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At least 991 records · Page 55Linked to original sources
PubMed2006

Minimal selfing, few clones, and no among-host genetic structure in a hermaphroditic parasite with asexual larval propagation.

Little is known about actual mating systems in natural populations of parasites or about what constitutes the limits of a parasite deme. These parameters are interesting because they affect levels of genetic diversity, opportunities for local adaptation, and other evolutionary processes. We expect that transmission dynamics and the distribution of parasites among hosts should have a large effect on mating systems and demic structure, but currently we have mostly speculation and very few data. For example, infrapopulations (all the parasites in a single host) should behave as demes if parasite offspring are transmitted as a clump from host to host over several generations. However, if offspring are well mixed, then the parasite component population (all the parasites among a host population) would function as the deme. Similarly, low mean intensities or a high proportion of worms in single infections should increase the selfing rate. For species having an asexual amplification stage, transmission between intermediate and definitive (final) hosts will control the variance in clonal reproductive success, which in turn could have a large influence on effective sizes and rates of inbreeding. We examined demic structure, selfing rates, and the variance in clonal reproductive success in natural populations of Plagioporus shawi, a hermaphroditic trematode that parasitizes salmon. Overall levels of genetic diversity were very high. An a posteriori inference of population structure overwhelmingly supports the component population as the deme, rather than individual infrapopulations. Only a single pair of 597 adult individuals was identified as clones. Thus, the variance in clonal reproductive success was almost zero. Despite being hermaphroditic, P. shawi appears to be almost entirely outcrossing. Genetic estimates of selfing (<5%) were in accordance with the proportion of parasites from single infections. Thus, it appears that individual flukes outcross whenever possible and only resort to selfing when alone. Finally, our data support the hypothesis that aquatic transmission and the use of several intermediate hosts promotes high genetic diversity and well-mixed infrapopulations.

Animals↗
PubMed1977

[Immunofluorescence study of the formation of evolutionary stable lens proteins in chick embryos].

In the lens of fishes (carp, spiny dogfish) beta-crystallins were identified which were characteristic also of reptiles, amphibians, birds and mammals (evolutionary stable beta-crystallins). The dynamics of the formation of such beta-crystallins in 5--14 days old chick embryos was studied by the indirect immunofluorescence method with antisera to fish lens. These proteins are reliably indentified first at the lens sections from 7--8days old chick embryos. At all stages under study these beta-crystallins are localized mainly in the epithelial cells and practically not found in the lens fibers. They were, however, found in the fibrous (central) part of developing lens as well by the method of immunoelectrophoresis.

Age Factors↗
PubMed2004

A cross-platform model for secure Electronic Health Record communication.

During the past decade, there have been many regional, national and European projects focused on the development of platforms for secure access and sharing of distributed patient information. A platform is needed because present local or enterprise-wide information systems are typically not intended for cross-organisational secure access of patient data. Most of the present secure platforms are local or regional. Commonly used platform types in the health care environment vary from secure point-to-point communication systems to internet-based portals. This paper defines an enhanced cross-security platform which makes it possible for different kinds of local, regional, and national health information systems to communicate in a secure way. The proposed evolutionary way interconnects regional or national security domains with the help of a cross-platform zone. A more revolutionary model based on peer-to-peer Grid like networks and dynamic security credentials is also discussed. The proposed evolutionary model uses cross-domain security and interoperability services to ensure secure communication and interoperability between different security domains. The platform supports both communication defined beforehand and adhoc dynamic access to distributed electronic health records (EHRs). The internet is proposed as the "glue" between different regional or national security domains.

Access to Information↗
PubMedDate not supplied

[Parameters adaptation in the populations models].

Ecology-evolutionary models of low dimensions were developed on the basis of competitive selection criteria. Dynamics of variables (number of individuals) and the search of evolutionary-stable values of parameters (biological characterictics of populations) were monitored in the suggested models. If the environmental temperature is changing periodically, the average (a) and width (d) of temperature tolerance range appears to be the important parameters. By model experiments it was established that stable values of temperature (a), favorable for development of highly specialized algae (d is low) were close to minimum and maximum of temperature curve. And for the low specialized algae (d is high) this values were close to the average temperature of environment. In a similar manner, a set of evolutionally stable parameters (a, d) was established for either of the two interacted populations (competitors and "predator-prey"). The hypotheses concerning it's geometric structure and the process of coevolution is formulated.

Adaptation, Physiological↗
PubMed2004

The evolutionary origin of cooperators and defectors.

Coexistence of cooperators and defectors is common in nature, yet the evolutionary origin of such social diversification is unclear. Many models have been studied on the basis of the assumption that benefits of cooperative acts only accrue to others. Here, we analyze the continuous snowdrift game, in which cooperative investments are costly but yield benefits to others as well as to the cooperator. Adaptive dynamics of investment levels often result in evolutionary diversification from initially uniform populations to a stable state in which cooperators making large investments coexist with defectors who invest very little. Thus, when individuals benefit from their own actions, large asymmetries in cooperative investments can evolve.

Altruism↗
PubMedDate not supplied

Evolution and analysis of model CPGs for walking: I. Dynamical modules.

Can one develop an abstract description of the dynamics of pattern generators that provides quantitative insight into their operation? We explored this question by examining the dynamics of a model central pattern generator that was created using an evolutionary algorithm. We propose an abstract description based on the concept of a dynamical module, a set of neurons that simultaneously make their transitions from one quasistable state to another while the synaptic inputs that they receive from other neurons remain essentially constant, thus temporarily reducing the dimensionality of the circuit dynamics. Using the mathematical tools of dynamical systems theory, we describe a method for identifying dynamical modules and demonstrate that this concept can be used to quantitatively characterize constraints on neural architecture, account for phase durations, and predict the effects of parameter changes. Moreover, this abstract description reveals coordinated parameter changes that leave the overall circuit dynamics essentially unchanged. In a companion article we employ this abstract description to examine the relationship between general principles and individual variability in large populations of evolved model pattern generators.

Gait↗
PubMed2005

Glassy dynamics in the adaptive immune response prevents autoimmune disease.

The immune system normally protects the human host against death by infection. However, when an immune response is mistakenly directed at self-antigens, autoimmune disease can occur. We describe a model of protein evolution to simulate the dynamics of the adaptive immune response to antigens. Computer simulations of the dynamics of antibody evolution show that different evolutionary mechanisms, namely, gene segment swapping and point mutation, lead to different evolved antibody binding affinities. Although a combination of gene segment swapping and point mutation can yield a greater affinity to a specific antigen than point mutation alone, the antibodies so evolved are highly cross reactive and would cause autoimmune disease, and this is not the chosen dynamics of the immune system. We suggest that in the immune system's search for antibodies, a balance has evolved between binding affinity and specificity.

Animals↗
PubMed1996

Frequency-Dependent Stability for Two-Species Interactions

Evolutionary game theory is extended to models of two-species interactions where fitnesses are based on individual characteristics (strategies) rather than on a population dynamic that assumes homogeneous species. It is shown that the coevolutionary theories in the literature that combine ecology with genetic viability selection are part of this extended theory and that the dynamic stability resulting from a separation of ecological and evolutionary processes actually follows from game-theoretic solution concepts. The main focus of the paper is to investigate the application of the ESS (evolutionarily stable strategy) solution concept to dynamic stability when fitnesses are given by random interactions between individuals as opposed to viability selection. For two-species frequency-dependent interactions, the ESS criterion that implies stability asserts that, in any system near the ESS, at least one of the species is better off (i.e., more fit) if it evolves towards the ESS. The global stability of a polymorphic two-species ESS that is shown for two-species matrix games gives a powerful tool to predict the course of evolution through static fitness comparisons.

Journal Article↗
PubMed1995

Enfolding health-as-wholeness-and-harmony: a theory of Rogerian nursing practice.

The theory of enfolding health-as-wholeness-and-harmony has been derived from the science of unitary human beings and provides an evolutionary understanding of Rogerian nursing practice that recognizes the existence of subtle and gross manifestations of field pattern and the dynamic matrix of subtle configurations of patterning identified as health-as-wholeness-and-harmony. Evolutionary change is specified to be a dynamic, non-linear, and acausal process characterized by the movement of gross manifestations of patterning to new syntheses of subtle manifestations of patterning. Rogerian nursing practice is identified as the nurse and client knowingly participating in evolutionary patterning of the human and environmental fields for the purpose of enfolding health-as-wholeness-and-harmony.

Holistic Health↗
PubMed1992

Fundamental issues in nonlinear urban population dynamic models: theory and a synthesis.

"The main purpose of this paper is to examine certain basic similarities between three nonlinear models of urban dynamics which appeared in [1980], and seem to be driven by somewhat different mechanisms.... By looking at their implicit assumptions we set the stage for addressing certain fundamental premises underlying the process of urban growth or decline. Searching for these fundamental assumptions permits the construction of a comprehensive-unifying theory of urban population dynamics placing urban evolution squarely within the domain of evolutionary theory."

Demography↗
PubMed2004

Knowing your enemies: seasonal dynamics of host-social parasite recognition.

Despite its evolutionary significance, behavioural flexibility of social response has rarely been investigated in insects. We studied a host-social parasite system: the slave-making ant Polyergus rufescens and its host Formica rufibarbis. Free-living host workers from parasitized and from unparasitized areas were compared in their level of aggression against the parasite and alien conspecifics. We expected that a seasonal change would occur in the acceptance threshold of F. rufibarbis workers from a parasitized area towards the parasite, whereas F. rufibarbis workers from an unparasitized area would not show substantial changes connected with the parasite's peak in activity (raiding and colony-founding season). The results showed a significant adaptive behavioural flexibility of host species workers and are consistent with the acceptance threshold model's (Reeve 1989) prediction that recognition systems are not fixed but context-dependent. In particular, host workers from the unparasitized area were highly aggressive towards the parasite regardless of the season, whereas host workers from the parasitized area significantly increased their aggression towards the parasite during its raiding and colony-founding season. Being able to detect and possibly kill a Polyergus scout searching for host nests can be an effective strategy for a Formica colony to avoid being raided or usurped by a parasite queen.

Aggression↗
PubMed2002

Evolutionary speed limits inferred from the fossil record.

The dynamics of extinction and diversification determine the long-term effects of extinction episodes. If rapid bursts of extinction are offset by equally rapid bursts of diversification, their biodiversity consequences will be transient. But if diversification rates cannot accelerate rapidly enough, pulses of extinction will lead to long-lasting depletion of biodiversity. Here I use spectral analysis of the fossil record to test whether diversification rates can accelerate as much as extinction rates, over both short and long spans of geological time. I show that although the long-wavelength variability of diversification rates equals or exceeds that of extinctions, diversification rates are markedly less variable than extinction rates at wavelengths shorter than roughly 25 million years. This implies that there are intrinsic speed limits that constrain how rapidly diversification rates can accelerate in response to pulses of extinction.

Animals↗
PubMed1975

Intrinsic asymmetry of oligomer transitions and biomolecular evolution.

Structural transitions in oligomeric proteins due to ligand binding are important in biomolecular regulatory processes. The transitions may occur on the secondary, tertiary or quarternary structure levels. Detailed consideration of the time sequence of ligand binding to the oligomer shows that there is an intrinsic dynamic asymmetry in all oligomer transitions, even if the initial and the final state are completely symmetric. This asymmetry has important bearing on the evolution and the divergence of the primary structure (amino acid sequence) of oligomeric proteins. It may explain (at least in part) the occurrence of oligomeric proteins with similar but not identical protomers. Certain specific groups of oligomers are shown to be under greater evolutionary pressure for protomer structure divergence. The dynamic asymmetry of oligomer transitions also results in higher complexity in reaction kinetics. Some implications on ribosome structural evolution are discussed.

Biological Evolution↗
PubMed2004

In vivo dynamics of an immune response in the bumble bee Bombus terrestris.

Concepts from evolutionary ecology have recently been applied to questions of immune defences. However, an important but often neglected aspect is the temporal dynamics of the simple immune measures used in ecological studies. Here, we present observations for workers of the bumble bee Bombus terrestris on the dynamics of the phenoloxidase (PO) system, antibacterial activity, and the total number of haemocytes following a challenge with immune elicitors (LPS, Laminarin), over a time-span ranging from 1min to 14 days. The dynamics of the PO measurement showed a complex pattern and was correlated with haemocyte counts. Antibacterial activity, on the other hand, increased sharply between 2 and 24h post-challenge followed by a slow decrease. Surprisingly, the effects of a challenge lasted up to 14 days.

Adjuvants, Immunologic↗
PubMed2005

Diverse evolutionary trajectories characterize a community of RNA-cleaving deoxyribozymes: a case study into the population dynamics of in vitro selection.

Two parallel in vitro selections (denoted Selection A and Selection B) were conducted under different selection-pressure regimes, yielding a diverse community of RNA-cleaving deoxyribozymes. In Selection A, the reaction time was reduced four times (from 5 h to 5 s) over the course of 24 generations, while in Selection B the reaction time was maintained at 5 h for 30 rounds of selective amplification. Sequence alignment was conducted on more than 800 clones assembled from 18 generations that span both selections. Many prominent catalytic sequence classes, including some that extend across both selections, were identified and used to construct fitness landscapes depicting their rise and fall over time. The landscapes from both selections exhibit similar global trends despite differences in population dynamics. Some deoxyribozymes were predominant in the early rounds of selection but gave way to other species that dominated in the middle rounds. Ultimately, these middle classes disappeared from the landscape in favor of new and presumably more fit deoxyribozyme sequence classes. The shape of these landscapes alludes to the presence of many latent deoxyribozymes in the initial library, which can only be accessed by changes in the selection pressure and/or by adaptive mutations. Basic computer simulations provide theoretical corroboration of the experimentally observed pattern of staggered sequence-class transitions across the fitness landscapes. These simulations model the influence of one or more contributing factors, including catalytic rate, folding efficiency, PCR amplification efficiency, and random mutagenesis. This is the first study which thoroughly documents the topography of a deoxyribozyme fitness landscape over many generations of in vitro selection.

Base Sequence↗
PubMed2006

Immunoepigenetics: the unseen side of cancer immunoediting.

Cancer immunosurveillance representing, till recently, the explanatory framework relating cancer and the immune system, does not convincingly explain tumor escape. At the beginning of the decade, a new theory emerged, namely the immunoediting theory, and it comprehensively defines the role of the immune system in carcinogenesis. The core of this theory embraces the concept that the immune system on the one hand protects the body from cancer and on the other it shapes the immunogenicity of these cancers, thus presents a persuasive rationalization of the resistance of tumors against the immune response. With the immune system playing, in this context, such a pivotal role in shaping the tumor immune profile and in subsequent oncogenesis, it seems rather paradoxical to accept the immunocompetent host's immune system as a constant moiety. While DNA mutations of immune genes create a rather polymorphic condition, their frequency is much lower than that of other genetic events. Of these, epigenetic alterations give rise to new epialleles, which can reach up to 100% per locus. Bearing in mind that cancer is characterized by a tremendous amount of epigenetic aberrations, in both gene and global level, it is reasonable to postulate that, for the same unknown causes, analogous aberrations could affect the immune genes. Should this be the case, the relation between oncogenesis and the immune system appears much more dynamic and complex. Such an immunoepigenetic approach to carcinogenesis could improve our understanding of a series of common cancer-related aspects, such as environmental risk factors, effectiveness of demethylating agents, failure of current immunotherapies, etc. Moreover, this immunoepigenetic paradigm will take the current perception of the immune system and cancer interrelation further and beyond, constituting that the immunoresistant cancer cell phenotype is not shaped by the immune system acting as a steady and rigid evolutionary pressure, but rather as an extremely dynamic variable.

Antigens, Neoplasm↗
PubMed2001

Why the null matters: statistical tests, random walks and evolution.

A number of statistical tests have been developed to determine what type of dynamics underlie observed changes in morphology in evolutionary time series, based on the pattern of change within the time series. The theory of the 'scaled maximum', the 'log-rate-interval' (LRI) method, and the Hurst exponent all operate on the same principle of comparing the maximum change, or rate of change, in the observed dataset to the maximum change expected of a random walk. Less change in a dataset than expected of a random walk has been interpreted as indicating stabilizing selection, while more change implies directional selection. The 'runs test' in contrast, operates on the sequencing of steps, rather than on excursion. Applications of these tests to computer generated, simulated time series of known dynamical form and various levels of additive noise indicate that there is a fundamental asymmetry in the rate of type II errors of the tests based on excursion: they are all highly sensitive to noise in models of directional selection that result in a linear trend within a time series, but are largely noise immune in the case of a simple model of stabilizing selection. Additionally, the LRI method has a lower sensitivity than originally claimed, due to the large range of LRI rates produced by random walks. Examination of the published results of these tests show that they have seldom produced a conclusion that an observed evolutionary time series was due to directional selection, a result which needs closer examination in light of the asymmetric response of these tests.

Animals↗
PubMed2005

Evolvability suppression to stabilize far-sighted adaptations.

The opportunistic character of adaptation through natural selection can lead to evolutionary pathologies--situations in which traits evolve that promote the extinction of the population. Such pathologies include imprudent predation and other forms of habitat overexploitation, or the tragedy of the commons, adaptation to temporally unreliable resources, cheating and other antisocial behavior, infectious pathogen carrier states, parthenogenesis, and cancer, an intraorganismal evolutionary pathology. It is known that hierarchical population dynamics can protect a population from invasion by pathological genes. Can it also alter the genotype so as to prevent the generation of such genes in the first place, that is, suppress the evolvability of evolutionary pathologies? A model is constructed in which one locus controls the expression of the pathological trait, and a series of modifier loci exist that can prevent the expression of this trait. It is found that multiple evolvability checkpoint genes can evolve to prevent the generation of variants that cause evolutionary pathologies. The consequences of this finding are discussed.

Adaptation, Physiological↗