Research

I work on a range of topics within – and beyond – the field of social evolution. Mostly, I focus on Darwinian adaptation: see “Adaptation as organism design” (Gardner 2009, Biol Lett) for a brief ‘manifesto’. Here, I outline some recent research themes.

 

Kin selection and inclusive fitness

Natural selection may operate both directly – via the impact that the individual has on her own fitness – and also indirectly – via the impact that she has on the fitness of her genetic relatives, i.e. “kin selection”. As a consequence of natural selection, the individual appears adapted to maximize the overall reproductive success of all her social partners, each increment or decrement being weighted by her genetic relatedness to that individual, i.e. her “inclusive fitness”. I develop theory on the topics of kin selection and inclusive fitness, and apply this theory to understand the evolution of social behaviours from microbes to insects to humans.

Find out more:

“The genetical theory of kin selection” (Gardner et al 2011, J Evol Biol)

“The meaning of death: evolution and ecology of apoptosis in protozoan parasites” (Reece et al 2011, PLoS Pathogens)

“Spite and virulence in the bacterium Pseudomonas aeruginosa (Inglis et al 2009, PNAS)

“Adaptation and the evolution of parasite virulence in a connected world” (Wild et al 2009, Nature)

 

Group selection and group adaptation

Natural selection may instead be conceptualised as operating within and between social groups, which provides an alternative – but equivalent – approach to understanding social adaptations. I develop theory on group selection, with a particular focus on the complexities associated with class-structured populations. I also investigate the conditions under which group selection may give rise to adaptation at the level of the social group, i.e. a “superorganism”. My work has highlighted that, whilst group selection is ubiquitous in nature, group adaptation is rare, on account of within-group selection acting to erode adaptive integrity at the group level.

Find out more:

“Group selection versus group adaptation” (Gardner 2015, Nature)

“The genetical theory of multilevel selection” (Gardner 2015, J Evol Biol)

“Adaptation of individuals and groups” (Gardner 2013, In: From Groups To Individuals, MIT Press)

“Capturing the superorganism: a formal theory of group adaptation” (Gardner & Grafen 2009, J Evol Biol)

 

Selfish genes and intragenomic conflict

I also study adaptation and conflicts of interest at the level of individual genes. These too become adapted, as a result of selection, to maximise their inclusive fitness, leading to the existence of altruistic and spiteful – and not just selfish – genes. I am particularly interested in understanding intragenomic conflicts of interest that arise in the absence of selection within the individual genome, such as in relation to genomic imprinting and greenbeard genes.

Find out more:

“The meaning of intragenomic conflict” (Gardner & Úbeda 2017, Nature Ecology & Evolution)

“Intrafamily and intragenomic conflicts in human warfare” (Micheletti et al 2017, Proc B)

“A formal theory of the selfish gene” (Gardner & Welch 2011, J Evol Biol)

“Greenbeards” (Gardner & West 2010, Evolution)

 

Haplodiploidy and eusociality

Perhaps the most famous kin selection hypothesis is that haplodiploid genetics has promoted the evolution of altruistic sib rearing in the social hymenoptera (ants, bees & wasps), on account of the inflated genetic relatedness between sisters. However, this hypothesis does not really make sense, either theoretically or empirically. I have developed theory to understand the impact that haplodiploidy has had on the evolution of eusociality via inflated sororal relatedness, showing that its effect is typically weak and that it may even be inhibitory. And I’ve investigated the role for haplodiploidy to have driven sex-biased helping in the social hymenoptera. Finally, I’ve suggested that haplodiploidy may have preadapted certain taxa to eusociality by enabling them to easily adjust sex allocation towards the more helpful sex, and shown that ease of sex-ratio adjustment is a major determinant of which sex or sexes help in arthropod societies.

Find out more:

“The ecology of sex explains patterns of helping in arthropod societies” (Davies et al 2016, Ecology Letters)

“Haplodiploidy, sex-ratio adjustment and eusociality” (Gardner & Ross 2013, American Naturalist)

“Ecology, not the genetics of sex determination, determines who helps in eusocial populations” (Ross et al 2013, Current Biology)

“Haplodiploidy and the evolution of eusociality: split sex ratios” (Gardner et al 2012, American Naturalist)

 

Kin selection and sexual selection

Darwin’s initial musings on kin selection and sexual selection first appeared in the Origin of Species, and have developed into two very large — but curiously disconnected — literatures. I’ve reviewed the broad connections between the two theories, investigated how kin selection may put the brakes on (and foment intragenomic conflicts in relation to) harmful sexual conflict in viscous populations, and shown the logical connections between the greenbeard and sexy-son effects.

Find out more:

“The relation between R. A. Fisher’s sexy-son hypothesis and W. D. Hamilton’s greenbeard effect” (Faria et al 2018, Evol Lett)

“Sexual selection modulates genetic conflicts and patterns of genomic imprinting” (Faria et al 2017, Evolution)

“Sex-biased dispersal, kin selection and the evolution of sexual conflict” (Faria et al 2015, J Evol Biol)

“The sociobiology of sex: inclusive fitness consequences of inter-sexual interactions” (Pizzari & Gardner 2012, Phil Trans B)

 

Evolution of reproductive systems

Whilst much of social evolution theory is concerned with understanding the impact of mode of reproduction upon the evolution of social behaviour, I have also been interested in understanding how social evolution may drive the evolution of reproductive systems themselves. I’ve considered how population demography and mode of sex determination may interact in driving the evolution of “paternal genome elimination”, a phenomenon exhibited by many small arthropods which resembles haplodiploidy in terms of patterns of genetic transmission. And I’ve developed theory to understand the evolution of a bizarre form of hermaphroditism in scale insects, via a parasitic sperm-producing cell lineage. I’ve also investigated the role for sibling conflict to drive the evolution of parental care, in the form of food provisioning, and whether haplodiploidy promotes or inhibits the evolution of maternal care, relative to diploid inheritance.

Find out more:

“Mating ecology explains patterns of genome elimination” (Gardner & Ross 2014, Ecology Letters)

“Evolution of maternal care in diploid and haplodiploid populations” (Gardner 2012, J Evol Biol)

“The evolution of hermaphroditism by an infectious male-derived cell lineage: an inclusive-fitness analysis” (Gardner & Ross 2011, American Naturalist)

“Evolution of parental care driven by mutual reinforcement of parental food provisioning and sibling competition” (Gardner & Smiseth 2011, Proc R Soc Lond B)

 

 Sex allocation

The proportional investment into males versus females is perhaps the classic social evolutionary trait. I’ve developed theory on this topic, in particular focusing on the demographic drivers of female-biased sex ratios and, for example, confirming that kin selection may be responsible for the extraordinary sex ratios observed in social spiders. I’ve also investigated facultative sex ratio adjustment in response to strain diversity in malaria infections, with an interplay of theoretical and experimental research, which has demonstrated that malaria parasites are able to discriminate their kin.

Find out more:

“Simultaneous failure of two sex-allocation invariants: implications for sex-ratio variation within and between populations” (Rodrigues & Gardner 2015, Proc B)

“Dynamics of sex ratio and female unmatedness under haplodiploidy” (Gardner 2014, Ecology & Evolution)

“Budding dispersal and the sex ratio” (Gardner et al 2009, J Evol Biol)

“Sex ratio adjustment and kin discrimination in malaria parasites” (Reece et al 2008, Nature)

 

Social evolution in viscous populations

One way for genetic relatedness to occur between social partners is if individuals tend not to disperse far during their lifetime, such that their neighbours tend to be genealogical kin. This has led to the expectation that indiscriminate cooperation will tend to be promoted by limited dispersal. However, in the simplest models of inelastic population structure, the cooperation-promoting effect of increased relatedness is exactly cancelled out by a cooperation-inhibiting effect of intensified local competition for resources. I develop theory to understand the demographic, genetic and cognitive modulators of cooperation in viscous populations, including the cooperation-promoting potential of sex-biased dispersal, dispersal-dependent social behaviour and weird ploidy.

Find out more:

“Evolution of helping and harming in heterogeneous groups” (Rodrigues & Gardner 2013, Evolution)

“A general ploidy model for the evolution of helping in viscous populations” (Yeh & Gardner 2012, J Theor Biol)

“Sex-biased dispersal of adults mediates the evolution of altruism among juveniles” (Gardner 2010, J Theor Biol)

“Nice natives and mean migrants: the evolution of dispersal-dependent social behaviour in viscous populations” (El Mouden & Gardner 2008, J Evol Biol)

 

Selection and adaptation in other media

One way to better understand the central Darwinian logic of selection and adaptation is to consider if and how it applies outwith the real-world biological realm. I’ve developed theory of kin selection under the assumption of blending inheritance, confirming that its logic does hold up and that Darwin’s musings on kin selection do make sense even though he didn’t know about genes. And, at the level of whole universes, I’ve used Price’s theorem to formalise hypotheses concerning the apparent fine-tuning of the fundamental constants of nature, including the “cosmological natural selection” and “anthropic bias” hypotheses. Much of this work is inevitably a bit tongue-in-cheek.

Find out more:

“Life, the universe and everything” (Gardner 2014, Biology & Philosophy)

“Cosmological natural selection and the purpose of the universe” (Gardner & Conlon 2013, Complexity)

“Kin selection under blending inheritance” (Gardner 2011, J Theor Biol)

“The Price equation” (Gardner 2008, Curr Biol)