are favoured to do so, and this drives the evolution of female-biased sex allocation. This result obtains because, whereas under density-independent dispersal, population viscosity is associated not only with higher relatedness—which promotes female bias—but also with higher kin competition—which inhibits female bias—under density-dependent dispersal, the kin-competition consequences of a female-biased sex ratio are entirely abolished. We derive analytical results for the full range of group sizes and costs of dispersal, under haploid, diploid and haplodiploid modes of inheritance. These results show that population viscosity promotes female-biased sex ratios in the context of density-dependent dispersal.

[New paper] Paternal genome elimination promotes altruism in viscous populations

Hitchcock TJ & Gardner A (in press) Paternal genome elimination promotes altruism in viscous populations. Evolution https://doi.org/10.1111/evo.14585

Image: https://www.craiyon.com

Population viscosity has long been thought to promote the evolution of altruism. However, in the simplest scenarios, the potential for altruism is invariant with respect to dispersal – a surprising result that holds for haploidy, diploidy, and haplodiploidy (arrhenotoky). Here we develop a kin-selection model to investigate how population viscosity affects the potential for altruism in species with male paternal genome elimination (PGE), exploring altruism enacted by both females and males, and both juveniles and adults. We find that: 1) PGE promotes altruistic behaviours relative to the other inheritance systems, and to a degree that depends on the extent of paternal genome expression. 2) Under PGE, dispersal increases the potential for altruism in juveniles and decreases it in adults. 3) The genetics of PGE can lead to striking differences in sex-specific potentials for altruism, even in the absence of any sex-differences in ecology.

Scots translation (by Ashley Douglas):

Paternal genome drap-oot forders altruism in stieve populations

Population stieveness has lang been thocht tae forder the evolution o altruism. Hooivver, in the maist straucht-forrit o scenarios, the potential fur altruism is invariant wi respeck tae skail – a stamagasterin ootcome that hauds fur haploidy, diploidy, and haplodiploidy (arrhenotoky). Here we pit forrit a kin-walin model fur tae airt-oot hoo population stieveness affects the potential fur altruism in species wi male paternal genome drap-oot (PGD), takkin tent o altruism enactit by baith females and males, forby baith young-anes and aulder-anes. We find that: 1) PGD forders altruistic ongauns relative tae the ither inheritance seestems, forby tae a degree that depends on the extent o paternal genome kythin. 2) Unner PGD, skail maks mair muckle the potential fur altruism amang young-anes and gars it less likely amang aulder-anes. 3) The genetics o PGD can lead tae kenspeckle differences in sex-specific potentials fur altruism, even wioot onie sex-differences in ecology.

[New Paper] The evolution of religiosity by kin selection

Stucky K & Gardner A (2022) The evolution of religiosity by kin selection. Religion, Brain & Behavior

Despite religion’s apparent ubiquity, hypotheses about the selection pressures that may have shaped its cognitive foundations remain controversial. Here, we develop and analyze a mathematical model inspired by Crespi and Summers’ suggestion that parent-offspring conflict has driven the evolution of religious beliefs to explore the causes and consequences of these selection pressures. To this end, we employ kin selection methodology to investigate how selection may mold an individual’s propensity for religiosity and corresponding patterns of gene expression, revealing that the evolution of religiosity is modulated by genetic relatedness between social partners, that selection in relation to religiosity may depend on an individual’s age and sex, and that religiosity can foment intragenomic conflicts of interest that give rise to parent-of-origin specific patterns of gene expression and concomitant clinical disorders. More generally, we develop a formal, theoretical framework that enables the derivation of clear-cut, comparative predictions about adaptive as well as maladaptive religiosity phenotypes.

[New Paper] Population viscosity promotes altruism under density-dependent dispersal

Kanwal JK & Gardner A (2022) Population viscosity promotes altruism under density-dependent dispersal Proceedings of the Royal Society of London Series B – Biological Sciences289, 20212668.

A basic mechanism of kin selection is population viscosity, whereby individuals do not move far from their place of birth and hence tend to be surrounded by relatives. In such circumstances, even indiscriminate altruism among neighbours will often involve interactions between kin, which has a promoting effect on the evolution of altruism. This has the potential to explain altruistic behaviour across the whole tree of life, including in taxa for which recognition of kin is implausible. However, population viscosity may also intensify resource competition among kin, which has an inhibitory effect on altruism. Indeed, in the simplest scenario, in which individuals disperse with a fixed probability, these two effects have been shown to exactly cancel such that there is no net impact of viscosity on altruism. Here, we show that if individuals are able to disperse conditionally upon local density, they are favoured to do so, with more altruistic neighbourhoods exhibiting a higher rate of dispersal and concomitant relaxation of kin competition. Comparing across different populations or species, this leads to a negative correlation between overall levels of dispersal and altruism. We demonstrate both analytically and using individual-based simulations that population viscosity promotes the evolution of altruism under density-dependent dispersal.