[Show abstract][Hide abstract] ABSTRACT: Trust is essential in virtually all economic interactions. In this paper, we build on previous work analyzing the “trust game” from the perspective of evolutionary dynamics. There are two players: an “investor” and a “trustee.” The investor begins with one monetary unit and can choose to transfer it to the trustee. The transfer, if made, is multiplied by a factor b>1 representing the gains that arise from cooperation. The trustee can then return a fraction of his choosing to the investor. Previous work has shown that adding information to this game can lead to trusting and trustworthy behavior. But in those models, information spreads instantaneously and investors never face conflicting information. Here, we relax both of these assumptions. We introduce delays in information propagation so that an investor may still be acting on old information after a trustee has changed his behavior. And we give investors “memories” and thereby allow for the possibility that they might face conflicting information about trustees. In both cases, we find that the trust and trustworthiness induced by information is robust to delays and conflicts. Even if it takes time for information to spread, and even if investors sometimes deem information to be unreliable, the benefits of trust are realized with just moderate levels of information about trustees. We conclude that information (or “reputation”) is a robust explanation for the trust and trustworthiness observed among humans.
[Show abstract][Hide abstract] ABSTRACT: Trust is a central component of social and economic interactions among humans. While rational self-interest dictates that "investors" should not be trusting and "trustees" should not be trustworthy in one-shot anonymous interactions, behavioral experiments with the "trust game" have found that people are both. Here we show how an evolutionary framework can explain this seemingly irrational, altruistic behavior. When individuals' strategies evolve in a context in which (1) investors sometimes have knowledge about trustees before transactions occur and (2) trustees compete with each other for access to investors, natural selection can favor both trust and trustworthiness, even in the subset of interactions in which individuals interact anonymously. We investigate the effects of investors having \fuzzy minds" and making irrationally large demands, finding that both improve outcomes for investors but are not evolutionarily stable. Furthermore, we often find oscillations in trust and trustworthiness instead of convergence to a socially optimal stable equilibrium, with increasing trustworthiness preceding trust in these cycles. Finally, we show how "partner choice," or competition among trustees in small group settings, can lead to arbitrarily equitable distributions of the game's proceeds. To complement our theoretical analysis, we performed a novel behavioral experiment with a modified version of the trust game. Our evolutionary framework provides an ultimate mechanism - not just a proximate psychological explanation - for the emergence of trusting behavior and can explain why trust and trustworthiness are sometimes stable and other times unstable.
[Show abstract][Hide abstract] ABSTRACT: The origins of life on Earth required the establishment of self-replicating chemical systems capable of maintaining and evolving biological information. In an RNA world, single self-replicating RNAs would have faced the extreme challenge of possessing a mutation rate low enough both to sustain their own information and to compete successfully against molecular parasites with limited evolvability. Thus theoretical analyses suggest that networks of interacting molecules were more likely to develop and sustain life-like behaviour. Here we show that mixtures of RNA fragments that self-assemble into self-replicating ribozymes spontaneously form cooperative catalytic cycles and networks. We find that a specific three-membered network has highly cooperative growth dynamics. When such cooperative networks are competed directly against selfish autocatalytic cycles, the former grow faster, indicating an intrinsic ability of RNA populations to evolve greater complexity through cooperation. We can observe the evolvability of networks through in vitro selection. Our experiments highlight the advantages of cooperative behaviour even at the molecular stages of nascent life.
[Show abstract][Hide abstract] ABSTRACT: Behavior in social dilemmas is often inconsistent with the predictions of classical game theory: people (and a wide variety of other organisms) are more cooperative than might be expected. Here we consider behavior in one such social dilemma, the Traveler's Dilemma, that has received considerable attention in the economics literature but is little known among theoretical biologists. The rules of the game are as follows. Two players each choose a value between R and M, where 0<R<M. If the players choose the same value, both receive that amount. If the players choose different values v(1) and v(2), where v(1)<v(2), then the player choosing v(1) receives v(1)+R and the player choosing v(2) receives v(1)-R. While the players would maximize their payoffs by both choosing the largest allowed value, M, the Nash equilibrium is to choose the smallest allowed value, R. In behavioral experiments, however, people generally choose values much larger than the minimum and the deviation from the expected equilibrium decreases with R. In this paper, we show that the cooperative behavior observed in the Traveler's Dilemma can be explained in an evolutionary framework. We study stochastic evolutionary dynamics in finite populations with varying intensity of selection and varying mutation rate. We derive analytic results showing that strategies choosing high values can be favored when selection is weak. More generally, selection favors strategies that choose high values if R is small (relative to M) and strategies that choose low values if R is large. Finally, we show that a two-parameter model involving the intensity of selection and the mutation rate can quantitatively reproduce data that from a Traveler's Dilemma experiment. These results demonstrate the power of evolutionary game theory for explaining human behavior in contexts that are challenging for standard economic game theory.
[Show abstract][Hide abstract] ABSTRACT: During the origin of life, the biological information of nucleic acid polymers must have increased to encode functional molecules (the RNA world). Ribozymes tend to be compositionally unbiased, as is the vast majority of possible sequence space. However, ribonucleotides vary greatly in synthetic yield, reactivity and degradation rate, and their non-enzymatic polymerization results in compositionally biased sequences. While natural selection could lead to complex sequences, molecules with some activity are required to begin this process. Was the emergence of compositionally diverse sequences a matter of chance, or could prebiotically plausible reactions counter chemical biases to increase the probability of finding a ribozyme? Our in silico simulations using a two-letter alphabet show that template-directed ligation and high concatenation rates counter compositional bias and shift the pool toward longer sequences, permitting greater exploration of sequence space and stable folding. We verified experimentally that unbiased DNA sequences are more efficient templates for ligation, thus increasing the compositional diversity of the pool. Our work suggests that prebiotically plausible chemical mechanisms of nucleic acid polymerization and ligation could predispose toward a diverse pool of longer, potentially structured molecules. Such mechanisms could have set the stage for the appearance of functional activity very early in the emergence of life.
Nucleic Acids Research 02/2012; 40(10):4711-22. DOI:10.1093/nar/gks065 · 9.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Conjugation is the main mode of horizontal gene transfer that spreads antibiotic resistance among bacteria. Strategies for inhibiting conjugation may be useful for preserving the effectiveness of antibiotics and preventing the emergence of bacterial strains with multiple resistances. Filamentous bacteriophages were first observed to inhibit conjugation several decades ago. Here we investigate the mechanism of inhibition and find that the primary effect on conjugation is occlusion of the conjugative pilus by phage particles. This interaction is mediated primarily by phage coat protein g3p, and exogenous addition of the soluble fragment of g3p inhibited conjugation at low nanomolar concentrations. Our data are quantitatively consistent with a simple model in which association between the pili and phage particles or g3p prevents transmission of an F plasmid encoding tetracycline resistance. We also observe a decrease in the donor ability of infected cells, which is quantitatively consistent with a reduction in pili elaboration. Since many antibiotic-resistance factors confer susceptibility to phage infection through expression of conjugative pili (the receptor for filamentous phage), these results suggest that phage may be a source of soluble proteins that slow the spread of antibiotic resistance genes.
PLoS ONE 05/2011; 6(5):e19991. DOI:10.1371/journal.pone.0019991 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Template-directed polymerization of nucleotides is believed to be a pathway for the replication of genetic material in the earliest cells. We assume that activated monomers are produced by prebiotic chemistry. These monomers can undergo spontaneous polymerization, a system that we call "prelife." Adding template-directed polymerization changes the equilibrium structure of prelife if the rate constants meet certain criteria. In particular, if the basic reproductive ratio of sequences of a certain length exceeds one, then those sequences can attain high abundance. Furthermore, if many sequences replicate, then the longest sequences can reach high abundance even if the basic reproductive ratios of all sequences are less than one. We call this phenomenon "subcritical life." Subcritical life suggests that sequences long enough to be ribozymes can become abundant even if replication is relatively inefficient. Our work on the evolution of replication has interesting parallels to infection dynamics. Life (replication) can be seen as an infection of prelife.
[Show abstract][Hide abstract] ABSTRACT: We study the origin of evolution. Evolution is based on replication, mutation, and selection. But how does evolution begin? When do chemical kinetics turn into evolutionary dynamics? We propose "prelife" and "prevolution" as the logical precursors of life and evolution. Prelife generates sequences of variable length. Prelife is a generative chemistry that proliferates information and produces diversity without replication. The resulting "prevolutionary dynamics" have mutation and selection. We propose an equation that allows us to investigate the origin of evolution. In one limit, this "originator equation" gives the classical selection equation. In the other limit, we obtain "prelife." There is competition between life and prelife and there can be selection for or against replication. Simple prelife equations with uniform rate constants have the property that longer sequences are exponentially less frequent than shorter ones. But replication can reverse such an ordering. As the replication rate increases, some longer sequences can become more frequent than shorter ones. Thus, replication can lead to "reversals" in the equilibrium portraits. We study these reversals, which mark the transition from prelife to life in our model. If the replication potential exceeds a critical value, then life replicates into existence.