Ecological drivers of the evolution of public-goods cooperation in bacteria

The role of ecological processes in the evolution of social traits is increasingly recognized. Here, we explore, using a general theoretical model and experiments with bacteria, the joint effects of disturbance frequency and resource supply on the evolution of cooperative biofilm formation. Our results demonstrate that cooperation tends to peak at intermediate frequencies of disturbance but that the peak shifts toward progressively higher frequencies of disturbance as resource supply increases. This appears to arise due to increased growth rates at higher levels of resource supply, which allows cooperators to more rapidly exceed the density threshold above which cooperation is beneficial following catastrophic disturbance. These findings demonstrate for the first time the importance of interactions between ecological processes in the evolution of public-goods cooperation and suggest that cooperation can be favored by selection across a wide range of ecological conditions.

Altruism, spite, and greenbeards

Hamilton's theory of inclusive fitness showed how natural selection could lead to behaviors that decrease the relative fitness of the actor and also either benefit (altruism) or harm (spite) other individuals. However, several fundamental issues in the evolution of altruism and spite have remained contentious. Here, we show how recent work has resolved three key debates, helping clarify how Hamilton's theoretical overview links to real-world examples, in organisms ranging from bacteria to humans: Is the evolution of extreme altruism, represented by the sterile workers of social insects, driven by genetics or ecology? Does spite really exist in nature? And, can altruism be favored between individuals who are not close kin but share a "greenbeard" gene for altruism?

The enforcement of cooperation by policing

Policing is regarded as an important mechanism for maintaining cooperation in human and animal social groups. A simple model providing a theoretical overview of the coevolution of policing and cooperation has been analyzed by Frank (1995, 1996b, 2003, 2009), and this suggests that policing will evolve to fully suppress cheating within social groups when relatedness is low. Here, we relax some of the assumptions made by Frank, and investigate the consequences for policing and cooperation. First, we address the implicit assumption that the individual cost of investment into policing is reduced when selfishness dominates. We find that relaxing this assumption leads to policing being favored only at intermediate relatedness. Second, we address the assumption that policing fully recovers the loss of fitness incurred by the group owing to selfishness. We find that relaxing this assumption prohibits the evolution of full policing. Finally, we consider the impact of demography on the coevolution of policing and cooperation, in particular the role for kin competition to disfavor the evolution of policing, using both a heuristic "open" model and a "closed" island model. We find that large groups and increased kin competition disfavor policing, and that policing is maintained more readily than it invades. Policing may be harder to evolve than previously thought.

Adaptation as organism design

The problem of adaptation is to explain the apparent design of organisms. Darwin solved this problem with the theory of natural selection. However, population geneticists, whose responsibility it is to formalize evolutionary theory, have long neglected the link between natural selection and organismal design. Here, I review the major historical developments in theory of organismal adaptation, clarifying what adaptation is and what it is not, and I point out future avenues for research.

Translaminar screw fixation of a kyphosis of the cervical and thoracic spine in neurofibromatosis

The spinal manifestations of neurofibromatosis include cervicothoracic kyphosis, in which scalloping of the vertebral body and erosion of the pedicles may render conventional techniques of fixation impossible. We describe a case of cervicothoracic kyphosis managed operatively with a vascularised fibular graft anteriorly across the apex of the kyphus, followed by a long posterior construct using translaminar screws, which allow segmental fixation in vertebral bodies where placement of the pedicle screws was impracticable.

Adaptation and the evolution of parasite virulence in a connected world

Adaptation is conventionally regarded as occurring at the level of the individual organism, where it functions to maximize the individual's inclusive fitness. However, it has recently been argued that empirical studies on the evolution of parasite virulence in spatial populations show otherwise. In particular, it has been claimed that the evolution of lower virulence in response to limited parasite dispersal provides proof of Wynne-Edwards's idea of adaptation at the group level. Although previous theoretical work has shown that limited dispersal can favour lower virulence, it has not clarified why, with five different suggestions having been given. Here we show that the effect of dispersal on parasite virulence can be understood entirely within the framework of inclusive fitness theory. Limited parasite dispersal favours lower parasite growth rates and, hence, reduced virulence because it (1) decreases the direct benefit of producing offspring (dispersers are worth more than non-dispersers, because they can go to patches with no or fewer parasites), and (2) increases the competition for hosts experienced by both the focal individual ('self-shading') and their relatives ('kin shading'). This demonstrates that reduced virulence can be understood as an individual-level adaptation by the parasite to maximize its inclusive fitness, and clarifies the links with virulence theory more generally.

Density dependence and cooperation: theory and a test with bacteria

Although cooperative systems can persist in nature despite the potential for exploitation by noncooperators, it is often observed that small changes in population demography can tip the balance of selective forces for or against cooperation. Here we consider the role of population density in the context of microbial cooperation. First, we account for conflicting results from recent studies by demonstrating theoretically that: (1) for public goods cooperation, higher densities are relatively unfavorable for cooperation; (2) in contrast, for self-restraint-type cooperation, higher densities can be either favorable or unfavorable for cooperation, depending on the details of the system. We then test our predictions concerning public goods cooperation using strains of the pathogenic bacterium Pseudomonas aeruginosa that produce variable levels of a public good-iron-scavenging siderophore molecules. As predicted, we found that the relative fitness of cheats (under-producers) was greatest at higher population densities. Furthermore, as assumed by theory, we show that this occurs because cheats are better able to exploit the cooperative siderophore production of other cells when they are physically closer to them.

Social evolution: this microbe will self-destruct

Phenotypic noise and social evolution in microbes have recently attracted huge interdisciplinary interest. A new study highlights the interaction between these phenomena and its implications for self-destructive cooperation.

Improvements in cognition, mood and behaviour following commencement of continuous subcutaneous insulin infusion therapy in children with type 1 diabetes mellitus: a pilot study

AIMS/HYPOTHESIS

Anecdotally, parents and teachers of children with type 1 diabetes mellitus report improvements in behaviour and learning following the commencement of continuous subcutaneous insulin infusion (CSII). This study aimed to investigate changes in cognition, mood and behaviour following commencement of CSII in children with type 1 diabetes.

METHODS

Children (n = 32) with type 1 diabetes aged 6-16 years and starting CSII at two Australian centres underwent behavioural, cognitive and glycaemic assessments prior to the commencement of CSII and 6-8 weeks after its start. A comprehensive cognitive test battery was administered, comprising measures of intelligence, attention, processing speed and executive skills. Behaviour and mood were assessed using the Behaviour Assessment System for Children--Second Edition. Continuous glucose monitoring was performed over a 72 h period and HbA(1c) was measured at both time-points.

RESULTS

After commencement of CSII, there were significant improvements in HbA(1c), a reduction in hyperglycaemia and blood glucose variation and an increase in normoglycaemia. Significant improvements were observed in perceptual reasoning, selective attention, divided attention, cognitive flexibility and working memory. Fewer mood-related symptoms were reported (parent, teacher and self-report) and fewer behavioural problems (parent reports)

CONCLUSIONS/INTERPRETATION

In this uncontrolled pilot study, children with type 1 diabetes demonstrated significant improvements in measures of metabolic control, mood and behaviour and in some complex cognitive skills after commencing CSII therapy.

Capturing the superorganism: a formal theory of group adaptation

Adaptation is conventionally regarded as occurring at the level of the individual organism. However, in recent years there has been a revival of interest in the possibility for group adaptations and superorganisms. Here, we provide the first formal theory of group adaptation. In particular: (1) we clarify the distinction between group selection and group adaptation, framing the former in terms of gene frequency change and the latter in terms of optimization; (2) we capture the superorganism in the form of a 'group as maximizing agent' analogy that links an optimization program to a model of a group-structured population; (3) we demonstrate that between-group selection can lead to group adaptation, but only in rather special circumstances; (4) we provide formal support for the view that between-group selection is the best definition for 'group selection'; and (5) we reveal that mechanisms of conflict resolution such as policing cannot be regarded as group adaptations.

Limited dispersal, budding dispersal, and cooperation: an experimental study

Numerous theoretical studies have investigated how limited dispersal may provide an explanation for the evolution of cooperation, by leading to interactions between relatives. However, despite considerable theoretical attention, there has been a lack of empirical tests. In this article, we test how patterns of dispersal influence the evolution of cooperation, using iron-scavenging in the bacterium Pseudomonas aeruginosa as our cooperative trait. We found that relatively limited dispersal does not favor cooperation. The reason for this is that although limited dispersal increases the relatedness between interacting individuals, it also leads to increased local competition for resources between relatives. This result supports Taylor's prediction that in the simplest possible scenario, the effects of increased relatedness and local competition exactly cancel out. In contrast, we show that one way for cooperation to be favored is if individuals disperse in groups (budding dispersal), because this maintains high relatedness while reducing local competition between relatives (relatively global competition).

Nice natives and mean migrants: the evolution of dispersal-dependent social behaviour in viscous populations

There has been much interest in the evolution of social behaviour in viscous populations. While low dispersal increases the relatedness of neighbours, which tends to promote the evolution of indiscriminate helping behaviour, it can also increase competition between neighbours, which tends to inhibit the evolution of helping and may even favour harming behaviour. In the simplest scenario, these two effects exactly cancel, so that dispersal rate has no impact on the evolution of helping or harming. Here, we show that dispersal rate does matter when individuals can adjust their social behaviour conditional on whether they have dispersed or whether they have remained close to their place of origin. We find that nondispersing individuals are weakly favoured to indiscriminately help their neighbours, whereas dispersing individuals are more readily favoured to indiscriminately harm their neighbours.

Evaluation of Nasopharyngeal Oxygen, Nasal Prongs and Facemask Oxygen Therapy Devices in Adult Patients: A Randomised Crossover Trial

Nasopharyngeal oxygen (NPO) therapy may overcome some of the difficulties associated with nasal prongs and facemask oxygen delivery devices. In response to a lack of published studies of NPO therapy in adults, we conducted a prospective randomised crossover trial to compare the effectiveness of NPO, nasal prongs (NP) and facemasks (FM) when used in an adult population (n=37) from the intensive care unit and general hospital wards. We measured oxygen saturation (SpO 2 using pulse oximetry, oxygen flow (litres per minute), respiration rate (per minute) and comfort using a horizontal visual analogue scale. All three devices were effective in maintaining a Sp0 2 of ≥95% (NP 97.0±1.9, NPO 97.7±1.7, FM 98.8±1.3%). NPO therapy consumed less oxygen than NP and FM therapy (NP 2.6±1.0, NPO 2.2±0.9, FM 6.1 ±0.4 l/min, P <0.001). There was no significant difference in patients’ respiratory rates (NP 19.9±3.2, NPO 19.9±3.0, FM 19.8±3.1 per minute, P=0.491). In terms of comfort, patients rated NP higher than NPO and FM using a horizontal visual analogue scale (100 mm=most comfortable) (NP 65.5±14.3, NPO 62.8±19.4, FM 49.4 ± 21.4 mm, P <0.001). We conclude that for adult patients, nasal prongs and nasopharyngeal oxygen therapy consume less oxygen and provide greater comfort than facemasks while still maintaining Sp0 2 ≥95%.

Sex ratio adjustment and kin discrimination in malaria parasites

Malaria parasites and related Apicomplexans are the causative agents of the some of the most serious infectious diseases of humans, companion animals, livestock and wildlife. These parasites must undergo sexual reproduction to transmit from vertebrate hosts to vectors, and their sex ratios are consistently female-biased. Sex allocation theory, a cornerstone of evolutionary biology, is remarkably successful at explaining female-biased sex ratios in multicellular taxa, but has proved controversial when applied to malaria parasites. Here we show that, as predicted by theory, sex ratio is an important fitness-determining trait and Plasmodium chabaudi parasites adjust their sex allocation in response to the presence of unrelated conspecifics. This suggests that P. chabaudi parasites use kin discrimination to evaluate the genetic diversity of their infections, and they adjust their behaviour in response to environmental cues. Malaria parasites provide a novel way to test evolutionary theory, and support the generality and power of a darwinian approach.

Resource supply and the evolution of public-goods cooperation in bacteria

Background

Explaining public-goods cooperation is a challenge for evolutionary biology. However, cooperation is expected to more readily evolve if it imposes a smaller cost. Such costs of cooperation are expected to decline with increasing resource supply, an ecological parameter that varies widely in nature. We experimentally tested the effect of resource supply on the evolution of cooperation using two well-studied bacterial public-good traits: biofilm formation by Pseudomonas fluorescens and siderophore production by Pseudomonas aeruginosa.

Results

The frequency of cooperative bacteria increased with resource supply in the context of both bacterial public-good traits. In both cases this was due to decreasing costs of investment into public-goods cooperation with increasing resource supply.

Conclusion

Our empirical tests with bacteria suggest that public-goods cooperation is likely to increase with increasing resource supply due to reduced costs of cooperation, confirming that resource supply is an important factor in the evolution of cooperation.

Pacing a small cage: mutation and RNA viruses

RNA viruses have an extremely high mutation rate, and we argue that the most plausible explanation for this is a trade-off with replication speed. We suggest that research into further increasing this mutation rate artificially as an antiviral treatment requires a theoretical reevaluation, especially relating to the so-called error threshold. The main evolutionary consequence of a high mutation rate appears to have been to restrict RNA viruses to a small genome; they thus rapidly exploit a limited array of possibilities. Investigating this constraint to their evolution, and how it is occasionally overcome, promises to be fruitful. We explain the many terms used in investigating RNA viral evolution and highlight the specific experimental and comparative work that needs to be done.

Mosaicism in autosomal dominant polycystic kidney disease revealed by genetic testing to enable living related renal transplantation

Patients with end-stage renal disease (ESRD) secondary to autosomal dominant polycystic kidney disease (ADPKD) receive fewer living-related kidney (LRK) transplants than other groups with ESRD. This relates to the difficulties in excluding the disease in potential donors. We report a case which highlights these difficulties and, by discovery of mosaicism for a new mutation, illustrates the role of clinical and molecular genetic resources in assessing young related kidney donors for patients with ADPKD.

Social evolution: the decline and fall of genetic kin recognition

Animals should benefit from the ability to recognise their kin, yet curiously this faculty is often absent. New theory confirms that genetic kin recognition is inherently unstable, explaining its rarity.

Evolutionary theory of bacterial quorum sensing: when is a signal not a signal?

The term quorum sensing (QS) is used to describe the communication between bacterial cells, whereby a coordinated population response is controlled by diffusible molecules produced by individuals. QS has not only been described between cells of the same species (intraspecies), but also between species (interspecies) and between bacteria and higher organisms (inter-kingdom). The fact that QS-based communication appears to be widespread among microbes is strange, considering that explaining both cooperation and communication are two of the greatest problems in evolutionary biology. From an evolutionary perspective, intraspecies signalling can be explained using models such as kin selection, but when communication is described between species, it is more difficult to explain. It is probable that in many cases this involves QS molecules being used as 'cues' by other species as a guide to future action or as manipulating molecules whereby one species will 'coerce' a response from another. In these cases, the usage of QS molecules cannot be described as signalling. This review seeks to integrate the evolutionary literature on animal signalling with the microbiological literature on QS, and asks whether QS within bacteria is true signalling or whether these molecules are also used as cues or for the coercion of other cells.

Is bacterial persistence a social trait?

The ability of bacteria to evolve resistance to antibiotics has been much reported in recent years. It is less well-known that within populations of bacteria there are cells which are resistant due to a non-inherited phenotypic switch to a slow-growing state. Although such ‘persister’ cells are receiving increasing attention, the evolutionary forces involved have been relatively ignored. Persistence has a direct benefit to cells because it allows survival during catastrophes–a form of bet-hedging. However, persistence can also provide an indirect benefit to other individuals, because the reduced growth rate can reduce competition for limiting resources. This raises the possibility that persistence is a social trait, which can be influenced by kin selection. We develop a theoretical model to investigate the social consequences of persistence. We predict that selection for persistence is increased when: (a) cells are related (e.g. a single, clonal lineage); and (b) resources are scarce. Our model allows us to predict how the level of persistence should vary with time, across populations, in response to intervention strategies and the level of competition. More generally, our results clarify the links between persistence and other bet-hedging or social behaviours.

Evolutionary Explanations for Cooperation

Natural selection favours genes that increase an organism's ability to survive and reproduce. This would appear to lead to a world dominated by selfish behaviour. However, cooperation can be found at all levels of biological organisation: genes cooperate in genomes, organelles cooperate to form eukaryotic cells, cells cooperate to make multicellular organisms, bacterial parasites cooperate to overcome host defences, animals breed cooperatively, and humans and insects cooperate to build societies. Over the last 40 years, biologists have developed a theoretical framework that can explain cooperation at all these levels. Here, we summarise this theory, illustrate how it may be applied to real organisms and discuss future directions.

Frequency dependence and cooperation: theory and a test with bacteria

Hamilton's inclusive fitness theory provides a leading explanation for the problem of cooperation. A general result from inclusive fitness theory is that, except under restrictive conditions, cooperation should not be subject to frequency-dependent selection. However, several recent studies in microbial systems have demonstrated that the relative fitness of cheaters, which do not cooperate, is greater when cheaters are rarer. Here we demonstrate theoretically that such frequency-dependent selection can occur in microbes when there is (1) sufficient population structuring or (2) an association between the level of cooperation and total population growth. We test prediction (2) and its underlying assumption, using the pathogenic bacterium Pseudomonas aeruginosa, by competing strains that produce iron-scavenging siderophore molecules (cooperators) with nonproducers (cheaters) at various ratios, under conditions that minimize population structuring. We found that both the relative fitness of cheaters and the productivity of the mixed culture were significantly negatively related to initial cheater frequency. Furthermore, when the period of population growth was experimentally shortened, the strength of frequency dependence was reduced. More generally, we argue that frequency-dependent selection on cooperative traits may be more common in microbes than in metazoans because strong selection, structuring, and cooperation-dependent growth will be more common in microbial populations.

Cooperation Peaks at Intermediate Disturbance

Explaining cooperation is a challenge for evolutionary biology. Surprisingly, the role of extrinsic ecological parameters remains largely unconsidered. Disturbances are widespread in nature and have evolutionary consequences. We develop a mathematical model predicting that cooperative traits most readily evolve at intermediate disturbance. Under infrequent disturbance, cooperation breaks down through the accumulation of evolved cheats. Higher rates of disturbance prevent this because the resulting bottlenecks increase genetic structuring (relatedness) promoting kin selection for cooperation. However, cooperation cannot be sustained under very frequent disturbance if population density remains below the level required for successful cooperation. We tested these predictions by using cooperative biofilm formation by the bacterium Pseudomonas fluorescens. The proportion of biofilm-forming bacteria peaked at intermediate disturbance, in a manner consistent with model predictions. Under infrequent and intermediate disturbance, most bacteria occupied the biofilm, but the proportion of cheats was higher under less frequent disturbance. Under frequent disturbance, many bacteria did not occupy the biofilm, suggesting that biofilm dwelling was not as beneficial under frequent versus intermediate disturbance. Given the ubiquity of disturbances in nature, these results suggest that they may play a major role in the evolution of social traits in microbes.

Social semantics: altruism, cooperation, mutualism, strong reciprocity and group selection

From an evolutionary perspective, social behaviours are those which have fitness consequences for both the individual that performs the behaviour, and another individual. Over the last 43 years, a huge theoretical and empirical literature has developed on this topic. However, progress is often hindered by poor communication between scientists, with different people using the same term to mean different things, or different terms to mean the same thing. This can obscure what is biologically important, and what is not. The potential for such semantic confusion is greatest with interdisciplinary research. Our aim here is to address issues of semantic confusion that have arisen with research on the problem of cooperation. In particular, we: (i) discuss confusion over the terms kin selection, mutualism, mutual benefit, cooperation, altruism, reciprocal altruism, weak altruism, altruistic punishment, strong reciprocity, group selection and direct fitness; (ii) emphasize the need to distinguish between proximate (mechanism) and ultimate (survival value) explanations of behaviours. We draw examples from all areas, but especially recent work on humans and microbes.

The causes and consequences of variation in offspring size: a case study using Daphnia

Offspring size can have large and direct fitness implications, but we still do not have a complete understanding of what causes offspring size to vary. Daphnia (water fleas) generally produce fewer and larger offspring when food is limited. Here, we use a mathematical model to show that this could be explained by either: (1) an advantage of producing larger eggs when food is limited; or (2) a lower boundary on egg volume (below which eggs do not have sufficient resources to be viable), that is similar in volume to the evolutionarily stable egg volume predicted by standard clutch size models. We tested the first possibilities experimentally by placing offspring from mothers kept at two food treatments (high and low - leading to relatively small and large eggs respectively) into two food treatments (same as maternal treatments, in a fully factorial design) and measuring their fitness (reproduction, age at maturity, and size at maturity). We also tested survival under starvation conditions of offspring produced from mothers at low and high food treatments. We found that (larger) offspring produced by low-food mothers actually had lower fitness as they took longer to reproduce, regardless of their current food treatment. Additionally, we found no survival advantage to being born of a food-stressed mother. Consequently, our results do not support the hypothesis that there is an advantage to producing larger eggs when food is limited. In contrast, data from the literature support the importance of a lower boundary on egg size.

Spiteful soldiers and sex ratio conflict in polyembryonic parasitoid wasps

The existence of spiteful behaviors remains controversial. Spiteful behaviors are those that are harmful to both the actor and the recipient, and they represent one of the four fundamental types of social behavior (alongside selfishness, altruism, and mutual benefit). It has generally been assumed that the conditions required for spite to evolve are too restrictive, and so spite is unlikely to be important. This idea has been challenged in recent years, with the realization that localized competition can relax the conditions required for spite to evolve. Here we develop a theoretical model for a prime candidate for a spiteful behavior, the production of the sterile soldier caste in polyembryonic wasps. Our results show that (a) the biology of these soldiers is consistent with their main role being to mediate conflict over the sex ratio and not to defend against competitors and (b) greater conflict will occur in more outbred populations. We also show that the production of the sterile soldier caste can be classed as a spiteful behavior but that, to an extent, this is merely a semantic choice, and other interpretations such as altruism or indirect altruism are valid. However, the spite interpretation is useful in that it can lead to a more natural interpretation of relatedness and facilitate the classification of behaviors in a way that emphasizes biologically interesting differences that can be empirically tested.

Direct fitness or inclusive fitness: how shall we model kin selection?

Two standard mathematical formulations of kin-selection models can be found. Inclusive fitness is an actor-centred approach, which calculates the fitness effect on a number of recipients of the behaviour of a single actor. Direct fitness is a recipient-centred approach, which calculates the fitness effect on the recipient of the behaviour of a number of actors. Inclusive fitness offers us a powerful heuristic, of choosing behaviour to maximize fitness, but direct fitness can be mathematically easier to work with and has recently emerged as the preferred approach of theoreticians. In this paper, we explore the fundamental connection between these two approaches in both homogeneous and class-structured populations, and we show that under simple assumptions (mainly fair meiosis and weak selection) they provide equivalent formulations, which correspond to the predictions of Price's equation for allele frequency change. We use a couple of examples to highlight differences in their conception and formulation, and we briefly discuss a two-species example in which we have a class of 'actor' that is never a 'recipient', which the standard direct fitness method can handle but the usual inclusive fitness cannot.

The relation between multilocus population genetics and social evolution theory

Evolution at multiple gene positions is complicated. Direct selection on one gene disturbs the evolutionary dynamics of associated genes. Recent years have seen the development of a multilocus methodology for modeling evolution at arbitrary numbers of gene positions with arbitrary dominance and epistatic relations, mode of inheritance, genetic linkage, and recombination. We show that the approach is conceptually analogous to social evolutionary methodology, which focuses on selection acting on associated individuals. In doing so, we (1) make explicit the links between the multilocus methodology and the foundations of social evolution theory, namely, Price's theorem and Hamilton's rule; (2) relate the multilocus approach to levels-of-selection and neighbor-modulated-fitness approaches in social evolution; (3) highlight the equivalence between genetical hitchhiking and kin selection; (4) demonstrate that the multilocus methodology allows for social evolutionary analyses involving coevolution of multiple traits and genetical associations between nonrelatives, including individuals of different species; (5) show that this methodology helps solve problems of dynamic sufficiency in social evolution theory; (6) form links between invasion criteria in multilocus systems and Hamilton's rule of kin selection; (7) illustrate the generality and exactness of Hamilton's rule, which has previously been described as an approximate, heuristic result.

Demography, altruism, and the benefits of budding

It is now widely appreciated that competition between kin inhibits the evolution of altruism. In standard population genetics models, it is difficult for indiscriminate altruism towards social partners to be favoured at all. The reason is that while limited dispersal increases the kinship of social partners it also intensifies local competition. One solution that has received very little attention is if individuals disperse as groups (budding dispersal), as this relaxes local competition without reducing kinship. Budding behaviour is widespread through all levels of biological organization, from early protocellular life to cooperatively breeding vertebrates. We model the effects of individual dispersal, budding dispersal, soft selection and hard selection to examine the conditions under which altruism is favoured. More generally, we examine how these various demographic details feed into relatedness and scale of competition parameters that can be included into Hamilton's rule.

Social evolution theory for microorganisms

Microorganisms communicate and cooperate to perform a wide range of multicellular behaviours, such as dispersal, nutrient acquisition, biofilm formation and quorum sensing. Microbiologists are rapidly gaining a greater understanding of the molecular mechanisms involved in these behaviours, and the underlying genetic regulation. Such behaviours are also interesting from the perspective of social evolution - why do microorganisms engage in these behaviours given that cooperative individuals can be exploited by selfish cheaters, who gain the benefit of cooperation without paying their share of the cost? There is great potential for interdisciplinary research in this fledgling field of sociomicrobiology, but a limiting factor is the lack of effective communication of social evolution theory to microbiologists. Here, we provide a conceptual overview of the different mechanisms through which cooperative behaviours can be stabilized, emphasizing the aspects most relevant to microorganisms, the novel problems that microorganisms pose and the new insights that can be gained from applying evolutionary theory to microorganisms.

Cooperation and the Scale of Competition in Humans

Explaining cooperation is one of the greatest challenges for evolutionary biology. It is particularly a problem in species such as humans, where there is cooperation between nonrelatives. Numerous possible solutions have been suggested for the problem of cooperation between nonrelatives, including punishment, policing, and various forms of reciprocity. Here, we suggest that local competition for resources can pose a problem for these hypotheses, analogous to how it can select against cooperation between relatives. We extend the prisoner's dilemma (PD) game to show that local competition between interacting individuals can reduce selection for cooperation between nonrelatives. This is because, with local competition, fitness is relative to social partners, and cooperation benefits social partners. We then test whether nonrelated humans adjust their level of cooperation facultatively in response to the scale of competition when playing the PD for cash prizes. As predicted, we found that individuals were less likely to cooperate when competition was relatively local. Cooperation between humans will therefore be most likely when repeated interactions take place on a local scale between small numbers of people, and competition for resources takes place on a more global scale among large numbers of people.

Evidence against the hypothesis that BCL-2 inhibits apoptosis through an anti-oxidant effect

We contrasted possible protection against apoptosis afforded by either BCL-2 expression or anti-oxidant inhibitors in the same tumor target challenged by two distinct triggers of apoptosis. Exposure of L929 fibroblasts to tumor necrosis factor (TNF) or etoposide (VP-16) induced apoptotic death with similar kinetics. Enforced expression of BCL-2 significantly protected against apoptosis induced by VP-16 but had no effect against TNF-induced apoptosis. In contrast, the anti-oxidants desferrioxamine, butylated hydroxyanisol and N-acetyl cysteine all inhibited TNF-induced apoptosis in a concentration-dependent fashion. Although exposure to VP-16 resulted in a significant generation of intracellular oxyradicals, the above three anti-oxidant inhibitors had no effect on VP-16-induced apoptotic death. Interestingly, enforced expression of BCL-2 also inhibited the ability of VP-16 to generate oxy-radicals and to depress intracellular glutathione levels. These results indicate that BCL-2 can exert anti-oxidant effects but argue against the hypothesis that these effects are critical to its protection against apoptosis.

Recombination and the evolution of mutational robustness

Mutational robustness is the degree to which a phenotype, such as fitness, is resistant to mutational perturbations. Since most of these perturbations will tend to reduce fitness, robustness provides an immediate benefit for the mutated individual. However, robust systems decay due to the accumulation of deleterious mutations that would otherwise have been cleared by selection. This decay has received very little theoretical attention. At equilibrium, a population or asexual lineage is expected to have a mutation load that is invariant with respect to the selection coefficient of deleterious alleles, so the benefit of robustness (at the level of the population or asexual lineage) is temporary. However, previous work has shown that robustness can be favoured when robustness loci segregate independently of the mutating loci they act upon. We examine a simple two-locus model that allows for intermediate rates of recombination and inbreeding to show that increasing the effective recombination rate allows for the evolution of greater mutational robustness.