On the role of collective sensing and evolution in group formation

Collective sensing is an emergent phenomenon which enables individuals to estimate a hidden property of the environment through the observation of social interactions. Previous work on collective sensing shows that gregarious individuals obtain an evolutionary advantage by exploiting collective sensing when competing against solitary individuals. This work addresses the question of whether collective sensing allows for the emergence of groups from a population of individuals without predetermined behaviors. It is assumed that group membership does not lessen competition on the limited resources in the environment, e.g., groups do not improve foraging efficiency. Experiments are run in an agent-based evolutionary model of a foraging task, where the fitness of the agents depends on their foraging strategy. The foraging strategy of agents is determined by a neural network, which does not require explicit modeling of the environment and of the interactions between agents. Experiments demonstrate that gregarious behavior is not the evolutionary-fittest strategy if resources are abundant, thus invalidating previous findings in a specific region of the parameter space. In other words, resource scarcity makes gregarious behavior so valuable as to make up for the increased competition over the few available resources. Furthermore, it is shown that a population of solitary agents can evolve gregarious behavior in response to a sudden scarcity of resources, thus individuating a possible mechanism that leads to gregarious behavior in nature. The evolutionary process operates on the whole parameter space of the neural networks; hence, these behaviors are selected among an unconstrained set of behavioral models.     

Mechanistically comparing reproductive manipulations caused by selfish chromosomes and bacterial symbionts

Insects naturally harbor a broad range of selfish agents that can manipulate their reproduction and development, often leading to host sex ratio distortion. Such effects directly benefit the spread of the selfish agents. These agents include two broad groups: bacterial symbionts and selfish chromosomes. Recent studies have made steady progress in uncovering the cellular targets of these agents and their effector genes. Here we highlight what is known about the targeted developmental processes, developmental timing, and effector genes expressed by several selfish agents. It is now becoming apparent that: (1) the genetic toolkits used by these agents to induce a given reproductive manipulation are simple, (2) these agents target sex-specific cellular processes very early in development, and (3) in some cases, similar processes are targeted. Knowledge of the molecular underpinnings of these systems will help to solve long-standing puzzles and provide new tools for controlling insect pests.Subject terms: Development, Genomic instability     

Adaptation in the face of internal conflict: the paradox of the organism revisited

The paradox of the organism refers to the observation that organisms appear to function as coherent purposeful entities, despite the potential for within-organismal components like selfish genetic elements and cancer cells to erode them from within. While it is commonly accepted that organisms may pursue fitness maximisation and can be thought to hold particular agendas, there is a growing recognition that genes and cells do so as well. This can lead to evolutionary conflicts between an organism and the parts that reside within it. Here, we revisit the paradox of the organism. We first outline its conception and relationship to debates about adaptation in evolutionary biology. Second, we review the ways selfish elements may exploit organisms, and the extent to which this threatens organismal integrity. To this end, we introduce a novel classification scheme that distinguishes between selfish elements that seek to distort transmission versus those that seek to distort phenotypic traits. Our classification scheme also highlights how some selfish elements elude a multi-level selection decomposition using the Price equation. Third, we discuss how the organism can retain its status as the primary fitness-maximising agent in the face of selfish elements. The success of selfish elements is often constrained by their strategy and further limited by a combination of fitness alignment and enforcement mechanisms controlled by the organism. Finally, we argue for the need for quantitative measures of both internal conflicts and organismality.     

Learning and stabilization of altruistic behaviors in multi-agent systems by reciprocity

Optimization of performance in collective systems often requires altruism. The emergence and stabilization of altruistic behaviors are difficult to achieve because the agents incur a cost when behaving altruistically. In this paper, we propose a biologically inspired strategy to learn stable altruistic behaviors in artificial multi-agent systems, namely reciprocal altruism. This strategy in conjunction with learning capabilities make altruistic agents cooperate only between themselves, thus preventing their exploitation by selfish agents, if future benefits are greater than the current cost of altruistic acts. Our multi-agent system is made up of agents with a behavior-based architecture. Agents learn the most suitable cooperative strategy for different environments by means of a reinforcement learning algorithm. Each agent receives a reinforcement signal that only measures its individual performance. Simulation results show how the multi-agent system learns stable altruistic behaviors, so achieving optimal (or near-to-optimal) performances in unknown and changing environments. Received: 1 August 1997 / Accepted in revised form: 28 November 1997     

Micro-economics based resource allocation in Grid-Federation environment

A Grid-Federation environment is composed of a collection of autonomous and selfish distributed cluster resource managers. These selfish managers participate in Grid-Federation to share their resources. Market models could be used to motivate the self-interested participants to share their resources. In this paper, firstly, a market for resource exchange in grid federation environment is established. Then, in order that the market reaches a Walrasian equilibrium, a computationally tractable mechanism is proposed. A Walrasian equilibrium problem consists of finding a set of prices and allocations of resources in such a way that the cluster resource managers could maximize their utilities and the market clears. Market clears when the resource supply equals to the demand. We show that in a Walrasian equilibrium, the Grid Federation market reaches an efficient resource allocation.     

Population biology of a selfish sex ratio distorting element in a booklouse (Psocodea: Liposcelis)

Arthropods harbour a variety of selfish genetic elements that manipulate reproduction to be preferentially transmitted to future generations. A major ongoing question is to understand how these elements persist in nature. In this study, we examine the population dynamics of an unusual selfish sex ratio distorter in a recently discovered species of booklouse, Liposcelis sp. (Psocodea: Liposcelididae) to gain a better understanding of some of the factors that may affect the persistence of this element. Females that carry the selfish genetic element only ever produce daughters, although they are obligately sexual. These females also only transmit the maternal half of their genome. We performed a replicated population cage experiment, varying the initial frequency of females that harbour the selfish element, and following female frequencies for 20 months. The selfish genetic element persisted in all cages, often reaching very high (and thus severely female‐biased) frequencies. Surprisingly, we also found that females that carry the selfish genetic element had much lower fitness than their nondistorter counterparts, with lower lifetime fecundity, slower development and a shorter egg‐laying period. We suggest that differential fitness plays a role in the maintenance of the selfish genetic element in this species. We believe that the genetic system in this species, paternal genome elimination, which allows maternal control of offspring sex ratio, may also be important in the persistence of the selfish genetic element, highlighting the need to consider species with diverse ecologies and genetic systems when investigating the effects of sex ratio manipulators on host populations.     

Modeling the Dynamics of a Non-Limited and a Self-Limited Gene Drive System in Structured Aedes aegypti Populations

Recently there have been significant advances in research on genetic strategies to control populations of disease-vectoring insects. Some of these strategies use the gene drive properties of selfish genetic elements to spread physically linked anti-pathogen genes into local vector populations. Because of the potential of these selfish elements to spread through populations, control approaches based on these strategies must be carefully evaluated to ensure a balance between the desirable spread of the refractoriness-conferring genetic cargo and the avoidance of potentially unwanted outcomes such as spread to non-target populations. There is also a need to develop better estimates of the economics of such releases. We present here an evaluation of two such strategies using a biologically realistic mathematical model that simulates the resident Aedes aegypti mosquito population of Iquitos, Peru. One strategy uses the selfish element Medea, a non-limited element that could permanently spread over a large geographic area; the other strategy relies on Killer-Rescue genetic constructs, and has been predicted to have limited spatial and temporal spread. We simulate various operational approaches for deploying these genetic strategies, and quantify the optimal number of released transgenic mosquitoes needed to achieve definitive spread of Medea-linked genes and/or high frequencies of Killer-Rescue-associated elements. We show that for both strategies the most efficient approach for achieving spread of anti-pathogen genes within three years is generally to release adults of both sexes in multiple releases over time. Even though females in these releases should not transmit disease, there could be public concern over such releases, making the less efficient male-only release more practical. This study provides guidelines for operational approaches to population replacement genetic strategies, as well as illustrates the use of detailed spatial models to assist in safe and efficient implementation of such novel genetic strategies.     

Evolution of cooperation by reciprocation within structured demes

The iterative two-person Prisoners’ Dilemma game has been generalised to theN-person case. The evolution of cooperation is explored by matching the Tit For Tat (TFT) strategy (Axelrod and Hamilton 1981) against the selfish strategy. Extension of TFT toN-person situations yields a graded set of strategies from the softest TFT, which continues cooperation even if only one of the opponents reciprocates it, to the hardest, which would do so only when all the remaining opponents cooperate. The hardest TFT can go to fixation against the selfish strategy provided it crosses a threshold frequencypc. All the other TFT are invadable by the selfish (D) or the pure defector strategy, while none can invadeD. Yet, provided a thresholdpc is crossed, they can coexist stably withD. AsN, the size of the group increases, the threshold p_c also increases, indicating that the evolution of cooperation is more difficult for larger groups. Under certain conditions, only the soft TFT can coexist stably against the selfish strategyD, while the harder ones cannot. An interesting possibility of a complete takeover of the selfish population by successive invasions by harder and harder TFT strategies is also presented.     

The costs of sex due to deleterious intracellular parasites

A major problem in evolutionary theory is to explain the widespread occurrence of sexual recombination. This is particularly difficult in anisogamous species where the familiar ‘two-fold cost of sex’ is encountered. Another cost has recently been identified: that fusion of gametes allows intracellular parasites or deleterious ‘selfish’ genomes to invade a population. These costs of anisogamy and the ability of cytoplasmic agents to invade a sexual population are quantified, allowing the costs and consequences of different modes of reproduction to be compared. It is found that the costs of selfish elements are likely to be very high and, in particular, that isogamous sexual reproduction (the putative ‘primitive’ form) is not cost-free, but incurs a fitness reduction of the order of 90%; thus a large selective disadvantage occurs in the initial evolution of sex which is ignored in standard analysis. Even once anisogamy has evolved, the low levels of ‘paternal leakage’ observed in many extant organisms may allow selfish cytoplasmic elements to spread, resulting in moderate to large decreases in host population fitness. However, much of the cost of selfish elements is avoided in sexual lifecycles with a large number of asexual cellular divisions between sexual reproduction: this greatly impedes the spread of selfish agents and reduces the fitness loss attributable to selfish elements.     

A parasitic selfish gene that affects host promiscuity

Selfish genes demonstrate transmission bias and invade sexual populations despite conferring no benefit to their hosts. While the molecular genetics and evolutionary dynamics of selfish genes are reasonably well characterized, their effects on hosts are not. Homing endonuclease genes (HEGs) are one well-studied family of selfish genes that are assumed to be benign. However, we show that carrying HEGs is costly for Saccharomyces cerevisiae, demonstrating that these genetic elements are not necessarily benign but maybe parasitic. We estimate a selective load of approximately 1–2% in ‘natural’ niches. The second aspect we examine is the ability of HEGs to affect hosts'' sexual behaviour. As all selfish genes critically rely on sex for spread, then any selfish gene correlated with increased host sexuality will enjoy a transmission advantage. While classic parasites are known to manipulate host behaviour, we are not aware of any evidence showing a selfish gene is capable of affecting host promiscuity. The data presented here show a selfish element may increase the propensity of its eukaryote host to undergo sex and along with increased rates of non-Mendelian inheritance, this may counterbalance mitotic selective load and promote spread. Demonstration that selfish genes are correlated with increased promiscuity in eukaryotes connects with ideas suggesting that selfish genes promoted the evolution of sex initially.     

Starve a competitor: evolution of luxury consumption as a competitive strategy

Organisms are often observed to acquire an excess of non-limiting resources, a process known as luxury consumption. Luxury consumption has been largely treated as a bet hedging strategy for temporal variation in resource supply, but may also function as a competitive strategy. We incorporate luxury resource consumption into a derivation of the classic resource ratio model for competition between terrestrial plant, and explore its consequences for population dynamics and competition. We show that luxury consumption reduces the potential for coexistence between two species competing for two resources. Furthermore, we demonstrate that luxury consumption can be selected for because of the competitive advantage that luxury consumers gain. Luxury consumption evolves when competition for resources is local rather than global, there is potential for coexistence between the two species and the competitive environment remains stable over a sufficient period of time to allow selection to act. The evolutionary outcome can be either extinction of one of the competing species or coexistence of the two species with maximum luxury consumption. The potential for selection to favor luxury consumption is well predicted by the competitive outcome between individuals of the two species with and without luxury consumption.     

Optimal growth strategies of larval helminths in their intermediate hosts

We consider optimal growth of larval stages in complex parasite life cycles where there is no constraint because of host immune responses. Our model predicts an individual's asymptotic size in its intermediate host, with and without competition from conspecific larvae. We match observed variations in larval growth patterns in pseudophyllid cestodes with theoretical predictions of our model. If survival of the host is vital for transmission, larvae should reduce asymptotic size as intensity increases, to avoid killing the host. The life history strategy (LHS) model predicts a size reduction <1/intensity, thus increasing the parasite burden on the host. We discuss whether body size of competing parasites is an evolved LHS or simply reflects resource constraints (RC) on growth fixed by the host, leading to a constant total burden with intensity. Growth under competition appears comparable with "the tragedy of the commons", much analysed in social sciences. Our LHS prediction suggests that evolution generates a solution that seems cooperative but is actually selfish.     

Commons and anticommons in a simple renewable resource harvest model

We consider a model where agents harvesting from a renewable resource can impose limitations on the harvesting efforts of other agents. Obstructing the harvesting of others comes at a cost, and is viewed as regulation of resource access. Thus, the agent population comprises agents that do not obstruct (“cooperators”) and agents that obstruct (“obstructors”). As the economically better performing strategy spreads in the population, the system self-organizes at a level of obstruction which depends on the costs of obstruction, the obstruction efficiency and the history of the system. We show that commons and anticommons can be considered as the end points of a continuum of varying degrees of obstruction and we identify three regimes for the stationary state of the evolution dynamics: (i) a state where the system ends up in a tragedy of the commons, (ii) a tragedy of the anticommons state and (iii) a moderately regulated state in between both extremes. The more a policy environment in the moderately regulated state is tuned for optimality, the higher the danger that a fluctuation destabilizes the system into severe overexploitation.     

Predator-driven fragmentation of fiddler crab droves into selfish miniherds of biased composition

An explanation for animal groups is the selfish herd, characterized by aggregation as each member tries to shield itself from a predator by moving into a tight gap between other members. We test the hypotheses that: (1) droves, the large feeding groups of fiddler crabs, are selfish herds; (2) the miniherds that form when droves fragment on approach of a large predator are selfish herds; (3) selfish herds form when refugia are unlikely to be reached before an approaching predator arrives; and (4) the composition of selfish miniherds is biased toward individuals most vulnerable to predation. The study was conducted in South Carolina (USA) by videotaping the movements of sand fiddler crabs Uca pugilator when approached by a human predator. In both droves and miniherds, interindividual distance decreases with predator approach, consistent with behavior in a selfish herd. However, two other expectations for selfish herds—herd cohesion and sacrificing distance from the predator in order to get closer to other herders—are only met in miniherds. Crabs farther from refugia are more likely to form and remain in miniherds, indicating that selfish herding is only favored when refugia cannot be quickly reached. The composition of the smallest miniherds, consisting of 2-18 crabs, is biased toward females and small males. These individuals may be more vulnerable to predation because they lack the enlarged claw of large males that deters some predators. The small miniherds are relatively homogeneous with respect to the size and sex of their members, which may enhance cohesion and effectiveness as selfish herds. Miniherds will be effective selfish groups when predator attack has a significant vertical component and when the strike distance is large relative to both the size of the prey and the distance between group members. Droves are not selfish herds but permit crabs to flee feeding grounds as members of selfish miniherds.     

Manipulation of parental nutritional condition reveals competition among family members

Parental care is thought to be costly, as it consumes time and energy. Such costs might be reduced in animal parents that raise their young on valuable food sources such as dung or carcasses, as parents are able to invest in self‐maintenance by feeding from the same resource. However, this might lower the nutritional value for other family members and, as a consequence, food competition might arise. To promote our understanding of the outcome of such competition, we manipulated the necessity of parents to feed from the resource. Using a full factorial design, we paired food‐deprived or well‐fed males with food‐deprived or well‐fed females of burying beetles, which are known to raise their young on vertebrate cadavers. We found that food‐deprived parents consumed more of the carrion than those that were well‐fed and this had a negative impact on other family members. However, the outcome of the competition depended on the sex of the parents, with females suffering when males fed more and offspring suffering when females fed more. Thus, family life involves selfish elements, as both parents remove resources for the purpose of self‐maintenance. However, females show altruistic aspects, as they appear to restrict their food consumption for the benefit of their offspring when paired with a food‐deprived male. Interestingly, males extend their stay with the brood when having faced food scarcity prior to reproduction, presumably to replenish their energy reserves. Our study therefore reveals that breeding on shared resources can promote family living, but also results in competition.     

A dynamic over games drives selfish agents to win–win outcomes

Understanding human institutions, animal cultures and other social systems requires flexible formalisms that describe how their members change them from within. We introduce a framework for modelling how agents change the games they participate in. We contrast this between-game ‘institutional evolution’ with the more familiar within-game ‘behavioural evolution’. We model institutional change by following small numbers of persistent agents as they select and play a changing series of games. Starting from an initial game, a group of agents trace trajectories through game space by navigating to increasingly preferable games until they converge on ‘attractor’ games. Agents use their ‘institutional preferences'' for game features (such as stability, fairness and efficiency) to choose between neighbouring games. We use this framework to pose a pressing question: what kinds of games does institutional evolution select for; what is in the attractors? After computing institutional change trajectories over the two-player space, we find that attractors have disproportionately fair outcomes, even though the agents who produce them are strictly self-interested and indifferent to fairness. This seems to occur because game fairness co-occurs with the self-serving features these agents do actually prefer. We thus present institutional evolution as a mechanism for encouraging the spontaneous emergence of cooperation among small groups of inherently selfish agents, without space, reputation, repetition, or other more familiar mechanisms. Game space trajectories provide a flexible, testable formalism for modelling the interdependencies of behavioural and institutional evolutionary processes, as well as a mechanism for the evolution of cooperation.     

GENETICS OF SEX RATIO VARIATION AMONG NATURAL POPULATIONS OF A DIOECIOUS PLANT

The female-biased sex ratio in the genus Silene is the most widely documented example of sex ratio bias in plants. It has recently been shown that the genetic basis of sex ratio in this species involves a system of sex ratio distorters and restorers, but it is not known if these genetic elements are of fundamental importance in natural populations. I crossed plants from natural populations with known testers to examine the frequencies of sex ratio distorters and restorers in eight populations of S. alba, and to determine the extent that these elements are responsible for the sex ratio variation in nature. The genetic basis of sex ratio varied from one population to another, suggesting that sex ratio evolution occurs over a relatively small spatial scale and is asynchronous among local demes. The resulting variation in the frequency of sex ratio alleles among populations explained most of the sex ratio variation seen in nature, which suggests that the ecology and evolution of this trait in natural populations is governed primarily by underlying dynamics of selfish genetic elements. The possible causes and the evolutionary consequences of population structure at genes controlling sex ratio are discussed, as are the implications of these results for the overall importance of selfish genetic elements in natural populations.     

Self-Fulfilling Prophecies as a Link between Men’s Facial Width-to-Height Ratio and Behavior

The facial width-to-height ratio (fWHR) has been identified as a reliable predictor of men’s behavior, with researchers focusing on evolutionary selection pressures as the underlying mechanism explaining these relationships. In this paper, we complement this approach and examine the extent to which social processes also determine the extent to which men’s fWHR serves as a behavioral cue. Specifically, we propose that observers’ treatment of target men based on the targets’ fWHR subsequently affects behavior, leading the targets to behave in ways that are consistent with the observers’ expectations (i.e., a self-fulfilling prophecy). Results from four studies demonstrate that individuals behave more selfishly when interacting with men with greater fWHRs, and this selfish behavior, in turn, elicits selfish behavior in others.     

The origins of cellular life

All life on earth can be naturally classified into cellular life forms and virus-like selfish elements, the latter being fully dependent on the former for their reproduction. Cells are reproducers that not only replicate their genome but also reproduce the cellular organization that depends on semipermeable, energy-transforming membranes and cannot be recovered from the genome alone, under the famous dictum of Rudolf Virchow, Omnis cellula e cellula. In contrast, simple selfish elements are replicators that can complete their life cycles within the host cell starting from genomic RNA or DNA alone. The origin of the cellular organization is the central and perhaps the hardest problem of evolutionary biology. I argue that the origin of cells can be understood only in conjunction with the origin and evolution of selfish genetic elements. A scenario of precellular evolution is presented that involves cohesion of the genomes of the emerging cellular life forms from primordial pools of small genetic elements that eventually segregated into hosts and parasites. I further present a model of the coevolution of primordial membranes and membrane proteins, discuss protocellular and non-cellular models of early evolution, and examine the habitats on the primordial earth that could have been conducive to precellular evolution and the origin of cells.     

A general model of the public goods dilemma

An individually costly act that benefits all group members is a public good. Natural selection favours individual contribution to public goods only when some benefit to the individual offsets the cost of contribution. Problems of sex ratio, parasite virulence, microbial metabolism, punishment of noncooperators, and nearly all aspects of sociality have been analysed as public goods shaped by kin and group selection. Here, I develop two general aspects of the public goods problem that have received relatively little attention. First, variation in individual resources favours selfish individuals to vary their allocation to public goods. Those individuals better endowed contribute their excess resources to public benefit, whereas those individuals with fewer resources contribute less to the public good. Thus, purely selfish behaviour causes individuals to stratify into upper classes that contribute greatly to public benefit and social cohesion and to lower classes that contribute little to the public good. Second, if group success absolutely requires production of the public good, then the pressure favouring production is relatively high. By contrast, if group success depends weakly on the public good, then the pressure favouring production is relatively weak. Stated in this way, it is obvious that the role of baseline success is important. However, discussions of public goods problems sometimes fail to emphasize this point sufficiently. The models here suggest simple tests for the roles of resource variation and baseline success. Given the widespread importance of public goods, better models and tests would greatly deepen our understanding of many processes in biology and sociality.