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Search Results: 1 - 10 of 44841 matches for " Michael Doebeli "
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A model for the evolutionary diversification of religions
Michael Doebeli,Iaroslav Ispolatov
Physics , 2008, DOI: 10.1016/j.jtbi.2010.09.013
Abstract: We address the problem of diversification in religions by studying selection on cultural memes that colonize humans hosts. In analogy to studying the evolution of pathogens or symbionts colonizing animal hosts, we use models for host-pathogen dynamics known from theoretical epidemiology. In these models, religious memes colonize individual humans. Rates of transmission of memes between humans, i.e., transmission of cultural content, and rates of loss of memes (loss of faith) are determined by the phenotype of the cultural memes, and by interactions between hosts carrying different memes. In particular, based on the notion that religion can lead to oppression of lower classes once a religious society has reached a certain size, we assume that the rate of loss increases as the number of humans colonized by a particular meme phenotype increases. This generates frequency-dependent selection on cultural memes, and we use evolutionary theory to show that this frequency dependence can generate the emergence of coexisting clusters of different meme types. The different clusters correspond to different religions, and hence our model describes the emergence of distinct descendent religions from single ancestral religions.
Omnivory can both enhance and dampen perturbations in food webs
Iaroslav Ispolatov,Michael Doebeli
Quantitative Biology , 2009, DOI: 10.1007/s12080-010-0074-0
Abstract: We investigate how perturbations propagate up and down a food chain with and without self-interaction and omnivory. A source of perturbation is a shift in death rate of a trophic level, and the measure of perturbation is the difference between the perturbed and unperturbed steady state populations. For Lotka-Volterra food chains with linear functional response, we show analytically that both intraspecific competition and intraguild predation can either dampen or enhance the propagation of perturbations, thus stabilizing or destabilizing the food web. The direction of the effect depend on the position of the source of perturbation, as well as on the position of the additional competitive and predatory links . These conclusions are confirmed numerically for a food chain with more realistic Type-II functional response. Our results support the positions of both sides in the long-standing debate on the effect of intraspecific competition and omnivory on the stability of trophic systems.
On the evolution of decoys in plant immune systems
Iaroslav Ispolatov,Michael Doebeli
Quantitative Biology , 2010, DOI: 10.1162/BIOT_a_00055
Abstract: The Guard-Guardee model for plant immunity describes how resistance proteins (guards) in host cells monitor host target proteins (guardees) that are manipulated by pathogen effector proteins. A recently suggested extension of this model includes decoys, which are duplicated copies of guardee proteins, and which have the sole function to attract the effector and, when modified by the effector, trigger the plant immune response. Here we present a proof-of-principle model for the functioning of decoys in plant immunity, quantitatively developing this experimentally-derived concept. Our model links the basic cellular chemistry to the outcomes of pathogen infection and resulting fitness costs for the host. In particular, the model allows identification of conditions under which it is optimal for decoys to act as triggers for the plant immune response, and of conditions under which it is optimal for decoys to act as sinks that bind the pathogen effectors but do not trigger an immune response.
Continuously stable strategies as evolutionary branching points
Michael Doebeli,Iaroslav Ispolatov
Quantitative Biology , 2010, DOI: 10.1016/j.jtbi.2010.06.036
Abstract: Evolutionary branching points are a paradigmatic feature of adaptive dynamics, because they are potential starting points for adaptive diversification. The antithesis to evolutionary branching points are Continuously stable strategies (CSS's), which are convergent stable and evolutionarily stable equilibrium points of the adaptive dynamics and hence are thought to represent endpoints of adaptive processes. However, this assessment is based on situations in which the invasion fitness function determining the adaptive dynamics have non-zero second derivatives at a CSS. Here we show that the scope of evolutionary branching can increase if the invasion fitness function vanishes to higher than first order at a CSS. Using a class of classical models for frequency-dependent competition, we show that if the invasion fitness vanishes to higher orders, a CSS may be the starting point for evolutionary branching, with the only additional requirement that mutant types need to reach a certain threshold frequency, which can happen e.g. due to demographic stochasticity. Thus, when invasion fitness functions vanish to higher than first order at equilibrium points of the adaptive dynamics, evolutionary diversification can occur even after convergence to an evolutionarily stable strategy.
Speciation due to hybrid necrosis in plant-pathogen models
I. Ispolatov,Michael Doebeli
Quantitative Biology , 2009, DOI: 10.1111/j.1558-5646.2009.00800.x
Abstract: We develop a model for speciation due to postzygotic incompatibility generated by autoimmune reactions. The model is based on predator-prey interactions between a host plants and their pathogens. Such interactions are often frequency-dependent, so that pathogen attack is focused on the most abundant plant phenotype, while rare plant types may escape pathogen attack. Thus, frequency dependence can generate disruptive selection, which can give rise to speciation if distant phenotypes become reproductively isolated. Based on recent experimental evidence from {\it Arabidopsis}, we assume that at the molecular level, incompatibility between strains is caused by epistatic interactions between two proteins in the plant immune system, the guard and the guardee. Within each plant strain, immune reactions occur when the guardee protein is modified by a pathogen effector, and the guard subsequently binds to the guardee, thus precipitating an immune response. However, when guard and guardee proteins come from phenotypically distant parents, a hybrid's immune system can be triggered by erroneous interactions between these proteins even in the absence of pathogen attack, leading to severe autoimmune reactions in hybrids. Our model shows how phenotypic variation generated by frequency-dependent host-pathogen interactions can lead to postzygotic incompatibility between extremal types, and hence to speciation.
Symmetric competition as a general model for single-species adaptive dynamics
Michael Doebeli,Iaroslav Ispolatov
Quantitative Biology , 2012,
Abstract: Adaptive dynamics is a widely used framework for modeling long-term evolution of continuous phenotypes. It is based on invasion fitness functions, which determine selection gradients and the canonical equation of adaptive dynamics. Even though the derivation of the adaptive dynamics from a given invasion fitness function is general and model-independent, the derivation of the invasion fitness function itself requires specification of an underlying ecological model. Therefore, evolutionary insights gained from adaptive dynamics models are generally model-dependent. Logistic models for symmetric, frequency-dependent competition are widely used in this context. Such models have the property that the selection gradients derived from them are gradients of scalar functions, which reflects a certain gradient property of the corresponding invasion fitness function. We show that any adaptive dynamics model that is based on an invasion fitness functions with this gradient property can be transformed into a generalized symmetric competition model. This provides a precise delineation of the generality of results derived from competition models. Roughly speaking, to understand the adaptive dynamics of the class of models satisfying a certain gradient condition, one only needs a complete understanding of the adaptive dynamics of symmetric, frequency-dependent competition. We show how this result can be applied to number of basic issues in evolutionary theory.
Chaos and Unpredictability in Evolution
Iaroslav Ispolatov,Michael Doebeli
Quantitative Biology , 2013, DOI: 10.1111/evo.12354
Abstract: The possibility of complicated dynamic behaviour driven by non-linear feedbacks in dynamical systems has revolutionized science in the latter part of the last century. Yet despite examples of complicated frequency dynamics, the possibility of long-term evolutionary chaos is rarely considered. The concept of "survival of the fittest" is central to much evolutionary thinking and embodies a perspective of evolution as a directional optimization process exhibiting simple, predictable dynamics. This perspective is adequate for simple scenarios, when frequency-independent selection acts on scalar phenotypes. However, in most organisms many phenotypic properties combine in complicated ways to determine ecological interactions, and hence frequency-dependent selection. Therefore, it is natural to consider models for the evolutionary dynamics generated by frequency-dependent selection acting simultaneously on many different phenotypes. Here we show that complicated, chaotic dynamics of long-term evolutionary trajectories in phenotype space is very common in a large class of such models when the dimension of phenotype space is large, and when there are epistatic interactions between the phenotypic components. Our results suggest that the perspective of evolution as a process with simple, predictable dynamics covers only a small fragment of long-term evolution. Our analysis may also be the first systematic study of the occurrence of chaos in multidimensional and generally dissipative systems as a function of the dimensionality of phase space.
Parallel Evolutionary Dynamics of Adaptive Diversification in Escherichia coli
Matthew D. Herron,Michael Doebeli
PLOS Biology , 2013, DOI: 10.1371/journal.pbio.1001490
Abstract: The causes and mechanisms of evolutionary diversification are central issues in biology. Geographic isolation is the traditional explanation for diversification, but recent theoretical and empirical studies have shown that frequency-dependent selection can drive diversification without isolation and that adaptive diversification occurring in sympatry may be an important source of biological diversity. However, there are no empirical examples in which sympatric lineage splits have been understood at the genetic level, and it is unknown how predictable this process is—that is, whether similar ecological settings lead to parallel evolutionary dynamics of diversification. We documented the genetic basis and the evolutionary dynamics of adaptive diversification in three replicate evolution experiments, in which competition for two carbon sources caused initially isogenic populations of the bacterium Escherichia coli to diversify into two coexisting ecotypes representing different physiological adaptations in the central carbohydrate metabolism. Whole-genome sequencing of clones of each ecotype from different populations revealed many parallel and some unique genetic changes underlying the derived phenotypes, including changes to the same genes and sometimes to the same nucleotide. Timelines of allele frequencies extracted from the frozen “fossil” record of the three evolving populations suggest parallel evolutionary dynamics driven at least in part by a co-evolutionary process in which mutations causing one type of physiology changed the ecological environment, allowing the invasion of mutations causing an alternate physiology. This process closely corresponds to the evolutionary dynamics seen in mathematical models of adaptive diversification due to frequency-dependent ecological interactions. The parallel genetic changes underlying similar phenotypes in independently evolved lineages provide empirical evidence of adaptive diversification as a predictable evolutionary process.
The Repeatability of Adaptive Radiation During Long-Term Experimental Evolution of Escherichia coli in a Multiple Nutrient Environment
Gerda Saxer,Michael Doebeli,Michael Travisano
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0014184
Abstract: Adaptive radiations occur when a species diversifies into different ecological specialists due to competition for resources and trade-offs associated with the specialization. The evolutionary outcome of an instance of adaptive radiation cannot generally be predicted because chance (stochastic events) and necessity (deterministic events) contribute to the evolution of diversity. With increasing contributions of chance, the degree of parallelism among different instances of adaptive radiations and the predictability of an outcome will decrease. To assess the relative contributions of chance and necessity during adaptive radiation, we performed a selection experiment by evolving twelve independent microcosms of Escherichia coli for 1000 generations in an environment that contained two distinct resources. Specialization to either of these resources involves strong trade-offs in the ability to use the other resource. After selection, we measured three phenotypic traits: 1) fitness, 2) mean colony size, and 3) colony size diversity. We used fitness relative to the ancestor as a measure of adaptation to the selective environment; changes in colony size as a measure of the evolution of new resource specialists because colony size has been shown to correlate with resource specialization; and colony size diversity as a measure of the evolved ecological diversity. Resource competition led to the rapid evolution of phenotypic diversity within microcosms. Measurements of fitness, colony size, and colony size diversity within and among microcosms showed that the repeatability of adaptive radiation was high, despite the evolution of genetic variation within microcosms. Consistent with the observation of parallel evolution, we show that the relative contributions of chance are far smaller and less important than effects due to adaptation for the traits investigated. The two-resource environment imposed similar selection pressures in independent populations and promoted parallel phenotypic adaptive radiations in all independently evolved microcosms.
Effects of neighbourhood size and connectivity on spatial Continuous Prisoner's Dilemma
Margarita Ifti,Timothy Killingback,Michael Doebeli
Quantitative Biology , 2004,
Abstract: The Prisoner's Dilemma, a 2-person game in which the players can either cooperate or defect, is a common paradigm for studying the evolution of cooperation, when individuals exhibit variable degrees of cooperation. It is known that in the presence of spatial structure, when individuals ``play against'' their neighbours, and ``compare to'' them, cooperative investments can evolve to considerable levels. Here we examine the effect of increasing the neighbourhood size: we find that the mean-field limit of no cooperation is reached for a critical neighbourhood size of about five neighbours. We also find the related result that in a network of players, the critical average degree (number of neighbours) of nodes for which defection is the final state depends only on the network topology. This critical average degree is considerably higher for clustered networks, than for distributed random networks. This result strengthens the argument that clustering is the mechanism which makes the development and maintenance of the cooperation possible.
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