Antagonistic interactions subdue inter‐species green‐beard cooperation in bacteria

Cooperation can be favoured through the green‐beard mechanism, where a set of linked genes encodes both a cooperative trait and a phenotypic marker (green beard), which allows carriers of the trait to selectively direct cooperative acts to other carriers. In theory, the green‐beard mechanism should favour cooperation even when interacting partners are totally unrelated at the genome level. Here, we explore such an extreme green‐beard scenario between two unrelated bacterial species—Pseudomonas aeruginosa and Burkholderia cenocepacia, which share a cooperative locus encoding the public good pyochelin (an iron‐scavenging siderophore) and its cognate receptor (green beard) required for iron–pyochelin uptake. We show that pyochelin, when provided in cell‐free supernatants, can be mutually exchanged between species and provide fitness benefits under iron limitation. However, in co‐culture we observed that these cooperative benefits vanished and communities were dominated by P. aeruginosa, regardless of strain background and species starting frequencies. Our results further suggest that P. aeruginosa engages in interference competition to suppress B. cenocepacia, indicating that inter‐species conflict arising from dissimilarities at the genome level overrule the aligned cooperative interests at the pyochelin locus. Thus, green‐beard cooperation is subdued by competition, indicating that interspecific siderophore cooperation is difficult to evolve and to be maintained.     

An evolutionary analysis of the relationship between spite and altruism

We investigate the selective pressures on a social trait when evolution occurs in a population of constant size. We show that any social trait that is spiteful simultaneously qualifies as altruistic. In other words, any trait that reduces the fitness of less related individuals necessarily increases that of related ones. Our analysis demonstrates that the distinction between "Hamiltonian spite" and "Wilsonian spite" is not justified on the basis of fitness effects. We illustrate this general result with an explicit model for the evolution of a social act that reduces the recipient's survival ("harming trait"). This model shows that the evolution of harming is favoured if local demes are of small size and migration is low (philopatry). Further, deme size and migration rate determine whether harming evolves as a selfish strategy by increasing the fitness of the actor, or as a spiteful/altruistic strategy through its positive effect on the fitness of close kin.     

Altruism and mental disorders

Data suggest that the theories of kin selection and reciprocal altruism are viable working models to explain altruistic behavior. It remains to be demonstrated if these models can explain the behavior of persons with mentaL disorders for whom altruistic behavior is reported to be reduced. This paper addresses this issue. Part I reviews proximate factors that are thought to influence both altruistic decision making and interindividual variation in altruistic behavior. The focus is on trait signaling by potential beneficiaries and the evaluation of signals and altruistic decision making by potential altruists. In Part II, points developed in Part I are combined with clinical and empirical findings to analyze data on personality disorders and dysthymic disorder. The analysis leads to three causal hypotheses: Reduced altruistic behavior may be an evolved strategy, a consequence of dysfunctional recognition systems or algorithms, and/or a secondary response to an increase in symptoms. Different disorders and features of disorders are explained by each hypothesis.     

Selective pressures for accurate altruism targeting: evidence from digital evolution for difficult-to-test aspects of inclusive fitness theory

Inclusive fitness theory predicts that natural selection will favour altruist genes that are more accurate in targeting altruism only to copies of themselves. In this paper, we provide evidence from digital evolution in support of this prediction by competing multiple altruist-targeting mechanisms that vary in their accuracy in determining whether a potential target for altruism carries a copy of the altruist gene. We compete altruism-targeting mechanisms based on (i) kinship (kin targeting), (ii) genetic similarity at a level greater than that expected of kin (similarity targeting), and (iii) perfect knowledge of the presence of an altruist gene (green beard targeting). Natural selection always favoured the most accurate targeting mechanism available. Our investigations also revealed that evolution did not increase the altruism level when all green beard altruists used the same phenotypic marker. The green beard altruism levels stably increased only when mutations that changed the altruism level also changed the marker (e.g. beard colour), such that beard colour reliably indicated the altruism level. For kin- and similarity-targeting mechanisms, we found that evolution was able to stably adjust altruism levels. Our results confirm that natural selection favours altruist genes that are increasingly accurate in targeting altruism to only their copies. Our work also emphasizes that the concept of targeting accuracy must include both the presence of an altruist gene and the level of altruism it produces.     

Charitable giving as a signal of trustworthiness: Disentangling the signaling benefits of altruistic acts

It has been shown that psychological predispositions to benefit others can motivate human cooperation and the evolution of such social preferences can be explained with kin or multi-level selection models. It has also been shown that cooperation can evolve as a costly signal of an unobservable quality that makes a person more attractive with regard to other types of social interactions. Here we show that if a proportion of individuals with social preferences is maintained in the population through kin or multi-level selection, cooperative acts that are truly altruistic can be a costly signal of social preferences and make altruistic individuals more trustworthy interaction partners in social exchange. In a computerized laboratory experiment, we test whether altruistic behavior in the form of charitable giving is indeed correlated with trustworthiness and whether a charitable donation increases the observing agents' trust in the donor. Our results support these hypotheses and show that, apart from trust, responses to altruistic acts can have a rewarding or outcome-equalizing purpose. Our findings corroborate that the signaling benefits of altruistic acts that accrue in social exchange can ease the conditions for the evolution of social preferences.     

Not only states but traits — Humans can identify permanent altruistic dispositions in 20 s

Humans behave altruistically in one-shot interactions under total anonymity. In search of explanations for such behavior, it has been argued that at least some individuals have a general tendency to behave altruistically independent of profitability. In fact, a stable altruistic trait would be adaptive if it were recognizable. Then, altruists could choose each other in order to retain benefits through mutual cooperation. Previous research has shown that individuals can predict the degree of altruistic behavior of strangers by reading signs of emotions evoked in significant social decisions. However, the identification of benevolent emotional states is no guarantee of the existence of permanent altruistic traits, though permanent traits are the preferable criterion for selection of good interaction partners. In this study, we tested whether individuals are able to identify altruistic traits. Judges watched 20-s silent video clips of unacquainted target persons and were asked to estimate the behavior of these target persons in a money-sharing task. As the videotapes of the target persons had been recorded in a setting unrelated to altruistic behavior, the judges could not base their estimates on situational cues related to the money-sharing task but instead had to draw on stable signals of altruism. Estimates were significantly better than chance, indicating that individuals can identify permanent altruistic traits in others. As this mechanism raises opportunities for selective interactions between altruists, our findings are discussed with respect to their relevance for explaining the evolution of altruism through assortment.     

Group selection in plant populations

Summary Several mechanisms have been proposed for group selection, to account for the evolution of altruistic traits. One type, Neighbourhood models, suggests that individuals react with those immediately around them, but with no recognition mechanism. The organization of plant populations seems especially favorable for this type of selection. The possibility of Neighbourhood selection was investigated by simulating a plant population. It was possible for an altruistic trait to evolve, though only under restricted conditions. The main requirement was gene flow only by very restricted pollen dispersal, and a high benefit : cost ratio in the altruistic relationship. Under conditions favourable for such evolution, the starting frequency of the allele, the initial pattern, and the population size, had little effect. Inbreeding tended to prevent the increase of the altruism allele, though this depended on the mechanism of selfing. Known ecological features of plants are discussed that could be considered altruistic and hence require some form of group selection for their evolution, and whether the benefit : cost requirements are likely to be met. Neighbourhood models of group selection are a possibility in plant populations, and we therefore cannot exclude the possibility of altruism in plants. However, Neighbourhood selection is weak force, unlikely to be effective in the face of opposing individual selection. It may be more important as reinforcement of individual selection.     

Dual Utility of a Melanin‐Based Ornament in Bearded Tits

A general mechanism to account for the maintenance of sexually selected traits is the armament–ornament model, which predicts that females exploit signals used in male–male aggressive contests. Melanin‐based ornaments are good candidates to be dual utility traits because they are usually involved in male–male competition and function as signals of male dominance or fighting ability. Despite this, very few studies have investigated the dual utility of melanin‐based plumage traits and results remain generally ambiguous. In this study, we investigated the dual function of a melanin‐based trait (the black beard) in the male bearded tit, Panurus biarmicus. In a controlled aviary experiment, beard length was the best predictor of dominance in male–male competition. In a choice experiment, females showed a proximity preference for males whose beard length was experimentally elongated. Our results indicate that beard length plays an important role in both male–male competition and female choice in bearded tits, as proposed by the dual utility model.     

The Role of Compassion in Altruistic Helping and Punishment Behavior

Compassion, the emotional response of caring for another who is suffering and that results in motivation to relieve suffering, is thought to be an emotional antecedent to altruistic behavior. However, it remains unclear whether compassion enhances altruistic behavior in a uniform way or is specific to sub-types of behavior such as altruistic helping of a victim or altruistic punishment of a transgressor. We investigated the relationship between compassion and subtypes of altruistic behavior using third-party paradigms where participants 1) witnessed an unfair economic exchange between a transgressor and a victim, and 2) had the opportunity to either spend personal funds to either economically a) help the victim or b) punish the transgressor. In Study 1, we examined whether individual differences in self-reported empathic concern (the emotional component of compassion) was associated with greater altruistic helping or punishment behavior in two independent samples. For participants who witnessed an unfair transaction, trait empathic concern was associated with greater helping of a victim and had no relationship to punishment. However, in those who decided to punish the transgressor, participants who reported greater empathic concern decided to punish less. In Study 2, we directly enhanced compassion using short-term online compassion meditation training to examine whether altruistic helping and punishment were increased after two weeks of training. Compared to an active reappraisal training control group, the compassion training group gave more to help the victim and did not differ in punishment of the transgressor. Together, these two studies suggest that compassion is related to greater altruistic helping of victims and is not associated with or may mitigate altruistic punishment of transgressors.     

Mutual benefit can promote the evolution of preferential interactions and in this way can lead to the evolution of true altruism

We analyse the evolution of the assortment of encounters through active choice of companions among individuals that interact cooperatively in a situation of mutual benefit. Using a simple mathematical model, we show that mutual benefit can favour the evolution of a preference to interact with individuals that are similar to themselves with respect to an arbitrary tag even when both the preference and the tag depend on two independent and unlinked genes. Two necessary requisites to obtain this result are: (i) a small population or a large subdivided metapulation and (ii) an asymmetry between partners in such a way that one of them (donor) proposes the cooperation and elects the partner, whereas the other (receiver) never rejects the offer. We also show that mutual benefit can be the starting point for the evolution of altruistic behaviours as long as there are preferential interactions. This requires that the tag used in the election of partners is the altruistic or selfish behaviour itself.     

Multilevel selection: the evolution of cooperation in non-kin groups

Hamiltons (1964a, 1964b) landmark papers are rightly recognized as the formal basis for our understanding of the evolution of altruistic traits. However, Hamiltons equation as he originally expressed it is simplistic. A genetically oriented approach to studying multilevel selection can provide insights into how the terminology and assumptions used by Hamilton can be generalized. Using contextual analysis I demonstrated that Hamiltons rule actually embodies three distinct processes, group selection, individual selection, and transmission genetics or heritability. Whether an altruistic trait will evolve depends the balance of all of these factors. The genetical approach, and particularly, contextual analysis provides a means of separating these factors and examining them one at a time. Perhaps the greatest issue with Hamiltons equation is the interpretation of r. Hamilton (1964a) interpreted this as relatedness. In this paper I show that what Hamilton called relatedness is more generally interpreted as the proportion for variance among groups, and that many processes in addition to relatedness can increase the variance among groups. I also show that the evolution of an altruistic trait is driven by the ratio of the heritability at the group level to the heritability at the individual level. Under some circumstances this ratio can be greater than 1. In this situation altruism can evolve even if selection favoring selfish behavior is stronger than selection favoring altruism.     

Does breeding season variation affect evolution of a sexual signaling trait in a tropical lizard clade?

Sexually selected traits can be expected to increase in importance when the period of sexual behavior is constrained, such as in seasonally restricted breeders. Anolis lizard male dewlaps are classic examples of multifaceted signaling traits, with demonstrated intraspecific reproductive function reflected in courtship behavior. Fitch and Hillis found a correlation between dewlap size and seasonality in mainland Anolis using traditional statistical methods and suggested that seasonally restricted breeding seasons enhanced the differentiation of this signaling trait. Here, we present two tests of the Fitch–Hillis Hypothesis using new phylogenetic and morphological data sets for 44 species of Mexican Anolis. A significant relationship between dewlap size and seasonality is evident in phylogenetically uncorrected analyses but erodes once phylogeny is accounted for. This loss of strong statistical support for a relationship between a key aspect of dewlap morphology and seasonality also occurs within a species complex (A. sericeus group) that inhabits seasonal and aseasonal environments. Our results fail to support seasonality as a strong driver of evolution of Anolis dewlap size. We discuss the implications of our results and the difficulty of disentangling the strength of single mechanisms on trait evolution when multiple selection pressures are likely at play.     

Social evolution of toxic metal bioremediation in Pseudomonas aeruginosa

Bacteria are often iron-limited, and hence produce extracellular iron-scavenging siderophores. A crucial feature of siderophore production is that it can be an altruistic behaviour (individually costly but benefitting neighbouring cells), thus siderophore producers can be invaded by non-producing social ‘cheats’. Recent studies have shown that siderophores can also bind other heavy metals (such as Cu and Zn), but in this case siderophore chelation actually reduces metal uptake by bacteria. These complexes reduce heavy metal toxicity, hence siderophore production may contribute to toxic metal bioremediation. Here, we show that siderophore production in the context of bioremediation is also an altruistic trait and can be exploited by cheating phenotypes in the opportunistic pathogen Pseudomonas aeruginosa. Specifically, we show that in toxic copper concentrations (i) siderophore non-producers evolve de novo and reach high frequencies, and (ii) producing strains are fitter than isogenic non-producing strains in monoculture, and vice versa in co-culture. Moreover, we show that the evolutionary effect copper has on reducing siderophore production is greater than the reduction observed under iron-limited conditions. We discuss the relevance of these results to the evolution of siderophore production in natural communities and heavy metal bioremediation.     

ON THE PARADIGM OF ALTRUISTIC SUICIDE IN THE UNICELLULAR WORLD

Altruistic suicide is best known in the context of programmed cell death (PCD) in multicellular individuals, which is understood as an adaptive process that contributes to the development and functionality of the organism. After the realization that PCD‐like processes can also be induced in single‐celled lineages, the paradigm of altruistic cell death has been extended to include these active cell death processes in unicellular organisms. Here, we critically evaluate the current conceptual framework and the experimental data used to support the notion of altruistic suicide in unicellular lineages, and propose new perspectives. We argue that importing the paradigm of altruistic cell death from multicellular organisms to explain active death in unicellular lineages has the potential to limit the types of questions we ask, thus biasing our understanding of the nature, origin, and maintenance of this trait. We also emphasize the need to distinguish between the benefits and the adaptive role of a trait. Lastly, we provide an alternative framework that allows for the possibility that active death in single‐celled organisms is a maladaptive trait maintained as a byproduct of selection on pro‐survival functions, but that could—under conditions in which kin/group selection can act—be co‐opted into an altruistic trait.     

The co-evolution of culturally inherited altruistic helping and cultural transmission under random group formation

Limited migration results in kin selective pressure on helping behaviors under a wide range of ecological, demographic and life-history situations. However, such genetically determined altruistic helping can evolve only when migration is not too strong and group size is not too large. Cultural inheritance of helping behaviors may allow altruistic helping to evolve in groups of larger size because cultural transmission has the potential to markedly decrease the variance within groups and augment the variance between groups. Here, we study the co-evolution of culturally inherited altruistic helping behaviors and two alternative cultural transmission rules for such behaviors. We find that conformist transmission, where individuals within groups tend to copy prevalent cultural variants (e.g., beliefs or values), has a strong adverse effect on the evolution of culturally inherited helping traits. This finding is at variance with the commonly held view that conformist transmission is a crucial factor favoring the evolution of altruistic helping in humans. By contrast, we find that under one-to-many transmission, where individuals within groups tend to copy a “leader” (or teacher), altruistic helping can evolve in groups of any size, although the cultural transmission rule itself hitchhikes rather weakly with a selected helping trait. Our results suggest that culturally determined helping behaviors are more likely to be driven by “leaders” than by popularity, but the emergence and stability of the cultural transmission rules themselves should be driven by some extrinsic factors.     

Effect of group selection on the evolution of altruistic behavior

By using a Monte Carlo simulation, we studied the effect of group selection on the altruistic trait that is controlled by a single locus. The altruistic trait is disadvantageous to the bearer but advantageous to the others. Group selection is defined as the differential reproductive rate among demes caused by genotypic difference among demes. We found that the simulation reproduced many results of former studies. Additionally, when the mutation rate and the migration rate are small enough, we observed two new phenomena: (1) When the effect of the group selection is as large as that of the individual selection, the gene frequency is quite unstable. We found two local stable states, the A- and the S-state. When the metapopulation is in the A-state, altruists are nearly fixed. When in the S-state, on the contrary, altruists are almost lost. The metapopulation shifted quickly from one state to another. We call this phenomenon as the S-A transition. (2) When the mutation rate and migration rate are small enough we found an extremely strong mechanism to stop the non-altruists from expanding no matter how strong the individual selection coefficient is. This is caused by a phenomenon, which we call SA splitting, in which most demes are fixed either by altruists or non-altruists; thus, the relatedness of the metapopulation becomes nearly equal to one. We show SA splitting plays an important role in S-A transition. We define a parameter d to see the degree of SA splitting. We found that d is roughly proportional to mutation rate and deme size.     

Female mate-choice behavior and sympatric speciation

Many models have investigated how the process of speciation may occur in sympatry. In these models, individuals are either asexual or mate choice is determined by very simple rules. Females, for example, may be assumed either to compare their phenotype to that of a potential mate, preferring to mate with similar males (phenotype matching), or to possess preference genes that determine which male phenotype they prefer. These rules often do not reflect the mate-choice rules found in empirical studies. In this paper, we compare these two modes of female choice with various types of sexual imprinting. We examine the efficacy of different mate-choice behavior in causing divergence in male traits under simple deterministic one-locus population genetic models as well as under polygenic, individual-based simulations based on the models of Dieckmann and Doebeli (1999). We find that the inheritance mechanism of mate choice can have a large effect on the ease of sympatric speciation. When females imprint on their mothers, the result of the model is similar to phenotype matching, where speciation can occur fairly easily. When females imprint on their fathers or imprint obliquely, speciation becomes considerably less likely. Finally, when females rely on preference genes, male trait evolution occurs easily, but the correlation between trait and preference can be weak, and interpreting these results as speciation may be suspect.     

Warning coloration in sawflyAthalia rosae larva and concealing coloration in butterflyPieris rapae larva feeding on similar plants evolved through individual selection

Green larvae of the butterfly Pieris rapae and black larvae of the sawfly Athalia rosae feed on green leaves of the same cruciferous plants. To demonstrate that P. rapae has concealing coloration and that A. rosae has warning coloration, the larvae of the two species were supplied to naive chicks Gallus gallus on white, green or black backgrounds. P. rapae larvae were palatable and their green body color acted as a concealing coloration. On the other hand, A. rosae larvae were unpalatable and their black body color acted as a warning coloration. There is a general consensus that warning coloration is an altruistic character which needs victims, and thus can evolve through kin selection or green beard selection. However, black A. rosae larvae were seldom injured by chicks' attack, in particular, on the green background. Therefore, the warning coloration of A. rosae larvae can be a selfish character and hence can evolve through individual selection as well as concealing coloration of P. rapae.     

The economics of altruistic punishment and the maintenance of cooperation

Explaining the evolution and maintenance of cooperation among unrelated individuals is one of the fundamental problems in biology and the social sciences. Recent findings suggest that altruistic punishment is an important mechanism maintaining cooperation among humans. We experimentally explore the boundaries of altruistic punishment to maintain cooperation by varying both the cost and the impact of punishment, using an exceptionally extensive subject pool. Our results show that cooperation is only maintained if conditions for altruistic punishment are relatively favourable: low cost for the punisher and high impact on the punished. Our results indicate that punishment is strongly governed by its cost-to-impact ratio and that its effect on cooperation can be pinned down to one single variable: the threshold level of free-riding that goes unpunished. Additionally, actual pay-offs are the lowest when altruistic punishment maintains cooperation, because the pay-off destroyed through punishment exceeds the gains from increased cooperation. Our results are consistent with the interpretation that punishment decisions come from an amalgam of emotional response and cognitive cost-impact analysis and suggest that altruistic punishment alone can hardly maintain cooperation under multi-level natural selection. Uncovering the workings of altruistic punishment as has been done here is important because it helps predicting under which conditions altruistic punishment is expected to maintain cooperation.     

Evolution of suicide as a defence strategy against pathogens in a spatially structured environment

Suicide upon infection by lytic phages is known in several bacteria species and represents an effective defence strategy to limit phage spread. However, the ecological conditions favouring the evolution of such a radically altruistic behaviour are unclear. Here, we model the feedback of epidemiology on host evolution in a spatially structured environment and we generate several specific predictions on altruistic suicide evolution. We test these predictions experimentally by competing E. coli cells carrying the suicide gene Lit against non‐carrier cells in the presence or in the absence of the lytic phage T6. We show that in accord with our theoretical analysis altruistic suicide is only favoured in the presence of the phage in spatially structured environments at intermediate levels of mixing. Our work provides a general explanation for the evolution of altruistic defence strategies against pathogens. We discuss the implications of these results for oncolytic virus therapy.