Population cycles caused by selection by density dependent competitive interactions

Several animal species have cyclic population dynamics with phase-related cycles in life history traits such as body mass, reproductive rate, and pre-reproductive period. Although many mechanisms have been proposed there is no agreement on the cause of these cycles, and no population equation that deduces both the abundance and the life history cycles from basic ecological constraints has been formulated. Here I deduce a population dynamic equation from the selection pressure of density dependent competitive interactions in order to explain the cyclic dynamics in abundance and life history traits. The model can explain cycles by evolutionary changes in the genotype or by plastic responses in the phenotype. It treats the population dynamic growth rate as an initial condition, and its density independent fundament is Fisher’s (1930, The Genetical Theory of Natural Selection, Oxford: Clarendon) fundamental theorem of natural selection that predicts a hyper-geometrical increase in abundance. The predicted periods coincide with the cyclic dynamics of Lepidoptera, and the Calder hypothesis, which suggests that the period of population cycles is proportional to the 1/4 power of body mass, follows from first principles of the proposed density dependent ecology.     

From asexual to eusocial reproduction by multilevel selection by density-dependent competitive interactions

A game theoretical model is developed to illustrate that multilevel selection by density-dependent competitive interactions in mobile organisms might have played a major role in the evolutionary transitions from asexual over sexual to eusocial reproduction. The model has four equilibria with selection occurring among interacting units of respectively one, two, three, and up to infinitely many individuals. The different equilibria are characterised by different levels of competitive interactions among interacting units, and these levels select for different levels of sexual and co-operative reproduction among the individuals of the units. The model predicts: (i) that low-energy organisms with negligible body masses have asexual reproduction; (ii) that high-energy organisms with non-negligible body masses in evolutionary equilibria have sexual reproduction between a female and a male; (iii) that high-energy organisms with non-negligible body masses that increase exponentially at an evolutionary steady state have co-operative reproduction between a sexual pair and a single sexually produced offspring; and (iv) that high-energy organisms with upward constrained body masses have eusocial reproduction between a sexual pair and up to an infinite number of sexually produced offspring workers.     

Resource dynamics and seasonal changes in competitive interactions among three cladoceran species

A new approach to measuring zooplankton feeding success on naturalseston assemblages enabled us to test laboratory studies inthe field without being confined to radiotracers and their associatedproblems. We compared changes in the nutritional status of threespecies of cladoceran - Daphnia rosea Sars, D. middendorffianaFischer and Holopedium gibberum Zaddach - exposing them to differentnatural seston compositions. A modified version of the lipid-ovary index was applied in time-series experiments at CastleLake, California, throughout the summer of 1982. We used thesame index in competition experiments which were designed todetect shifts in competitive interactions among all of the abovespecies as a consequence of changes in the resource base. Thetime-series experiments with various seston compositions indicatedthat temporal and vertical distribution patterns of grazerswere strongly affected by the availability of suitable food.Daphnia rosea and H. gibberum had no noticeable effect on eachother's growth in early and midsummer, whereas the decline ofthe D. rosea population appeared to be accelerated by H. gibberumlater in the season. Daphnia middendorffiana, spatially separatedfrom the other two species, was unable to survive on food preferredby D. rosea and H. gibberum, whereas the latter two cladoceranswere negatively affected by D. middendorffiana when placed inwater containing hypolimnetic seston. Our results suggest thatresource abundance and composition has a much larger influenceon some zooplankton communities than previously acknowledged. ¹Present address: Lockheed EMSCO, Environmental Programs Office,1050 E. Flamingo Road, Las Vegas, NV 89109, USA     

Non-additive costs and interactions alter the competitive dynamics of co-occurring ecologically distinct plasmids

Plasmids play an important role in shaping bacterial evolution and adaptation to heterogeneous environments. As modular genetic elements that are often conjugative, the selective pressures that act on plasmid-borne genes are distinct from those that act on the chromosome. Many bacteria are co-infected by multiple plasmids that impart niche-specific phenotypes. Thus, in addition to host–plasmid dynamics, interactions between co-infecting plasmids are likely to be important drivers of plasmid population dynamics, evolution and ecology. Agrobacterium tumefaciens is a facultative plant pathogen that commonly harbours two distinct megaplasmids. Virulence depends on the presence of the tumour-inducing (Ti) plasmid, with benefits that are primarily restricted to the disease environment. Here, we demonstrate that a second megaplasmid, the At plasmid, confers a competitive advantage in the rhizosphere. To assess the individual and interactive costs of these plasmids, we generated four isogenic derivatives: plasmidless, pAt only, pTi only and pAtpTi, and performed pairwise competitions under carbon-limiting conditions. These studies reveal a low cost to the virulence plasmid when outside of the disease environment, and a strikingly high cost to the At plasmid. In addition, the costs of pAt and pTi in the same host were significantly lower than predicted based on single plasmid costs, signifying the first demonstration of non-additivity between naturally occurring co-resident plasmids. Based on these empirically demonstrated costs and benefits, we developed a resource–consumer model to generate predictions about the frequencies of these genotypes in relevant environments, showing that non-additivity between co-residing plasmids allows for their stable coexistence across environments.     

The effects of enrichment on the dynamics of apparent competitive interactions in stage-structured systems

In the absence of other limiting factors, assemblages in which species share a common, effective natural enemy are not expected to persist. Although a variety of mechanisms have been postulated to explain the coexistence of species that share natural enemies, the role of productivity gradients has not been explored in detail. Here, we examine how enrichment can affect the outcome of apparent competition. We develop a structured resource/consumer/natural enemy model in which the prey are exposed to attacks during a vulnerable life phase, the length of which depends on resource availability. With a single prey species, the model exhibits the "paradox of enrichment," with unstable dynamics at high levels of resource productivity. We extend this model to consider two prey species linked by a shared predator, each with their own distinct resource base. We derive invasion and stability conditions and examine how enrichment influences prey species exclusion and coexistence. Contrary to expectations from simpler, prey-dependent models, apparent competition is not necessarily strong at high productivity, and prey species coexistence may thus be more likely in enriched environments. Further, the coexistence of apparent competitors may be facilitated by unstable dynamics. These results contrast with the standard theory that apparent competition in productive environments leads to nonpersistent interactions and that coexistence of multispecies interactions is more likely under equilibrial conditions.     

On detecting interactions between species in population dynamics

If we are to progress out of our current state of uncertainty about the role of interspecific competition in community structure, it is essential that we can distinguish competition from other types of population interaction, and from lack of interaction, in particular case studies. To make such distinctions, it is necessary to quantify the effect of species on each other. One way to do this is to calculate interaction coefficients, and another (only readily applicable in experimental systems) is to plot graphs of N against time and to contrast monocultures with mixed cultures. We show (1) that these methods often appear to give contradictory results, and (2) that the problems are most pronounced when one species has a low equilibrium population size in mixed culture. To resolve the question of whether an interspecific interaction is taking place (and if so, what kind of interaction), it is necessary to apply tests of significance which overcome the problems of serial correlation inherent in all long-term population experiments. We illustrate the use of such tests in the analysis of the results from an experimental Drosophiia system. In the past, this kind of test has generally not been applied, and this raises the question of whether some of the 'classic cases of competition' in the experimental literature were really competition at all.     

Variation in selection pressures on the goldenrod gall fly and the competitive interactions of its natural enemies

Summary Larvae of the tephritid fly Eurosta solidaginis induce ball-shaped galls on the stem of tall goldenrod, Solidago altissima. Survival probability depends on gall size; in small galls the larva is vulnerable to parasitoid oviposition, whereas larvae in large galls are more frequently eaten by avian predators. Fly populations from 20 natural old fields in central Pennsylvania were monitored in 1983 and 1984 to examine the distribution of the selection intensity imposed by natural enemies, the parasitoids Eurytoma gigantea and E. obtusiventris, the inquiline Mordellistena unicolor, and the predatory birds Dendrocopus pubescens and Parus atricapillus. Mordellistena and E. obtusiventris are able to attack galls of all diameters while E. gigantea and the predatory birds preferentially assaulted small and large diameter galls, respectively. Eurosta in intermediate sized galls had the highest survivorship, hence selection had a stabilizing component. However, parasitoid attack was more frequent than bird attack, and the two did not exactly balance, thus there was also a directional component. The mean directional selection intensity on gall size was 0.21 standard deviations of the mean, indicating that larger gall size was favored. Interactions among the insect members of the Eurosta natural enemy guild are complex and frequent.     

Species-level selection reduces selfishness through competitive exclusion

Adaptation does not necessarily lead to traits which are optimal for the population. This is because selection is often the strongest at the individual or gene level. The evolution of selfishness can lead to a 'tragedy of the commons', where traits such as aggression or social cheating reduce population size and may lead to extinction. This suggests that species-level selection will result whenever species differ in the incentive to be selfish. We explore this idea in a simple model that combines individual-level selection with ecology in two interacting species. Our model is not influenced by kin or trait-group selection. We find that individual selection in combination with competitive exclusion greatly increases the likelihood that selfish species go extinct. A simple example of this would be a vertebrate species that invests heavily into squabbles over breeding sites, which is then excluded by a species that invests more into direct reproduction. A multispecies simulation shows that these extinctions result in communities containing species that are much less selfish. Our results suggest that species-level selection and community dynamics play an important role in regulating the intensity of conflicts in natural populations.     

On mathematical theory of selection: continuous time population dynamics

Mathematical theory of selection is developed within the frameworks of general models of inhomogeneous populations with continuous time. Methods that allow us to study the distribution dynamics under natural selection and to construct explicit solutions of the models are developed. All statistical characteristics of interest, such as the mean values of the fitness or any trait can be computed effectively, and the results depend in a crucial way on the initial distribution. The developed theory provides an effective method for solving selection systems; it reduces the initial complex model to a special system of ordinary differential equations (the escort system). Applications of the method to the Price equations are given; the solutions of some particular inhomogeneous Malthusian, Ricker and logistic-like models used but not solved in the literature are derived in explicit form.     

Endogenous timing in competitive interactions among relatives

Most evolutionary game theory models solve for equilibrium levels of some behaviour on the restrictive assumptions that players choose their actions simultaneously, and that a player cannot change its action after observing that of its opponent. An alternative framework is provided by sequential or 'Stackelberg' games in which one player commits to a 'first move' and the other has an opportunity to observe this move before choosing its response. Recent interest in the economic literature has focused on Stackelberg games which exhibit 'endogenous timing', i.e. games in which a leader and a follower arise spontaneously as a consequence of each player attempting to maximize its reward. Here, we provide the first demonstration of endogenous timing in an evolutionary context using a simple model of resource competition (the 'tug-of-war' model). We show that whenever two related individuals compete for a share of communal resources, both do best to adopt distinct roles in a sequential game rather than engage in simultaneous competition. Somewhat counterintuitively, the stable solution is for the weaker individual to act as leader and commit to a first move, because this arrangement leads to a lower total effort invested in competition. Endogenous timing offers a new explanation for the spontaneous emergence of leaders and followers in social groups, and highlights the benefits of commitment in social interaction.     

Optimal competitive hopfield network with stochastic dynamics for maximum cut problem

In this paper, introducing stochastic dynamics into an optimal competitive Hopfield network model (OCHOM), we propose a new algorithm that permits temporary energy increases which helps the OCHOM escape from local minima. The goal of the maximum cut problem, which is an NP-complete problem, is to partition the node set of an undirected graph into two parts in order to maximize the cardinality of the set of edges cut by the partition. The problem has many important applications including the design of VLSI circuits and design of communication networks. Recently, Galán-Marín et al. proposed the OCHOM, which can guarantee convergence to a global/local minimum of the energy function, and performs better than the other competitive neural approaches. However, the OCHOM has no mechanism to escape from local minima. The proposed algorithm introduces stochastic dynamics which helps the OCHOM escape from local minima, and it is applied to the maximum cut problem. A number of instances have been simulated to verify the proposed algorithm.     

Intraspecific competitive interactions rapidly evolve via spontaneous mutations

Using a mechanistic spatially explicit trait-based neighborhood-model, we quantify the impact of mutations on intraspecific spatial interactions to better understand mechanisms underlying the maintenance of genetic variation and the potential effects of these evolved interactions on the population dynamics of Arabidopsis thaliana. We use 100 twenty-fifth generation mutation accumulation (MA) lines (genotypes) derived from one founder genotype to study mutational effects on neighbor responses in a field experiment. We created individual-based maps (15,000 individuals), including phenotypic variation, to quantify mutational effects within genotypes versus between genotypes on reproduction and survival. At small-scale (within 80 cm of the focal plant), survival is enhanced but seed-set is decreased when a genotype is surrounded by different genotypes. At large-scale (within 200 cm of the focal plant), seed set is facilitated by different genotypes while the same genotype has either no effect or negative effects. The direction of the interactions among MA lines suggests that at small scale these interactions may contribute to the maintenance of genetic variation and at large scale contribute to the survival of the population. This may suggest, that, mutations potentially have immediate effects on population and community dynamics by influencing the outcome of competitive and faciliatory interactions among conspecifics.

     

Evaporative cooling feature selection for genotypic data involving interactions

MOTIVATION: The development of genome-wide capabilities for genotyping has led to the practical problem of identifying the minimum subset of genetic variants relevant to the classification of a phenotype. This challenge is especially difficult in the presence of attribute interactions, noise and small sample size. METHODS: Analogous to the physical mechanism of evaporation, we introduce an evaporative cooling (EC) feature selection algorithm that seeks to obtain a subset of attributes with the optimum information temperature (i.e. the least noise). EC uses an attribute quality measure analogous to thermodynamic free energy that combines Relief-F and mutual information to evaporate (i.e. remove) noise features, leaving behind a subset of attributes that contain DNA sequence variations associated with a given phenotype. RESULTS: EC is able to identify functional sequence variations that involve interactions (epistasis) between other sequence variations that influence their association with the phenotype. This ability is demonstrated on simulated genotypic data with attribute interactions and on real genotypic data from individuals who experienced adverse events following smallpox vaccination. The EC formalism allows us to combine information entropy, energy and temperature into a single information free energy attribute quality measure that balances interaction and main effects. AVAILABILITY: Open source software, written in Java, is freely available upon request.     

Genetic selection for dissociative inhibitors of designated protein-protein interactions

Many biological processes rely on protein-protein interactions. These processes include signal transduction, cell cycle regulation, gene regulation, and viral assembly and replication. Moreover, many proteins and enzymes manifest their function as oligomers. We describe here an efficient means to sift through large combinatorial libraries and identify molecules that block the interaction of target proteins in vivo. The power of this approach is demonstrated by the identification of nine-residue peptides from a combinatorial library that inhibit the intracellular dimerization of HIV-1 protease. Fewer than 1 in 106 peptides do so. In vitro biochemical analyses of one such peptide demonstrate that it acts by dissociating HIV-1 protease into monomers, which are inactive catalysts. Inhibition is enhanced further by dimerizing the peptide. This approach enables the facile identification of new molecules that control cellular processes.     

Foraging information affects the nature of competitive interactions

If animals can learn environmental cues, how might their use of information affect competitive interactions between species? I used shared‐preference isoleg theory to generate four different predictions regarding possible ways information could affect density‐dependent habitat selection. To test these predictions, I conducted field experiments on the foraging behavior of three coexisting species of hummingbirds. I studied black‐chinned (Archilochus alexandri), blue‐throated (Lampornis clemenciae), and magnificent (Eugenes fulgens) hummingbirds at the Southwestern Research Station in the Chiricahua Mts. of Southeastern Arizona, USA. Blue‐throated hummingbirds behaviorally dominate the other two species. I found that birds foraging with complete information (via learned color cues) avoid some of the negative effects from competition. Birds that foraged with complete information remained highly selective on rich feeders even with high competitor densities. When birds suffered an information deficit, however, the two subordinate species shifted their foraging preferences. The dominant species did not shift preference. Each subordinate's shift reflects its unique place in this competitively structured guild. With high competitor densities, black‐chinneds shifted from selective to opportunistic foraging when they also suffered from an information deficit. Thus, an information deficit caused black‐chinneds to cross over their first isoleg. Rather than reduce foraging efficiency, the magnificents shortened their foraging time. I use the magnificents’ response to competition and an information deficit to explore the possibility that this guild of hummingbirds is centrifugally organized. My results require us to consider the information‐gathering (e.g., learning) abilities of individual decision‐makers when we evaluate density‐dependent habitat selection. Differences in the level of usable information can alter the way coexisting species respond to competitive interactions that potentially structure communities.     

The dynamics of cohort selection

The dynamics of cohort selection in age-structured populations is explored in a series of computer simulations using a single-locus two-allele model. Cohort selection is a directional process which is a consequence of genetically determined inter-cohort variation in the magnitude of expression of interference behavior; its essential premise is that all genotypes of a given age are equally susceptible to interference and that no advantage accrues to the interfering genotype. In these simulations, allele frequency, population size, intensity of interference, and reproductive apportionment are all major determinants of evolutionary rates. The generality of this process is demonstrated and an analysis of the observed frequency-dependent effects is presented. Cohort selection is distinct from other kinds of selection and may be an important cause of the persistence of interference behavior (culminating in acts of spite) within animal populations.     

Natural selection and population dynamics

To what extent, and under which circumstances, are population dynamics influenced by concurrent natural selection? Density dependence and environmental stochasticity are generally expected to subsume any selective modulation of population growth rate, but theoretical considerations point to conditions under which selection can have an appreciable impact on population dynamics. By contrast, empirical research has barely scratched the surface of this fundamental question in population biology. Here, we present a diverse body of mostly empirical evidence that demonstrates how selection can influence population dynamics, including studies of small populations, metapopulations, cyclical populations and host-pathogen interactions. We also discuss the utility, in this context, of inferences from molecular genetic data, placing them within the broader framework of quantitative genetics and life-history evolution.     

Multilocus dynamics under haploid selection

 A general haploid selection model with arbitrary number of multiallelic loci and arbitrary linkage distribution is considered. The population is supposed to be panmictic. A dynamically equivalent diploid selection model is introduced. There is a position effect in this model if the original haploid selection is not multiplicative. If haploid selection is additive then the fundamental theorem is established even with an estimate for the change in the mean fitness. On this basis exponential convergence to an equilibrium is proved. As rule, the limit states are single-gamete ones. If, moreover, linkage is tight, then the single-gamete state with maximal fitness attracts the population for almost all initial states. Received 27 November 1995; received in revised form 17 January 1996