PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Extinction, Biological”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30Linked to original sources
PubMed1998

Late eocene sivaladapid primate from Guangxi Zhuang Autonomous Region, People's Republic of China.

A new genus and species of Sivaladapidae is described from the late Eocene Gongkang Formation, Yongle Basin, western Guangxi Zhuang Autonomous Region, southern China. Guangxilemur tongi, new genus and species, shows a combination of traits that occur separately in earlier and more primitive Asian adapiforms (Hoanghonius and Rencunius) and in Miocene sivaladapines (Sivaladapis and Sinoadapis). Phylogenetic analysis of dental characters suggests that Guangxilemur is closely related to the Miocene sivaladapine clade. Miocene sivaladapines were the latest surviving members of a broad radiation of Eocene adapiforms in Asia that included Hoanghonius, Rencunius, and Wailekia in addition to Guangxilemur. European Periconodon may also be specially related to this primarily Asian clade, but current anatomical data are insufficient to test this possibility adequately. Sivaladapine adapiforms and tarsiid tarsiiforms maintained relictual distributions in southern and/or southeastern Asia far beyond the extirpation of their closest relatives on other Holarctic continents near the Eocene-Oligocene boundary. This temporal persistence was mediated by Asian paleogeography, which allowed virtually continuous access to tropical refugia during a middle Cenozoic interval of climatic deterioration that coincided with the extinction of adapiforms and tarsiiforms in Europe and North America.

Animals↗
PubMed1997

A model of mass extinction.

In the last few years a number of authors have suggested that evolution may be a so-called self-organized critical phenomenon, and that critical processes might have a significant effect on the dynamics of ecosystems. In particular it has been suggested that mass extinction may arise through a purely biotic mechanism as the result of "coevolutionary avalanches". In this paper we first explore the empirical evidence which has been put forward in favor of this conclusion. The data center principally around the existence of power-law functional forms in the distribution of the sizes of extinction events and other quantities. We then propose a new mathematical model of mass extinction which does not rely on coevolutionary effects and in which extinction is caused entirely by the action of environmental stress on species. In combination with a simple model of species adaption we show that this process can account for all the observed data without the need to invoke coevolution and critical processes. The model also makes some independent predictions, such as the existence of "aftershock" extinctions in the aftermath of large mass extinction events, which should in theory be testable against the fossil record.

Animals↗
PubMed2002

A markov chain approach to calculate r(0) in stochastic epidemic models.

R(0) has been defined as "The expected number of secondary infections originated by a "typical" infective individual when introduced into a population of susceptibles", and it is perhaps the single most important parameter in epidemic models. A general framework to calculate R(0) that can be applied to complicated stochastic epidemic models that may include demography, several strains, latent or carrier-like states, with or without density-dependent parameters is introduced. This framework helps us to understand the concept of a "typical" infective individual used in the deterministic definition of R(0). The method is illustrated with applications to several epidemic models, including some in which it has been found that the disease may persist even if R(0)<1. It is shown that although the probability of extinction is difficult to calculate in these latter cases, it is possible to give general conditions on the parameters under which eventual extinction is certain.

Algorithms↗
PubMed2003

Extinction and re-evolution of similar adaptive types (ecomorphs) in Cenozoic North American ungulates and carnivores reflect van der Hammen's cycles.

Numerous patterns in periodicity (e.g., climate, extinction, and sedimentary cycles) and evolutionary change (e.g., chronofaunas and coordinated stasis) have been described based on aspects of the geologic record. Recently, convergent occurrences of faunal types or "repeating faunas" have received attention, but a highly specific, iterative pattern was first reported over 40 years ago. In the late 1950s, van der Hammen described climatic/floral cycles on the order of six million years based on a succession of A, B, and C pollen community types in South America. These A-B-C cycles are also seen in the replacement pattern of particular carnivore and ungulate adaptive types in Cenozoic North America as reported by Martin in the 1980s. For example, in the last 36 million years, there were four iterations of a sabertooth cat ecomorph independently evolving, dominating the niche through an A-B-C cycle, and then going extinct. Here we show further support for the existence of these cycles in the dominance turnover in hippo and dog ecomorphs in the North American Cenozoic. Shared patterns of extinction and re-evolution of adaptive types among plants and mammals across two continents suggest a global mechanism, which appears to be climatic change. Iterative climatic cycles of various scales may form a predictive framework for understanding fundamental patterns in the geologic record, such as radiations, extinction, rates of change, convergence, and sedimentary cycles.

Acclimatization↗
PubMed1994

A co-evolutionary model of mutualism from a commensal association on Lotka-Volterra dynamics.

This paper considers a Lotka-Volterra type of model of competition between a commensal pair of species and a mutualistic pair, presumed to have descended from a single ancestral pair of species. The species are behaviourally isolated but compete for resources. We have studied in detail the effects of the benefits of mutualism over commensalism. Specifically, it is shown that co-evolution of the mutalistic pair from the commensal association is not possible if (i) some of the interacting parameters coincide for the two pairs, or (ii) mutualists suffer more from competition, or (iii) mutualists produce a smaller competitive effect. If the costs of mutualism are density-independent, co-evolution of the mutualism is possible provided (i) the specific growth rate(s) of one or both of the mutualistic species is positive at the steady state of the commensal pair and (ii) the specific growth rate(s) of one or both of the commensal species is positive at the steady state of the mutualistic pair. Lastly, whenever the mutualists co-exist with the commensalists and the feasible interior equilibrium of the model is unstable, we have shown that the co-evolutionary model of mutualism collapses to the stable two-species mutualistic relationship, causing the ultimate extinction of both the species of the original commensal association.

Animals↗
PubMed2005

Diversity in times of adversity: probabilistic strategies in microbial survival games.

Population diversification strategies are ubiquitous among microbes, encompassing random phase-variation (RPV) of pathogenic bacteria, viral latency as observed in some bacteriophage and HIV, and the non-genetic diversity of bacterial stress responses. Precise conditions under which these diversification strategies confer an advantage have not been well defined. We develop a model of population growth conditioned on dynamical environmental and cellular states. Transitions among cellular states, in turn, may be biased by possibly noisy readings of the environment from cellular sensors. For various types of environmental dynamics and cellular sensor capability, we apply game-theoretic analysis to derive the evolutionarily stable strategy (ESS) for an organism and determine when that strategy is diversification. We find that: (1) RPV, effecting a sort of Parrondo paradox wherein random alternations between losing strategies produce a winning strategy, is selected when transitions between different selective environments cannot be sensed, (2) optimal RPV cell switching rates are a function of environmental lifecycle asymmetries and environmental autocorrelation, (3) probabilistic diversification upon entering a new environment is selected when sensors can detect environmental transitions but have poor precision in identifying new environments, and (4) in the presence of excess additive noise, low-pass filtering is required for evolutionary stability. We show that even when RPV is not the ESS, it may minimize growth rate variance and the risk of extinction due to 'unlucky' environmental dynamics.

Bacteria↗
PubMed2005

Nutrient-limited food webs with up to three trophic levels: feasibility, stability, assembly rules, and effects of nutrient enrichment.

Community structure is controlled, among multiple factors, by competition and predation. Using the R* rule and graphical analysis, we analyse here the feasibility, stability and assembly rules of resource-based food webs with up to three trophic levels. In particular, we show that (1) the stability of a food web with two plants and two generalist herbivores does not require that plants' resource exploitation abilities trade-off with resistance to the two herbivores, and (2) food webs with two plants and either one generalist herbivore and a carnivore or two generalist herbivores and two generalist carnivores are not feasible because of cascade competition between top consumers. The relative strength of species interactions and the relative impacts of plants and herbivores on factors which control their growth also play a critical role. We discuss how community structure constrains assembly rules and yields cascades of extinctions in food webs.

Animals↗
PubMed1998

Optimal harvesting under stochastic fluctuations and critical depensation.

We consider the derivation of the optimal harvesting strategy maximizing the expected cumulative yield from present up to extinction, under the assumption that the harvested population fluctuates stochastically and is subjected to an Allee-effect. By relying on both stochastic calculus and the classical theory of linear diffusions, we derive both the optimal harvesting thresholds at which harvesting should be initiated at full capacity and the value of the optimal strategy. In contrast to ordinary models which are absent of critical depensation, we show that the presence of an Allee-effect leads to the introduction of a lower harvesting threshold at which the population should be immediately depleted under the optimal policy. Moreover, we demonstrate that discounting increases the incentives to harvest and, therefore, increases the probability of a soon extinction of the harvested population.

Animals↗
PubMed2002

The rise and fall of SRY.

Comparisons between species reveal when and how SRY, the testis-determining gene, evolved. SRY is younger than the Y chromosome, and so was probably not the original mammal sex-determining gene that defined the Y. SRY is typical of genes on the Y chromosome. It arose from a gene on the proto-sex chromosome pair with a function (possibly brain-determination) in both sexes. It has been buffeted in evolution, and shows variation in copy number, structure and expression. And it is dispensable, having been lost at least twice independently in different rodent lineages. At the observed rate of attrition, the human Y chromosome will be gone in 5-10 million years. This could lead to the extinction of our species or to a burst of hominid speciation.

Animals↗
PubMed1993

Madagascar's lemurs.

These primates can tell us a great deal about our own evolutionary past. But many species are already extinct, and the habitats of those that remain are shrinking fast.

Animals↗
PubMed2003

Spatial grain and the causes of regional diversity gradients in ants.

Gradients of species richness (S; the number of species of a given taxon in a given area and time) are ubiquitous. A key goal in ecology is to understand whether and how the many processes that generate these gradients act at different spatial scales. Here we evaluate six hypotheses for diversity gradients with 49 New World ant communities, from tundra to rain forest. We contrast their performance at three spatial grains from S(plot), the average number of ant species nesting in a m2 plot, through Fisher's alpha, an index that treats our 30 1-m2 plots as subsamples of a locality's diversity. At the smallest grain, S(plot), was tightly correlated (r2 = 0.99) with colony abundance in a fashion indistinguishable from the packing of randomly selected individuals into a fixed space. As spatial grain increased, the coaction of two factors linked to high net rates of diversification--warm temperatures and large areas of uniform climate--accounted for 75% of the variation in Fisher's alpha. However, the mechanisms underlying these correlations (i.e., precisely how temperature and area shape the balance of speciation to extinction) remain elusive.

Animals↗
PubMed2026

Predictive evolutionary genomics: principles, validation, and practice.

Climate change and habitat loss are driving rapid evolutionary responses in populations world-wide, which creates an urgent need for evolutionary forecasting in conservation and agriculture. Such forecasting can be categorized into three time scales: trait-based models that use multivariate quantitative genetic equations to project correlated phenotypic responses up to c.&#xa0;20 generations, allele-based analyses that model allele frequency dynamics up to 100 generations, and composite adaptation scores that aggregate many small effects to yield predictions across longer horizons. However, these approaches have remained largely disconnected. Here, we present a Bayesian framework that integrates these three complementary approaches for evolutionary prediction. Our framework combines genomic, phenotypic, and environmental data to yield probabilistic predictions with explicit uncertainty. We show how predictive evolutionary forecasts can be validated with experimental evolution, field experimentation, historical specimens, and reciprocal transplants. These validated forecasts can help advance conservation and agricultural programmes by helping predict which populations are at risk of future extinction, optimizing breeding programmes for future climates, and planning ecosystem management under environmental change. By supporting a shift towards more predictive approaches in evolutionary biology, this framework may help improve our ability to manage biodiversity and food security in a changing world.

Genomics↗
PubMed2000

Evolutionary exploitation of design options by the first animals with hard skeletons.

The set of viable design elements available for animals to use in building skeletons has been fully exploited. Analysis of animal skeletons in relation to the multivariate, theoretical "Skeleton Space" has shown that a large proportion of these options are used in each phylum. Here, we show that structural elements deployed in the skeletons of Burgess Shale animals (Middle Cambrian) incorporate 146 of 182 character pairs defined in this morphospace. Within 15 million years of the appearance of crown groups of phyla with substantial hard parts, at least 80 percent of skeletal design elements recognized among living and extinct marine metazoans were exploited.

Animals↗
PubMed2007

The costs of carnivory.

Mammalian carnivores fall into two broad dietary groups: smaller carnivores (<20 kg) that feed on very small prey (invertebrates and small vertebrates) and larger carnivores (>20 kg) that specialize in feeding on large vertebrates. We develop a model that predicts the mass-related energy budgets and limits of carnivore size within these groups. We show that the transition from small to large prey can be predicted by the maximization of net energy gain; larger carnivores achieve a higher net gain rate by concentrating on large prey. However, because it requires more energy to pursue and subdue large prey, this leads to a 2-fold step increase in energy expenditure, as well as increased intake. Across all species, energy expenditure and intake both follow a three-fourths scaling with body mass. However, when each dietary group is considered individually they both display a shallower scaling. This suggests that carnivores at the upper limits of each group are constrained by intake and adopt energy conserving strategies to counter this. Given predictions of expenditure and estimates of intake, we predict a maximum carnivore mass of approximately a ton, consistent with the largest extinct species. Our approach provides a framework for understanding carnivore energetics, size, and extinction dynamics.

Animals↗
PubMedDate not supplied

[The active electrogenesis of the command neurons in the defensive behavior of the mollusk during conditioning].

In the paper changes of active electrogenesis of the command neurones of defensive closure of snail pneumostome at elaboration, extinction and repeated elaboration of classic conditioned defensive reflex to tactile stimulus was described; the tactile stimulation of other point of the body served as a differentiating stimulus. During the increase of biological significance of conditioned stimulus as a result of learning the stimulation of the command neurones in response to this stimulus was raised. At the same time the neurones showed decreased excitability in response to differentiating stimulus. Possible mechanisms of quick reconstruction of neurones excitability and functional value of PA generation threshold changes were discussed.

Animals↗
PubMed1988

[Species-forming involution of pathogenic bacteria as a biological pattern (exemplified by bacteria of the genus Moraxella)].

The author advances a hypothesis stating that a species disappearing as a result of new conditions evolving in a colonized animal or human body, unfavorable for the existence of this species, does not become extinct, but reverts into newly developing species due to the loss of its properties and acquiring new properties corresponding to new conditions. Thus the genus is preserved through the loss of species characteristics by individual organisms. The advanced hypothesis is substantiated by the analysis of the relevant processes observed in cases of infectious keratoconjunctivitis in humans and cattle, caused by bacteria of the genus Moraxella. The data indicating the possibility of such reversion in the genera Neisseria and Bordetella are presented.

Animals↗
PubMed2006

Extinct mammalian biodiversity of the ancient New World tropics.

The origins of mammalian biodiversity in the New World tropics extend back >25 million years, represented by clades that were originally endemic to South America, North America or Africa. Since then, these mammalian clades have been greatly affected by climatic, physiographic and biological changes. The Isthmian land bridge, which formed approximately 4 million years ago between North and South America, resulted in the maximum diversity of 17 New World tropical mammalian orders during the Great American Interchange. This diversity was subsequently reduced to 12 orders as a result of competition, climate change and human impacts. Here, I discuss how the fossil record is now providing a rich archive of past biodiversity, presenting unique evidence of the origins, macroevolution, macro-ecology and extinction of New World tropical mammals.

Animals↗