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At least 793 records · Page 44Linked to original sources
PubMed1997

Disease extinction and community size: modeling the persistence of measles.

A basic issue in ecology is the relation between extinction and population size. One of the clearest manifestations of a population threshold for extinction is the critical community size below which infections like measles do not persist. The current generation of stochastic models overestimates the observed critical community size for measles, generating much less persistence of infection than is observed. The inclusion of a more biologically realistic model for the duration of infection produced a much closer fit to the actual critical community size and explains previously undescribed high-frequency oscillations in measles incidence.

Adolescent↗
PubMed2005

Character displacement as the "best of a bad situation": fitness trade-offs resulting from selection to minimize resource and mate competition.

Character displacement has long been considered a major cause of adaptive diversification. When species compete for resources or mates, character displacement minimizes competition by promoting divergence in phenotypes associated with resource use (ecological character displacement) or mate attraction (reproductive character displacement). In this study, we investigated whether character displacement can also have pleiotropic effects that lead to fitness trade-offs between the benefits of avoiding competition and costs accrued in other fitness components. We show that both reproductive and ecological character displacement have caused spadefoot toads to evolve smaller body size in the presence of a heterospecific competitor. Although this shift in size likely arose as a by-product of character displacement acting to promote divergence between species in mating behavior and larval development, it concomitantly reduces offspring survival, female fecundity, and sexual selection on males. Thus, character displacement may represent the "best of a bad situation" in that it lessens competition, but at a cost. Individuals in sympatry with the displaced phenotype will have higher fitness than those without the displaced trait because they experience reduced competition, but they may have reduced fitness relative to individuals in allopatry. Such a fitness trade-off can limit the conditions under which character displacement evolves and may even increase the risk of "Darwinian extinction" in sympatric populations. Consequently, character displacement may not always promote diversification in the manner that is often expected.

Animals↗
PubMed1984

A homeostatic model of Pavlovian conditioning: tolerance to scopolamine-induced adipsia.

Several experiments support a model proposing that tolerance to scopolamine-induced adipsia involves an interplay between processes of homeostatic regulation and Pavlovian conditioning. In Experiment 1a, rats given access to water while under the influence of scopolamine developed adipsic tolerance, whereas rats denied access to water in the drug state did not. In Experiment 1b, rats displayed adipsic tolerance only when scopolamine was administered with cues associated with previous drug injections. In Experiment 1c, adipsic-tolerant rats showed a polydipsic response to an injection of phenobarbital (which unconditionally augments water consumption) relative to nontolerant animals with the same pharmacological history. Experiment 2 assessed the effect of deprivation level on the Pavlovian extinction of adipsic tolerance. Rats satiated with water during extinction showed a loss of adipsic tolerance, whereas water-deprived rats did not. The present model is discussed in relation to (a) tolerance in other response systems such as morphine analgesia, (b) theories of extinction for nonpharmacological Pavlovian conditioning, and (c) homeostatic regulation and incentive motivation.

Animals↗
PubMed2005

Stress and adaptation in conservation genetics.

Stress, adaptation and evolution are major concerns in conservation biology. Stresses from pollution, climatic changes, disease etc. may affect population persistence. Further, stress typically occurs when species are placed in captivity. Threatened species are usually managed to conserve their ability to adapt to environmental changes, whilst species in captivity undergo adaptations that are deleterious upon reintroduction into the wild. In model studies using Drosophila melanogaster, we have found that; (a) inbreeding and loss of genetic variation reduced resistance to the stress of disease, (b) extinction rates under inbreeding are elevated by stress, (c) adaptive evolutionary potential in an increasingly stressful environment is reduced in small population, (d) rates of inbreeding are elevated under stressful conditions, (e) genetic adaptation to captivity reduces fitness when populations are reintroduced into the 'wild', and (f) the deleterious effects of adaptation on reintroduction success can be reduced by population fragmentation.

Adaptation, Biological↗
PubMed1999

Dynamics of inbreeding depression due to deleterious mutations in small populations: mutation parameters and inbreeding rate.

A multilocus stochastic model is developed to simulate the dynamics of mutational load in small populations of various sizes. Old mutations sampled from a large ancestral population at mutation-selection balance and new mutations arising each generation are considered jointly, using biologically plausible lethal and deleterious mutation parameters. The results show that inbreeding depression and the number of lethal equivalents due to partially recessive mutations can be partly purged from the population by inbreeding, and that this purging mainly involves lethals or detrimentals of large effect. However, fitness decreases continuously with inbreeding, due to increased fixation and homozygosity of mildly deleterious mutants, resulting in extinctions of very small populations with low reproductive rates. No optimum inbreeding rate or population size exists for purging with respect to fitness (viability) changes, but there is an optimum inbreeding rate at a given final level of inbreeding for reducing inbreeding depression or the number of lethal equivalents. The interaction between selection against partially recessive mutations and genetic drift in small populations also influences the rate of decay of neutral variation. Weak selection against mutants relative to genetic drift results in apparent overdominance and thus an increase in effective size (Ne) at neutral loci, and strong selection relative to drift leads to a decrease in Ne due to the increased variance in family size. The simulation results and their implications are discussed in the context of biological conservation and tests for purging.

Animals↗
PubMed1997

Clonality and life cycles of intestinal crypts explained by a state dependent stochastic model of epithelial stem cell organization.

The organization and control of stem cells is a key issue in epithelial cell biology. The small intestinal murine crypt is a useful tissue to study such problems since stem cells are known to be located at specific positions at the bottom of the crypt where they are self maintaining. Recent data suggest, that (1) the number of active stem cells in a crypt can fluctuate, (2) the immediate progeny of a single stem cell can replace other stem cells eventually leading to monoclonality and (3) the life cycle of crypts may be linked to stem cell dynamics. It is the objective of this paper to suggest a stochastic state-dependent model of stem cell and crypt growth which can explain and-link these phenomena into one comprehensive framework. Monte Carlo simulations are performed to show consistently with available data. The model explains the size distribution of small intestinal crypts in steady state, the observations of stem cell fluctuations and monoclonality conversion, recovery of the crypt population after moderate damage and the rate of crypt fission and extinction. The key assumption of this model is an autoregulatory control of stem cell growth.

Animals↗
PubMed2005

Qualitative differences between naïve and scientific theories of evolution.

Philosophers of biology have long argued that Darwin's theory of evolution was qualitatively different from all earlier theories of evolution. Whereas Darwin's predecessors and contemporaries explained adaptation as the transformation of a species' "essence," Darwin explained adaptation as the selective propagation of randomly occurring mutations within a population. The present study explored the possibility of a parallel between early "transformational" theories of evolution and modern naïve theories. Forty-two high school and college students and three evolutionary biologists were tested on their understanding of six evolutionary phenomena: variation, inheritance, adaptation, domestication, speciation, and extinction. As predicted, a plurality of participants demonstrated transformational reasoning inconsistent with natural selection. Correlational analyses revealed that participants who demonstrated transformational reasoning were as internally consistent as participants who demonstrated an understanding of natural selection, with the exception of one group of participants who appeared to have assimilated two heuristics--"survival of the fittest" and "acquired traits are not inherited"--into an otherwise transformational framework. These findings suggest that the widespread and early-developing tendency to essentialize biological kinds precludes students from conceptualizing species as populations of individuals differentially affected by the environment.

Adaptation, Physiological↗
PubMed1988

Effects of English admixture and geographic distance on anthropometric variation and genetic structure in 19th-century Ireland.

The analysis of anthropometric data often allows investigation of patterns of genetic structure in historical populations. This paper focuses on interpopulational anthropometric variation in seven populations in Ireland using data collected in the 1890s. The seven populations were located within a 120-km range along the west coast of Ireland and include islands and mainland isolates. Two of the populations (the Aran Islands and Inishbofin) have a known history of English admixture in earlier centuries. Ten anthropometric measures (head length, breadth, and height; nose length and breadth; bizygomatic and bigonial breadth; stature; hand length; and forearm length) on 259 adult Irish males were analyzed following age adjustment. Discriminant and canonical variates analysis were used to determine the degree and pattern of among-group variation. Mahalanobis' distance measure, D2, was computed between each pair of populations and compared to distance measures based on geographic distance and English admixture (a binary measure indicating whether either of a pair of populations had historical indications of admixture). In addition, surname frequencies were used to construct distance measures based on random isonymy. Correlations were computed between distance measures, and their probabilities were derived using the Mantel matrix permutation method. English admixture has the greatest effect on anthropometric variation among these populations, followed by geographic distance. The correlation between anthropometric distance and geographic distance is not significant (r = -0.081, P = .590), but the correlation of admixture and anthropometric distance is significant (r = 0.829, P = .047). When the two admixed populations are removed from the analysis the correlation between geographic and anthropometric distance becomes significant (r = 0.718, P = .025). Isonymy distance shows a significant correlation with geographic distance (r = 0.425, P = .046) but not with admixture distance (r = -0.052, P = .524). The fact that anthropometrics show past patterns of gene flow and surnames do not reflects the greater impact of stochastic processes on surnames, along with the continued extinction of surnames. This study shows that 1) anthropometrics can be extremely useful in assessing population structure and history, 2) differential gene flow into populations can have a major impact on local genetic structure, and 3) microevolutionary processes can have different effects on biological characters and surnames.

Adult↗
PubMed2005

Cycle and duration of the seminiferous epithelium in puma (Puma concolor).

Puma or sussuarana (Puma concolor) is the second largest feline in the American continent and has an ample latitudinal distribution in very diverse habitats. In relation to its conservation status, the puma is considered an extinction-threatened species. The study of the testis morphology and the spermatogenic process in a species is fundamental for establishing the physiologic patterns that will make possible the selection of the protocols for assisted reproduction. A number of peculiarities associated with the reproductive biology of specific species such as the duration of spermatogenic process can be used to determine the frequency of sperm collection. Nine adult male pumas maintained in captivity were used to determine the relative frequency of stages in the seminiferous epithelium cycle. Three of them received intra-testicular injections of 0.1ml tritiated thymidine to determine the duration of the seminiferous epithelium cycle, and were subjected to biopsy 7 days later. The cycle of the seminiferous epithelium in puma was didactically described into eight stages by the tubular morphology method. The total duration of one seminiferous epithelium cycle in puma was calculated to be 9.89 days, and approximately 44.5 days are required for development of spermatozoon from spermatogonia. The duration of spermiogenesis, prophase and other events of meiosis were 14.08, 15.20 and 1.79 days, respectively. The relative frequency of the pre-meiotic, meiotic and post-meiotic phases were 3.98, 1.79 and 4.12 days, respectively.

Animals↗
PubMed2006

Vibrational signalling in a Gondwanan relict insect (Hemiptera: Coleorrhyncha: Peloridiidae).

Ancient, long-extinct floras and faunas can be reassembled through fossils and phylogenetics, and even palaeo-environments can be reconstructed with the aid of palaeoclimatology. However, very little is known about the sound-scape of the past. Of what kind were the first biologically meaningful sounds and vibrations ever emitted and perceived? The earliest signals in the history of life were probably produced by arthropods making use of the mechanical properties of their exoskeleton. Here, we report an observation of vibrational signalling in the coleorrhynchan Hackeriella veitchi, a representative of a Gondwanan relict insect lineage which is still extant in the Queensland rainforest. Our finding suggests that vibrational signalling by tymbal organs is ancestral for the Hemiptera (exclusive of Sternorrhyncha)--the song of the Coleorrhyncha was a likely element of the acoustic environment in the Permian moss forests and had possibly changed little since.

Animals↗
PubMedDate not supplied

[Local forms of amyloidosis as a manifestation of age-related pathology: a new look at the problem].

Endocrine (isolated atrium amyloidosis and insular amyloidosis of the pancreas) and non-endocrine (aorta, eye, prostate and/or seminal vesicles, cerebral amyloidosis) forms should be distinguished among senile local amyloidosis. Senile polyorgan amyloidosis is also possible: combination of endocrine forms with aorta amyloidosis or that of insular amyloidosis with cerebral and eye amyloidosis. These data reject Schwartz tetrad as an obligatory manifestation of an old age. Peptide hormones, serum and specialized proteins are precursors of amyloid fibrillary protein; each of them is able to beta-cross-conformation and formation of the fibrillar amyloid protein. This implies that conformational protein disturbances in increasing senile tissue bradytrophia and extinction of the diffuse endocrine system function are a basis of amyloidogenesis in senile local amyloidosis. Clinical manifestations of senile amyloidosis are very variable, but local senile amyloidosis is not recognized clinically. Its diagnosis will probably become possible with clinical use of molecular biology achievements and, first of all, those of molecular genetics.

Aged↗
PubMed2005

Using finite-element analysis to investigate suture morphology: a case study using large carnivorous dinosaurs.

Finite-element analysis (FEA) can be used to investigate the mechanical significance of sutures and regions of intracranial flexibility in skulls. By modeling the stress response to feeding forces in a finite-element skull model (with appropriate boundary conditions), one can compare the axis of distortion and orientation of stress and strain in the model to the degree of movement at actual sutural contacts in the real skull. Hypotheses detailing the effect of introducing patency or flexibility on mechanical performance can be constructed and subsequently tested. In this study, the correlation between stress environment, cranial strength, and sutural morphology and mobility is investigated in the cranium of the large theropod dinosaur Allosaurus fragilis. Theropods are an especially interesting model system as their skulls were massive (over 100 cm in some cases), may have generated extremely large bite forces, yet patent sutures persisted between many of the facial bones. In this analysis, it was discovered that Allosaurus cranial sutures appear generally capable of accommodating stress and strain patterns generated during biting. This study highlights the potential of FEA in devising and testing hypotheses of form and function and argues that useful information can be obtained from finite-element models of extinct animals, providing that adequate assumptions are made and appropriate questions asked.

Animals↗
PubMed1983

Effects of toxicants on populations: a qualitative approach II. First order kinetics.

System level effects exhibited by a population subjected to a chronic or an acute dose of toxicant are the emphasis of this study. A three dimensional model of a toxicant and a population, with state variables (the population biomass, the concentration of toxicant in an organism, and the concentration of toxicant in the environment) coupled by a linear dose-response function, is analyzed analytically. One of the main results presents sufficient conditions, in terms of a system level parameter, for the persistence, and for the extinction, of a population exposed to a chronic dose of toxicant. When depuration and degradation are negligible processes, the effects of toxicant accumulation associated with an acute exposure of a population are analyzed in some detail. Both persistence and extinction are shown to be viable behavior modes of a population in this biochemical setting.

Environmental Pollutants↗
PubMed2006

Competition between plasmid-bearing and plasmid-free organisms in a chemostat with nutrient recycling and an inhibitor.

The asymptotic behavior of solutions of a model for competition between plasmid-bearing and plasmid-free organisms in the chemostat with two distributed delays and an external inhibitor is considered. The model presents a refinement of the one considered by Lu and Hadeler [Z. Lu, K.P. Hadeler, Model of plasmid-bearing plasmid-free competition in the chemostat with nutrient recycling and an inhibitor, Math. Biosci. 167 (2000) p. 177]. The delays model the fact that the nutrient is partially recycled after the death of the biomass by bacterial decomposition. Furthermore, it is assumed that there is inter-specific competition between the plasmid-bearing and plasmid-free organisms as well as intra-specific competition within each population. Conditions for boundedness of solutions and existence of non-negative equilibrium are given. Analysis of the extinction of the organisms, including plasmid-bearing and plasmid-free organisms, and the uniform persistence of the system are also carried out. By constructing appropriate Liapunov-like functionals, some sufficient conditions of global attractivity to the extinction equilibria are obtained and the combined effects of the delays and the inhibitor are studied.

Bioreactors↗
PubMed2004

Extinction times and moment closure in the stochastic logistic process.

We investigate the statistics of extinction times for an isolated population, with an initially modest number M of individuals, whose dynamics are controlled by a stochastic logistic process (SLP). The coefficient of variation in the extinction time V is found to have a maximum value when the death and birth rates are close in value. For large habitat size K we find that Vmax is of order K1/4 / M1/2, which is much larger than unity so long as M is small compared to K1/2. We also present a study of the SLP using the moment closure approximation (MCA), and discuss the successes and failures of this method. Regarding the former, the MCA yields a steady-state distribution for the population when the death rate is low. Although not correct for the SLP model, the first three moments of this distribution coincide with those calculated exactly for an adjusted SLP in which extinction is forbidden. These exact calculations also pinpoint the breakdown of the MCA as the death rate is increased.

Animals↗
PubMed2002

Evolution of cannibalistic traits: scenarios derived from adaptive dynamics.

The evolution of cannibalistic traits in consumer populations is studied in this paper with the approach of adaptive dynamics theory. The model is kept at its minimum complexity by eliminating some environmental characteristics, like heterogeneity and seasonalities, and by hiding the size-structure of the population. Evolutionary dynamics are identified through numerical bifurcation analysis, applied both to the ecological (resident-mutant) model and to the canonical equation of adaptive dynamics. The result is a rich catalog of evolutionary scenarios involving evolutionary stable strategies and branching points both in the monomorphic and dimorphic dynamics. The possibility of evolutionary extinction of highly cannibalistic populations is also ascertained. This allows one to explain why cannibalism can be a transient stage of evolution.

Adaptation, Physiological↗
PubMed1992

Molecular evolution of the human interleukin-8 receptor gene cluster.

Interleukin-8 (IL-8) is the prototype for a family of at least eight neutrophil chemoattractants whose genes map to human chromosome 4q13-q21. Two human IL-8 receptors, IL8RA and IL8RB, are known from cDNA cloning; IL8RA is a promiscuous receptor for at least two other related ligands, GRO alpha and NAP-2. We now report cloning of the genes for IL8RA, IL8RB and a recently inactivated pseudogene of receptor A (IL8RAP). These form a cluster of only three genes in the superfamily of G protein-coupled receptors (GPCRs) and map to 2q34-q35. The coevolutionary diversity displayed by the IL-8 ligand-receptor complex--ligand promiscuity for IL-8, receptor promiscuity for IL8RA, gene duplication for both ligands and receptors and gene extinction in the case of IL8RAP--is unprecedented for the GPCR superfamily.

Amino Acid Sequence↗