Invasion Fitness Research Articles

Within-host competition and diversification of macro-parasites.

Although competitive speciation is more and more regarded as a plausible mechanism for sympatric speciation of non-parasite species, virtually no empirical or theoretical study has considered this evolutionary process to explain intra-host diversification of parasites. We expanded the theory of competitive speciation to parasite species looking at the effect of macro-parasite life history on the conditions for sympatric speciation under the so-called pleiotropic scenario. We included within-host competition in the classical Anderson and May framework assuming that individuals exploit within-host resources according to a quantitative trait. We derived the invasion fitness function of mutants considering different distributions of individuals among hosts. Although the mutant fitness depends on parameters describing the key features of macro-parasite life history, and on the relative distributions of mutant and residents in hosts, the conditions for competitive speciation of macro-parasites are exactly the same as those previously established for free-living species. As an interesting by-product, within-host competitive speciation is expected not to depend on the aggregation level of the parasites. This theoretical pattern is confirmed by comparing the speciation rate of weakly and strongly aggregated monogenean parasites.

Symmetric competition as a general model for single-species adaptive dynamics

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.

On invasion boundaries and the unprotected coexistence of two strategies

In this paper we present, in terms of invasion fitness functions, a sufficient condition for a coexistence of two strategies which are not protected from extinction when rare. In addition, we connect the result to the local characterization of singular strategies in the theory of adaptive dynamics. We conclude with some illustrative examples.

Continuously stable strategies as evolutionary branching points

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 CSS. Here we show that the scope of evolutionary branching can increase if the invasion fitness function vanishes to higher than first order at CSS. Using 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. 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.

Measurement and Optimization of Minimally Invasive Intervention Device Design Fitness Using a Multiobjective Weighted Isotropy Index

The recent transition from multiple-port to single-port systems in minimally invasive intervention (MII) procedures has created a need for more flexible, dexterous robotic manipulation devices capable of spanning an entire surgical workspace without the risk of collateral damage. The design of such devices requires a careful balance of the mechanical complexity needed to facilitate clinical functionality and the cost of manufacturing and operating the device. This paper presents a novel metric for measuring the design fitness of kinematically redundant robotic MII devices and for optimizing them to achieve that balance. The proposed fitness metric rewards designs that are conducive to collision avoidance and energy conservation while penalizing those with exorbitant design complexities that adversely affect the economic feasibility of an MII system. The authors’ metric is used here to design a kinematically redundant, single-port MII device capable of accessing the cardiothoracic cavity through a single subxiphoid port and reaching several regions of interest, consistent with procedures such as epicardial ablation and therapeutic substance injection, with minimal physiologic disturbance. The design of this device is determined by a morphological optimization process, which searches a discrete mechanical design parameter space, consisting of linkage parts, part dimensions, and actuator types, using genetic algorithms. The execution of specific surgical maneuvers is simulated for each candidate MII device design, and the design is improved until the fitness metric is maximized. The results of this optimization study demonstrate that redesigning a 20 degree-of-freedom (DOF) MII device using the proposed metric decreased the DOF in the design by 45% while ensuring near-optimal levels of kinematic flexibility. The results also demonstrate the ability of the fitness metric to elucidate the relationship between functionality and complexity and to produce suitable device designs over a broad range of performance and cost goals. The authors conclude that this new design fitness metric, while heuristic in nature, holds the potential to improve both the clinical value and the economy of a wide variety of single-port MII devices, including those used in cardiothoracic surgery.

Next-generation tools for evolutionary invasion analyses.

Evolutionary invasion analysis is a powerful technique for modelling in evolutionary biology. The general approach is to derive an expression for the growth rate of a mutant allele encoding some novel phenotype, and then to use this expression to predict long-term evolutionary outcomes. Mathematically, such 'invasion fitness' expressions are most often derived using standard linear stability analyses from dynamical systems theory. Interestingly, there is a mathematically equivalent approach to such stability analyses that is often employed in mathematical epidemiology, and that is based on so-called 'next-generation' matrices. Although this next-generation matrix approach has sometimes also been used in evolutionary invasion analyses, it is not yet common in this area despite the fact that it can sometimes greatly simplify calculations. The aim of this article is to bring the approach to a wider evolutionary audience in two ways. First, we review the next-generation matrix approach and provide a novel, and easily intuited, interpretation of how this approach relates to more standard techniques. Second, we illustrate next-generation methods in evolutionary invasion analysis through a series of informative examples. Although focusing primarily on evolutionary invasion analysis, we provide several insights that apply to biological modelling in general.

The evolution of juvenile-adult interactions in populations structured in age and space

We study the evolution of a spatially structured population with two age classes using spatial moment equations. In the model, adults can either help juveniles by increasing their survival, or adopt a cannibalistic behaviour and consume juveniles. While cannibalism is the sole evolutionary outcome when the population is well-mixed, both cannibalism and parental care can be evolutionarily stable if the population is viscous. Our analysis allows us to make two main technical points. First, we present a method to define invasion fitness in class-structured viscous populations, which allows us to apply adaptive dynamics methodology. Second, we show that ordinary pair approximation introduces an important quantitative bias in the evolutionary model, even on random networks. We propose a correction to the ordinary pair approximation that yields quantitative accuracy, and discuss how the bias associated with this approach is precisely what allows us to identify subtle aspects associated with the evolutionary dynamics of spatially structured populations.

Evolution of age-dependent sex-reversal under adaptive dynamics

We investigate the evolution of the age (or size) at sex-reversal in a model of sequential hermaphroditism, by means of the function-valued adaptive dynamics. The trait is the probability law of the age at sex-reversal considered as a random variable. Our analysis starts with the ecological model which was first introduced and analyzed by Calsina and Ripoll (Math Biosci 208(2), 393-418, 2007). The structure of the population is extended to a genotype class and a new model for an invading/mutant population is introduced. The invasion fitness functional is derived from the ecological setting, and it turns out to be controlled by a formula of Shaw-Mohler type. The problem of finding evolutionarily stable strategies is solved by means of infinite-dimensional linear optimization. We have found that these strategies correspond to sex-reversal at a single particular age (or size) even if the set of feasible strategies is considerably broader and allows for a probabilistic sex-reversal. Several examples, including in addition the population-dynamical stability, are illustrated. For a special case, we can show that an unbeatable size at sex-reversal must be larger than 69.3% of the expected size at death.

ASSORTATIVE MATE CHOICE AND DOMINANCE MODIFICATION: ALTERNATIVE WAYS OF REMOVING HETEROZYGOTE DISADVANTAGE

In genetic polymorphisms of two alleles, heterozygous individuals may contribute to the next generation on average more or fewer descendants than the homozygotes. Two different evolutionary responses that remove a disadvantageous heterozygote phenotype from the population are the evolution of strictly assortative mate choice, and that of a modifier making one of the two alleles completely dominant. We derive invasion fitness of mutants introducing dominance or assortative mate choice in a randomly mating population with a genetic polymorphism for an ecological trait. Mutations with small effects as well as mutants introducing complete dominance or perfect assorting are considered. Using adaptive dynamics techniques, we are able to calculate the ratio of fitness gradients for the effects of a dominance modifier and a mate choice locus, near evolutionary branching points. With equal resident allele frequencies, selection for mate choice is always stronger. Dominance is more strongly selected than assortative mating when the resident (common) alleles have very unequal frequencies at equilibrium. With female mate choice the difference in frequencies where dominance is more strongly selected is smaller than when mutants of both sexes can choose without costs. A symmetric resource-competition model illustrates the results.

Competition and parasitism in the native White Clawed Crayfish Austropotamobius pallipes and the invasive Signal Crayfish Pacifastacus leniusculus in the UK

Many crayfish species have been introduced to novel habitats worldwide, often threatening extinction of native species. Here we investigate competitive interactions and parasite infections in the native Austropotamobius pallipes and the invasive Pacifastacus leniusculus from single and mixed species populations in the UK. We found A. pallipes individuals to be significantly smaller in mixed compared to single species populations; conversely P. leniusculus individuals were larger in mixed than in single species populations. Our data provide no support for reproductive interference as a mechanism of competitive displacement and instead suggest competitive exclusion of A. pallipes from refuges by P. leniusculus leading to differential predation. We screened 52 P. leniusculus and 12 A. pallipes for microsporidian infection using PCR. We present the first molecular confirmation of Thelohania contejeani in the native A. pallipes; in addition, we provide the first evidence for T. contejeani in the invasive P. leniusculus. Three novel parasite sequences were also isolated from P. leniusculus with an overall prevalence of microsporidian infection of 38% within this species; we discuss the identity of and the similarity between these three novel sequences. We also screened a subset of fifteen P. leniusculus and three A. pallipes for Aphanomyces astaci, the causative agent of crayfish plague and for the protistan crayfish parasite Psorospermium haeckeli. We found no evidence for infection by either agent in any of the crayfish screened. The high prevalence of microsporidian parasites and occurrence of shared T. contejeani infection lead us to propose that future studies should consider the impact of these parasites on native and invasive host fitness and their potential effects upon the dynamics of native-invader systems.

An Analytically Tractable Model for Competitive Speciation

Several recent models have shown that frequency-dependent disruptive selection created by intraspecific competition can lead to the evolution of assortative mating and, thus, to competitive sympatric speciation. However, since most of these results rely on limited numerical analyses, their generality has been debated. Here, we consider one of the standard models (the so-called Roughgarden model) with a simplified genetics where the selected trait is determined by a single diallelic locus. This model is sufficiently complex to maintain key properties of the general multilocus case but simple enough to allow for comprehensive analytical treatment by means of invasion fitness arguments. Depending on (1) the strength and (2) the shape of stabilizing selection, (3) the strength and (4) the shape of pairwise competition, (5) the shape of the mating function, and (6) whether assortative mating leads to sexual selection, we find five different evolutionary regimes. In one of these regimes, complete reproductive isolation can evolve through arbitrarily small steps in the strength of assortative mating. Our approach provides a mechanistic understanding of several phenomena that have been found in previous models. The results demonstrate how even in a simple model, the evolutionary outcome depends in a complex way on ecological and genetic parameters.

Adaptive dynamics for physiologically structured population models

We develop a systematic toolbox for analyzing the adaptive dynamics of multidimensional traits in physiologically structured population models with point equilibria (sensu Dieckmann et al. in Theor. Popul. Biol. 63:309-338, 2003). Firstly, we show how the canonical equation of adaptive dynamics (Dieckmann and Law in J. Math. Biol. 34:579-612, 1996), an approximation for the rate of evolutionary change in characters under directional selection, can be extended so as to apply to general physiologically structured population models with multiple birth states. Secondly, we show that the invasion fitness function (up to and including second order terms, in the distances of the trait vectors to the singularity) for a community of N coexisting types near an evolutionarily singular point has a rational form, which is model-independent in the following sense: the form depends on the strategies of the residents and the invader, and on the second order partial derivatives of the one-resident fitness function at the singular point. This normal form holds for Lotka-Volterra models as well as for physiologically structured population models with multiple birth states, in discrete as well as continuous time and can thus be considered universal for the evolutionary dynamics in the neighbourhood of singular points. Only in the case of one-dimensional trait spaces or when N = 1 can the normal form be reduced to a Taylor polynomial. Lastly we show, in the form of a stylized recipe, how these results can be combined into a systematic approach for the analysis of the (large) class of evolutionary models that satisfy the above restrictions.

From Infanticide to Parental Care: Why Spatial Structure Can Help Adults Be Good Parents

We investigate the evolution of parental care and cannibalism in a spatially structured population where adults can either help or kill juveniles in their neighborhood. We show that spatial structure can reverse the selective pressures on adult behavior, leading to the evolution of parental care, whereas the nonspatial model predicts that cannibalism is the sole evolutionary outcome. Our analysis emphasizes that evolution of such spatially structured populations is best understood at the level of the cluster of invading mutants, and we define invasion fitness as the growth rate of that cluster. We derive an analytical expression for the selective pressures on the trait and show that relatedness and Hamilton's rule are recovered as emergent properties of the spatial ecological dynamics. When adults can also help other adults, the benefits to each class of recipients are weighted by the class reproductive value, a result consistent with that of other models of kin selection. Finally, we advocate a different approach to moment equations and argue that even though the development of moment closure approximations is a necessary line of research, much-needed ecological and evolutionary insight can be gained by studying the unclosed moment equations.

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of <I>Cyphocleonus achates</I> (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

The root-feeding weevil Cyphocleonus achates (Fahraeus) is a promising biological control agent for managing the exotic, invasive weed spotted knapweed. The objective of this study was to compare the relative and potentially interactive effects of competition and specialized herbivory on spotted knapweed fitness. Competition was assessed through three grass seeding treatments: a nonseeded control and seeding to long-lived wheatgrasses originating either from North America, Pseudoroegneria spicata (Pursh) A. Love ssp. spicata, or Europe, Thinopyrum intermedium (Host) Barkworth and D.R. Dewey. Comparisons were made of above-ground biomass, stem length, number of flowers, and life stage densities for spotted knapweed plants either infested or free of C. achates and growing under one of the grass seeding treatments. Infested adult spotted knapweed plants had lower shoot weight, stem length, and flowers/plant. The density of adult spotted knapweed plants was lower in grass-seeded plots than in nonseeded control plots. Results indicate C. achates herbivory is correlated with a reduction in measures of fitness and reproductive potential in field populations of spotted knapweed in North America. Furthermore, these results suggest that this agent might play a significant role in reducing spotted knapweed populations when competitive grasses are also present.

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of Cyphocleonus achates (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of Cyphocleonus achates (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

Spatially extended host–parasite interactions: The role of recovery and immunity

Techniques for determining the long-term dynamics of host–parasite systems are well established for mixed populations. The field of spatial modelling in ecology is more recent but a number of key advances have been made. In this paper, we use state-of-the-art approximation techniques, supported by simulations, in order to investigate the role of recovery and immunity in spatially structured populations. Our approach is to use correlation models, namely pair-wise models, to capture the spatial relationships of contacts and interactions between individuals. We use the pair-wise framework to address a number of key ecological questions; including, the persistence of endemic limit cycles and regions of parasite-driven extinction—features which differentiate spatial from non-spatial models—and the effects on invasion fitness. We demonstrate a loss of limit cycle behaviour, in addition to an increase in the critical transmissibility and extinction thresholds, when recovery is included. This approach allows for a better analytical understanding of the dynamics of host–parasite interactions and demonstrates the importance of recovery and immunity in local interactions.

Advantage of storage in a fluctuating environment

We will elaborate the evolutionary course of an ecosystem consisting of a population in a chemostat environment with periodically fluctuating nutrient supply. The organisms that make up the population consist of structural biomass and energy storage compartments. In a constant chemostat environment a species without energy storage always out-competes a species with energy reserves. This hinders evolution of species with storage from those without storage. Using the adaptive dynamics approach for non-equilibrium ecological systems we will show that in a fluctuating environment there are multiple stable evolutionary singular strategies ( ss's): one for a species without, and one for a species with energy storage. The evolutionary end-point depends on the initial evolutionary state. We will formulate the invasion fitness in terms of Floquet multipliers for the oscillating non-autonomous system. Bifurcation theory is used to study points where due to evolutionary development by mutational steps, the long-term dynamics of the ecological system changes qualitatively. To that end, at the ecological time scale, the trait value at which invasion of a mutant into a resident population becomes possible can be calculated using numerical bifurcation analysis where the trait is used as the free parameter, because it is just a bifurcation point. In a constant environment there is a unique stable equilibrium for one species following the “competitive exclusion” principle. In contrast, due to the oscillatory dynamics on the ecological time scale two species may coexist. That is, non-equilibrium dynamics enhances biodiversity. However, we will show that this coexistence is not stable on the evolutionary time scale and always one single species survives.

Evolution of discrete populations and the canonical diffusion of adaptive dynamics

The biological theory of adaptive dynamics proposes a description of the long-term evolution of a structured asexual population. It is based on the assumptions of large population, rare mutations and small mutation steps, that lead to a deterministic ODE describing the evolution of the dominant type, called the 'canonical equation of adaptive dynamics'. Here, in order to include the effect of stochasticity (genetic drift), we consider self-regulated randomly fluctuating populations subject to mutation, so that the number of coexisting types may fluctuate. We apply a limit of rare mutations to these populations, while keeping the population size finite. This leads to a jump process, the so-called 'trait substitution sequence', where evolution proceeds by successive invasions and fixations of mutant types. Then we apply a limit of small mutation steps (weak selection) to this jump process, that leads to a diffusion process that we call the 'canonical diffusion of adaptive dynamics', in which genetic drift is combined with directional selection driven by the gradient of the fixation probability, also interpreted as an invasion fitness. Finally, we study in detail the particular case of multitype logistic branching populations and seek explicit formulae for the invasion fitness of a mutant deviating slightly from the resident type. In particular, second-order terms of the fixation probability are products of functions of the initial mutant frequency, times functions of the initial total population size, called the invasibility coefficients of the resident by increased fertility, defence, aggressiveness, isolation, or survival.

Health and Fitness Programming for Fire Fighters

Firefighting is a job with unique physical demands that puts fire fighters (FF) at increased risk for cardiovascular events and musculoskeletal injuries. Despite this, many FF are resistant to physical fitness programs on and off the job designed to improve health and increase years of service. PURPOSE: The purpose of this investigation was to determine the long term impact of a minimally invasive health and fitness (H&F) program on chronic disease risk factors and physical fitness in a small fire department. METHODS: Sixteen line FF (16 males; Age= 42.6 ±8.6 yrs) participated in H&F testing followed by two year lifestyle intervention programming. All FF completed physical exams, pulmonary function testing, and physical fitness testing at baseline and after one year. The one year intervention consisted of monthly educational H&F talks; on-duty, exercise programming; and quarterly body-fat assessments. Repeated-measures ANOVA was used to compare cardiovascular and metabolic risk factors and physical fitness levels after one year of intervention. RESULTS: As listed in the table below, improvements made in several H&F areas were maintained over the two years. The department also maintained a 12 lbs weight loss (baseline: 215.7 ± 40.4; 2 yr: 203.3 ± 34.9; P= 0.009). Moreover, there was a significant improvement in lipid profiles, particularly HDL cholesterol (baseline: 41.2 ± 11.9 mg/dl; 2 yr: 50.9 ± 15.9 mg/dl).TableCONCLUSION: This study is further evidence that even minor adjustments in health and lifestyle programming at fire departments can result in major improvements in overall health that can persist over an extended period of time.

Apoptosis-like death as a feature of malaria infection in mosquitoes

Malaria parasites of the genus Plasmodium make a hazardous journey through their mosquito vectors. The majority die in the process, many as a result of the action of mosquito defence mechanisms. The mosquito too is not unscathed by the encounter with these parasites. Tissue damage occurs as a result of mid-gut invasion and reproductive fitness is lost when many developing ovarian follicles are resorbed. Here we discuss some of the mechanisms that are involved in killing the parasite and in the self-defence mechanisms employed by the mosquito to repair the mid-gut epithelium and to manipulate resources altering the trade-off position that balances reproduction and survival. In all cases, cells die by apoptotic-like mechanisms. In the midgut cells, apoptosis-induction pathways are being elucidated, the molecules involved in apoptosis are being recognised and Drosophila homologues sought. The death of ookinetes in the mosquito mid-gut lumen is associated with caspase-like activity and, although homologues of mammalian caspases are not present in the malaria genome, other cysteine proteases that are potential candidates have been discussed. In the ovary, apoptosis of patches of follicular epithelial cells is followed by resorption of the developing follicle and a subsequent loss of egg production in that follicle.