PubMed Health⌕ Search

Biomedical subjects

M Doebeli

Publications and source records attributed to M Doebeli.

At least 19 recordsLinked to original sources
PubMed2001

A bit of sex stabilizes host-parasite dynamics.

To date, only a few studies have focused on the effects of sex on population dynamics. Previous models have typically found that sexual reproduction dampens population fluctuations. Although asexual and sexual reproduction are just the two endpoints along a continuum of varying rates of sex, previous work has ignored the effects of intermediate degrees of sex on population dynamics. Here we study the effects of partial sexual reproduction (i.e. sex occurs only every few generations or with small probability in each generation) on the coupled population dynamics of a Nicholson-Bailey host-parasite model. We show that complex dynamics are simplified for high host population growth rates if the frequency of sex is sufficiently high in both host and parasite: sex decreases fluctuations in population density, and leads to non-chaotic dynamics for population growth rates that would result in chaotic dynamics in the absence of sexual reproduction. However, the simplification does not increase gradually with an increasing frequency of sex but appears abruptly at low-to-intermediate frequencies of sex. For some parameter settings, intermediate frequencies of sexual reproduction can simplify the dynamics more than lower or higher frequencies. Thus, in agreement with earlier results, sexual reproduction typically stabilizes complex population dynamics in our models. Additionally, our results suggest that low-to-intermediate frequencies of sex may often be as (or even more) stabilizing as high frequencies.

Animals↗
PubMed2000

Experimental evolution of aging, growth, and reproduction in fruitflies.

We report in this paper an evolutionary experiment on Drosophila that tested life-history theory and the evolutionary theory of aging. As theory predicts, higher extrinsic mortality rates did lead to the evolution of higher intrinsic mortality rates, to shorter lifespans, and to decreased age and size at eclosion; peak fecundity also shifted earlier in life. These results confirm the key role of extrinsic mortality rates in the evolution of growth, maturation, reproduction, and aging, and they do so with a selection regime that maintained selection on fertility throughout life while holding population densities constant.

Aging↗
PubMed1999

Evolution of cooperation in spatially structured populations

Using a spatial lattice model of the Iterated Prisoner's Dilemma we studied the evolution of cooperation within the strategy space of all stochastic strategies with a memory of one round. Comparing the spatial model with a randomly mixed model showed that (1) there is more cooperative behaviour in a spatially structured population, (2) PAVLOV and generous variants of it are very successful strategies in the spatial context and (3) in spatially structured populations evolution is much less chaotic than in unstructured populations. In spatially structured populations, generous variants of PAVLOV are found to be very successful strategies in playing the Iterated Prisoner's Dilemma. The main weakness of PAVLOV is that it is exploitable by defective strategies. In a spatial context this disadvantage is much less important than the good error correction of PAVLOV, and especially of generous PAVLOV, because in a spatially structured population successful strategies always build clusters. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Variable investment, the Continuous Prisoner's Dilemma, and the origin of cooperation.

Cooperation is fundamental to many biological systems. A common metaphor for studying the evolution of cooperation is the Prisoner's Dilemma, a game with two strategies: cooperate or defect. However, cooperation is rare all or nothing, and its evolution probably involves the gradual extension of initially modest degrees of assistance. The inability of the Prisoner's Dilemma to capture this basic aspect limits its use for understanding the evolutionary origins of cooperation. Here we consider a framework for cooperation based on the concept of investment: an act which is costly, but which benefits other individuals, where the cost and benefit depend on the level of investment made. In the resulting Continuous Prisoner's Dilemma the essential problem of cooperation remains: in the absence of any additional structure non-zero levels of investment cannot evolve. However, if investments are considered in a spatially structured context, selfish individuals who make arbitrarily low investments can be invaded by higher-investing mutants. This results in the mean level of investment evolving to significant levels, where it is maintained indefinitely. This approach provides a natural solution to the fundamental problem of how cooperation gradually increases from a non-cooperative state.

Biological Evolution↗
PubMed1999

On the origin of species by sympatric speciation.

Understanding speciation is a fundamental biological problem. It is believed that many species originated through allopatric divergence, where new species arise from geographically isolated populations of the same ancestral species. In contrast, the possibility of sympatric speciation (in which new species arise without geographical isolation) has often been dismissed, partly because of theoretical difficulties. Most previous models analysing sympatric speciation concentrated on particular aspects of the problem while neglecting others. Here we present a model that integrates a novel combination of different features and show that sympatric speciation is a likely outcome of competition for resources. We use multilocus genetics to describe sexual reproduction in an individual-based model, and we consider the evolution of assortative mating (where individuals mate preferentially with like individuals) depending either on an ecological character affecting resource use or on a selectively neutral marker trait. In both cases, evolution of assortative mating often leads to reproductive isolation between ecologically diverging subpopulations. When assortative mating depends on a marker trait, and is therefore not directly linked to resource competition, speciation occurs when genetic drift breaks the linkage equilibrium between the marker and the ecological trait. Our theory conforms well with mounting empirical evidence for the sympatric origin of many species.

Animals↗
PubMed1999

Population dynamics and the evolution of virulence in epidemiological models with discrete host generations

Chaos is the likely outcome of the interaction between a parasite and a host with discrete generations, if the parasite's virulence is high and if transmission from one generation to the next is held constant. We studied two alternative routes of transmission-vertical transmission from infecteds to their offspring, and transmission via long-lasting spores produced in those individuals that were killed by the infection-to investigate the influence of the transmission route on the system's evolution and population dynamics. The major results are: (1) vertical transmission often leads to low virulence, thus confirming most epidemiological models. However, if hosts can become super-infected, the evolutionary dynamics of virulence can have several equilibrium points, including 100% disease-induced mortality; (2) when parasites are transmitted with long-lasting spores, the evolutionary dynamics of virulence can become unstable, leading to the repeated bifurcation of two sub-populations with high and low virulence or to punctuated equilibria with sudden changes in the average level of virulence; (3) in general, the evolution of virulence moves the system to an area where the population dynamics are stable. When evolution leads to chaos, the system most often becomes extinct. Only for a restricted parameter space in the system where transmission from one generation to the next is via long-lasting spores do the dynamics become chaotic without extinction of the system. Copyright 1999 Academic Press.

Journal Article↗
PubMed1999

Genetic variability in sensitivity to population density affects the dynamics of simple ecological models.

Many 1-dimensional discrete time ecological models contain a sensitivity parameter that does not affect the dynamic complexity of these models. We show that genetic variability in this parameter can have a strong effect on population dynamics. We incorporate ecological dynamics in two different population genetic models with one locus and two alleles. The first is the classical model of a randomly mating population in Hardy-Weinberg equilibrium, and the second is a model of differential selection in males and females. In populations in Hardy-Weinberg equilibrium, variability in the sensitivity parameter can be maintained by overdominance. In this case, the dynamics of the polymorphic population tend to be much simpler than those of monomorphic populations. In the model with different selection in males and females, polymorphisms can be maintained in various ways, e.g., by opposing directional selection in males and females. Polymorphism in the sensitivity parameter tends to simplify population dynamics in the model with different selection in males and females as well. A number of interesting dynamic effects can be observed, e.g., multiple attractors with complicated basins of attraction. Then the final state of the system after a successful invasion by mutant alleles may depend on the mutation rate and on the distribution of mutational steps. In addition, there are situations in which genetic variability destabilizes a stable population dynamic equilibrium in the monomorphic model. There is an analogy between genetic variability and variability imposed by the environment. If differences in sensitivity are caused by the environment, dynamic effects similar to those in the genetic models can be observed. In addition, source-sink structures that are known to occur in spatially structured models can be seen in the genetic model if one of the genotypes is inviable. The results suggest that combining ecological and population genetic models can lead to a number of new insights. More work is needed, e.g., with fertility models, in which fitnesses are not assigned to individuals, but to mating pairs.

Alleles↗
PubMed1998

The evolution of interspecific mutualisms.

Interspecific mutualisms are widespread, but how they evolve is not clear. The Iterated Prisoner's Dilemma is the main theoretical tool to study cooperation, but this model ignores ecological differences between partners and assumes that amounts exchanged cannot themselves evolve. A more realistic model incorporating these features shows that strategies that succeed with fixed exchanges (e. g., Tit-for-Tat) cannot explain mutualism when exchanges vary because the amount exchanged evolves to 0. For mutualism to evolve, increased investments in a partner must yield increased returns, and spatial structure in competitive interactions is required. Under these biologically plausible assumptions, mutualism evolves with surprising ease. This suggests that, contrary to the basic premise of past theoretical analyses, overcoming a potential host's initial defenses may be a bigger obstacle for mutualism than the subsequent recurrence and spread of noncooperative mutants.

Biological Evolution↗
PubMed1998

Self-organized criticality in spatial evolutionary game theory.

Self-organized criticality is an important framework for understanding the emergence of scale-free natural phenomena. Cellular automata provide simple interesting models in which to study self-organized criticality. We consider the dynamics of a new class of cellular automata which are constructed as natural spatial extensions of evolutionary game theory. This construction yields a discrete one-parameter family of cellular automata. We show that there is a range of parameter values for which this system exhibits complex dynamics with long range correlations between states in both time and space. In this region the dynamics evolve to a self-organized critical state in which structures exist on all time and length scales, and the relevant statistical measures have power law behaviour.

Animals↗
PubMed1997

Population dynamics, demographic stochasticity, and the evolution of cooperation.

A basic evolutionary problem posed by the Iterated Prisoner's Dilemma game is to understand when the paradigmatic cooperative strategy Tit-for-Tat can invade a population of pure defectors. Deterministically, this is impossible. We consider the role of demographic stochasticity by embedding the Iterated Prisoner's Dilemma into a population dynamic framework. Tit-for-Tat can invade a population of defectors when their dynamics exhibit short episodes of high population densities with subsequent crashes and long low density periods with strong genetic drift. Such dynamics tend to have reddened power spectra and temporal distributions of population size that are asymmetric and skewed toward low densities. The results indicate that ecological dynamics are important for evolutionary shifts between adaptive peaks.

Biological Evolution↗
PubMed1997

High molecular weight forms of IGF-II ('big-IGF-II') released by Wilms' tumor cells.

Insulin-like growth factor-II (IGF-II) is thought to play a critical role in the development of embryonic tumors such as Wilms' tumor. However, despite highly elevated IGF-II mRNA levels in tumors, IGF-II is not elevated in the serum of patients with Wilms' tumors. Recently high molecular weight forms of IGF-II ('big'- or pro-IGF-II) have been found to be produced by some tumors. In order to prove whether or not high molecular weight forms of IGF-II are produced by Wilms' tumor cells and secreted into the culture medium, we established Wilms' tumor cell lines. After column chromatography of the culture medium, IGF-II and pro-IGF-II concentrations were measured. For pro-IGF-II measurement we established a pro-IGF-II RIA using a rabbit polyclonal antiserum directed against amino acids 7-21 (E7-21) of the E-domain of pro-IGF-II. Gel electrophoresis and Western blotting with anti-IGF-II antibodies revealed a band at 7.5 kDa corresponding to fully processed IGF-II and bands between 10 and 20 kDa. Using pro-IGF-II antiserum, bands between 10 and 25 kDa were detected. We conclude that in vitro cultured Wilms' tumor cells produce and release various forms of 'big IGF-II' with molecular masses between 10 and 25 kDa. It remains uncertain whether these high molecular weight forms of IGF-II represent normal precursors of IGF-II or incorrectly processed IGF-II.

Animals↗
PubMed1996

In the red zone.

Explore the source record for details and available documents.

Models, Biological↗
PubMed1995

Diagnosing senescence: inferring evolutionary causes from phenotypic patterns can be misleading.

Based on the predictions of two theories for the evolution of senescence, the 'antagonistic pleiotropy' and the 'mutation accumulation' theory, an age-specific increase in mortality and a decrease in fecundity are widely used criteria to diagnose senescence in natural, and laboratory populations. In this study we question the reliability of these criteria. Using a simple model we show that similar phenotypic patterns result from optimal life histories without senescence. With a tradeoff between reproduction and period survival, optimal life histories produce patterns of increasing mortality and decreasing fecundity as organisms age, even if the tradeoff does not deteriorate with age, so that we are not forced to invoke genetic effects such as antagonistic pleiotropy or accumulation of deleterious mutations to explain such patterns. Furthermore, if optimal life history theory is applied to senescent organisms, phenotypic patterns can result that are usually not associated with senescence. We conclude that the reliability of a diagnosis of senescence based on phenotypic patterns and the comprehension of the phenomenon senescence depends critically on understanding to what extent tradeoffs are determined by the effects of segregating genes.

Aging↗
PubMed1995

Evolutionary predictions from invariant physical measures of dynamic processes.

The invariant physical measure of a dynamic process comprises the information necessary to compute statistical quantities of the system. It allows us to replace time averages over the system's trajectory by integrals over state-space. In particular, it allows us to compute the long-term growth rate of a fluctuating population and hence to make evolutionary predictions. This is explained here using a one-dimensional difference equation. The invariant physical measure reflects three types of selection in this model: K-selection when the resident has a stable equilibrium; r-selection when the resident exhibits complex dynamics; and c-selection, i.e. selection for lower complexity when the resident undergoes large fluctuations. For all three evolutionary scenarios the interaction of the physical measure with the higher moments of the distribution of offspring numbers is crucial. It is also shown how stochastic noise can affect the invariant physical measure and the evolutionary predictions made from it.

Animals↗
PubMed1994

Intermittent chaos in population dynamics.

In modelling single species with discrete, non-overlapping generations, one usually assumes that the density at time t + .1 is a function of the density at time t: Nt+1 = f(Nt). The dynamical behaviour of this system depends on the parameters in the function f. It commonly changes, as a parameter increases, from a stable equilibrium through a series of bifurcations into stable cycles, to chaotic motion. It is implicit in the assumptions of the model that the population consists of identical individuals. In this paper it is shown that variation within the population can lead to a different route to chaos. Invasion of a mutant phenotype into a resident population can elicit intermittency. This kind of chaotic behaviour consists of regular motion most of the time with short intermittent periods in which the system fluctuates wildly.

Animals↗
PubMed1993

Determination of ovarian cycle in Goeldi's monkey (Callimico goeldii) via the measurement of steroids and peptides in plasma and urine.

Goeldi's monkey (Callimico goeldii) is an endangered species of New World primate. The present study provides the first description of the non-conception ovarian cycle in this species based on circulating reproductive steroid and peptide hormones. The data obtained were used to validate a non-invasive system for monitoring cyclicity based on urinary reproductive steroid metabolites. Nine sexually mature females were studied. In three females, matched blood and urine samples were collected once every 2-3 days for 90-120 days; in three other females, matched blood and urine samples were collected daily for 14-20 days for one peri-ovulatory period; and in the remaining three females, urine samples only were collected once every 1-3 days for 40-60 days. Plasma progesterone, oestrone-3-conjugates and bioactive LH were measured, in addition to urinary pregnanediol-3 alpha-glucuronide and oestrone-3-conjugates. The mean maximum concentration of plasma LH occurred 1-2 days before a significant rise in plasma progesterone, which was considered to occur 1 day after ovulation. On the basis of plasma progesterone titres, the duration of the ovarian cycle was estimated as 23.9 +/- 0.4 days (n = 9), and constituted a follicular phase of 10.7 +/- 0.3 days and a luteal phase of 13.5 +/- 0.3 days. Urinary pregnanediol-3 alpha-glucuronide demonstrated a high correlation with plasma progesterone (r = 0.8), and demonstrated a significant rise at the same time as plasma progesterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Streptomycin-resistant Escherichia coli as a marker of vulvovestibular contamination of endometrial culture swabs in the mare.

To investigate the vulvovestibular contamination of endometrial culture swabs in the mare, a liquid culture of a streptomycin-resistant strain of Escherichia coli was applied to the vulvovestibular area of mares and used as a marker of contamination of endometrial culture swabs. Prior to taking endometrial swabs, the perineal area was washed with soap, rinsed with water, and dried. Endometrial culture swabs were taken from mares that were in anestrus or diestrus and from mares that were in estrus. When a manual transvaginal swabbing technique was used, 22 of 24 endometrial swab specimens from 12 mares were contaminated with the experimental bacterial strain; culture of only one endometrial swab yielded more than nine colonies. When a speculum approach was employed, three of 12 swab specimens from 12 mares yielded between one and three colonies. The stage of cycle had no effect on the extent of contamination, but the proportion of positive cultures was significantly smaller when swabs were taken via a vaginal speculum approach, compared to a manual transvaginal approach. Complete preclusion of vulvovestibular contamination of endometrial swab specimens was not achieved; however, fewer than ten colonies can be expected even in mares in which the vulvovestibular area has been thoroughly contaminated with a broth culture, provided that the perineal area is adequately cleaned prior to swabbing.

Anestrus↗