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At least 307 records · Page 17Linked to original sources
PubMed1999

Resource partition history and evolutionary specialization of subunits in complex systems.

Specialization for part of the resources available is a common adaptation leading to diversification in biological systems. In complex ecological communities, the history of diversification of parts of a system correlates with the structure of the system in present times. Abundances and sequences of resource partition of species expected by a sequential broken stick model are compared with observed abundances of species and their sequences of evolutionary divergence. Where resources are abundant, first resource partitions correspond to most ancient speciations, and last to recent speciations. The inverse is true when resources are scarce, agreeing with the occupation of extreme habitats by recent species. Diverging species seem to specialize by dividing the niche of common ancestors. Similar trends exist inside genera as well as between families, suggesting similar patterns in diversification and specialization at other levels of biological organization, such as evolution of functional complexes of macromolecules and organs embryogenesis.

Animals↗
PubMed1996

Longevity in the united states: age and sex-specific evidence on life span limits from mortality patterns 1960-1990.

Determining the biological limits to human longevity is more difficult than for most other species because humans are long-lived. Consequently, mortality data, such as from the U.S. vital statistics system, which have been available for a long time (relative to most epidemiological studies) and have large numbers of cases, including deaths reported to advanced ages, are important in studying human longevity-though care must be exercised in dealing with error in age reporting. Furthermore, it is unlikely that free-living humans can realize as much of their biological endowment for longevity as animals living in a highly controlled experimental environment. We examined changes, 1960 to 1990, in U.S. White male and female extinct cohort life tables and age at death distributions to (a) examine evidence for the effects of a biological life span limit in current U.S. mortality patterns and (b) produce lower bound estimates of that limit.

Aged↗
PubMed2001

Chaos and population disappearances in simple ecological models.

A class of truncated unimodal discrete-time single species models for which low or high densities result in extinction in the following generation are considered. A classification of the dynamics of these maps into five types is proven: (i) extinction in finite time for all initial densities, (ii) semistability in which all orbits tend toward the origin or a semistable fixed point, (iii) bistability for which the origin and an interval bounded away from the origin are attracting, (iv) chaotic semistability in which there is an interval of chaotic dynamics whose compliment lies in the origin's basin of attraction and (v) essential extinction in which almost every (but not every) initial population density leads to extinction in finite time. Applying these results to the Logistic, Ricker and generalized Beverton-Holt maps with constant harvesting rates, two birfurcations are shown to lead to sudden population disappearances: a saddle node bifurcation corresponding to a transition from bistability to extinction and a chaotic blue sky catastrophe corresponding to a transition from bistability to essential extinction.

Animals↗
PubMed1990

Carbon dioxide emissions from Deccan volcanism and a K/T boundary greenhouse effect.

A greenhouse warming caused by increased emissions of carbon dioxide from the Deccan Traps volcanism has been suggested as the cause of the terminal Cretaceous extinctions on land and in the sea. We estimate total eruptive and noneruptive CO2 output by the Deccan eruptions (from 6 to 20 x 10(16) moles) over a period of several hundred thousand years based on best estimates of the CO2 weight fraction of the original basalts and basaltic melts, the fraction of CO2 degassed, and the volume of the Deccan Traps eruptions. Results of a model designed to estimate the effects of increased CO2 on climate and ocean chemistry suggest that increases in atmospheric pCO2 due to Deccan Traps CO2 emissions would have been less than 75 ppm, leading to a predicted global warming of less than 1 degree C over several hundred thousand years. We conclude that the direct climate effects of CO2 emissions from the Deccan eruptions would have been too weak to be an important factor in the end-Cretaceous mass extinctions.

Atmosphere↗
PubMed1983

Gene extinction and allelic origins in complex genealogies.

With the increasing emphasis on data analysis in mathematical genetics, problems of parametrizing genealogical structure become of practical importance. A complete specification of the genetic effects of genealogical structure is provided by the probabilities of genetically distinct states of gene identity by descent. Although this provides a direct parametrization for the joint distribution of traits on a set of related individuals, it is an unwieldy tool in the analysis of large and complex genealogies. Probabilities of joint descent of founder genes and likely ancestries of alleles provide alternative characterizations of relationship and have direct application in practical problems. Joint extinction probabilities of founder genes can also be derived as ancestral likelihoods: evolutionarily, the most significant characteristic of a genealogical structure must be its effect on the survival and extinction of genes.

Alleles↗
PubMed2002

Coevolution of costly mate choice and condition-dependent display of good genes.

Females often choose their mates, instead of mating at random, even when a father contributes nothing but genes to his offspring. Costly female preferences for males with exaggerated traits that reduce viability, such as the peacock's tail, are particularly puzzling. Such preferences can evolve if directly favoured by natural selection or when the exaggerated trait, although maladaptive per se, indicates high overall quality of the male's genotype. Two recent analyses suggested that the advantage to mate choice based on genetic quality is too weak to explain extreme cases of exaggeration of display traits and the corresponding preferences. We studied coevolution of a female mate-preference function and a genotype-dependent male display function where mutation supplies variation in genotype quality and mate preference is costly. Preference readily evolves, often causing extreme exaggeration of the display. Mate choice and trait expression can approach an equilibrium, or a limit cycle, or exaggeration can proceed forever, eventually causing extinction.

Animals↗
PubMed1995

Selectivity of end-Cretaceous marine bivalve extinctions.

Analyses of the end-Cretaceous or Cretaceous-Tertiary mass extinction show no selectivity of marine bivalve genera by life position (burrowing versus exposed), body size, bathymetric position on the continental shelf, or relative breadth of bathymetric range. Deposit-feeders as a group have significantly lower extinction intensities than suspension-feeders, but this pattern is due entirely to low extinction in two groups (Nuculoida and Lucinoidea), which suggests that survivorship was not simply linked to feeding mode. Geographically widespread genera have significantly lower extinction intensities than narrowly distributed genera. These results corroborate earlier work suggesting that some biotic factors that enhance survivorship during times of lesser extinction intensities are ineffectual during mass extinctions.

Animals↗
PubMed1980

Silent nucleotide substitutions and the molecular evolutionary clock.

Half of the nucleotide substitutions during the evolutionary divergence of genes in animals, bacteria, and viruses are silent changes. These result from an inherent biochemical property of DNA and are fixed by genetic drift. Evolution may be viewed as a device for protecting DNA molecules from extinction.

Animals↗
PubMed2005

Species loss and aboveground carbon storage in a tropical forest.

Tropical forest biodiversity is declining, but the resulting effects on key ecosystem services, such as carbon storage and sequestration, remain unknown. We assessed the influence of the loss of tropical tree species on carbon storage by simulating 18 possible extinction scenarios within a well-studied 50-hectare tropical forest plot in Panama, which contains 227 tree species. Among extinction scenarios, aboveground carbon stocks varied by more than 600%, and biological insurance varied by more than 400%. These results indicate that future carbon storage in tropical forests will be influenced strongly by future species composition.

Biodiversity↗
PubMed2002

A biologic basis for integrated malaria control.

In a series of models of Plasmodium falciparum dynamics, spontaneous local extinctions of the parasite sometimes occurred under steady, perennial-transmission conditions. These extinctions occurred only with extremely low mosquito densities or when the parameter describing the duration of human infection-blocking immunity was at its maximum value, and, simultaneously, those describing vector survivorship and the duration of human infectivity were at their minimum values. The range and frequency of extinctions increased with seasonal transmission, and decreased with the emergence of recombinant genotypes. Here we extend the immunity parameter up to levels that would describe a successful vaccine, and examine the combined influences of seasonality, genotype cross-reactivity, meiotic recombination, and human population turnover on parasite persistence. As Ross did 90 years ago, we conclude that malaria control programs that encompass several methods and targets of intervention are the most likely to succeed. Success is more likely if programs are cognizant of local circumstances of transmission, and, within that context, aim to reduce vector survivorship and human infectivity as well as augment human immunity.

Animals↗
PubMed2004

Cladogenesis, coalescence and the evolution of the three domains of life.

In this article, we explore the large-scale structure of the tree of life by using a simple model with a constant number of species and rates of speciation that equal the rates of extinction. In addition, we discuss the consequences of horizontal gene transfer for the concept of a most recent common ancestor of all living organisms (cenancestor). A simple null hypothesis based on coalescence theory explains some features of the observed topologies of the tree of life. Simulations of genes and organismal lineages suggest that there was no single common ancestor that contained all the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history that traces back to an individual molecular cenancestor. However, these molecular ancestors were likely to be present in different organisms and at different times.

Alleles↗
PubMed2005

Extinction dynamics in experimental metapopulations.

Metapopulation theory provides a framework for understanding population persistence in fragmented landscapes and as such has been widely used in conservation biology to inform management of fragmented populations. However, classical metapopulation theory [Levins, R. (1970) Lect. Notes Math. 2, 75-107] ignores metapopulation structure and local population dynamics, both of which may affect extinction dynamics. Here, we investigate metapopulation dynamics in populations that are subject to different migration rates by using experimental metapopulations of the annual plant Cardamine pensylvanica. As predicted by classical metapopulation theory, connected populations persisted longer than did isolated populations, but the relationship between migration and persistence time was nonlinear. Extinction risk sharply increased as the distance between local populations increased above a threshold value that was consistent with stochastic simulations and calculation of metapopulation capacity [Hanski, I. & Ovaskainen, O. (2000) Nature 404, 755-758]. In addition, the most connected metapopulations did not have the highest persistence levels. Stochastic simulations indicated an increase in extinction risk with the highest migration rates. Moreover, calculation of population coherence [Earn, D. J. D., Levin, S. A. & Rohani, P. (2000) Science 290, 1360-1364], a metric that predicts synchronous cycles, indicated that continuous populations should cycle in phase, resulting in an increased extinction risk. Determining empirically the optimal migration level to improve survival chances will be challenging for any natural population. Migration rates that would not increase migration above the threshold value would be ineffectual, but migration rates that would homogenize local densities could increase the risk of coherent oscillations and enhance extinction risk.

Ecology↗
PubMed1990

Extinction of a conditioned emotional response: massed and distributed exposures.

Using the lick-suppression methodology of Jacobs, Buttrick & Kennedy (Pavlovian Journal of Biological Science, 23, 29-34, 1988), a conditioned emotional response (CER) was established in 24 rats using off-the-baseline pairings of a light (the conditional stimulus) and brief footshock (the unconditioned stimulus). Following conditioning, the rats were assigned to one of three extinction groups differing in whether they received massed or distributed off-the-baseline exposure to the conditional stimulus. The effects of differential treatment were assessed on-the-baseline on test days, when the extinction of the CER was monitored. Rats receiving a single, long exposure to the conditional stimulus showed greater resistance to extinction than the rats in the distributed groups. They also showed a difference pattern of CER extinction. The results were discussed and compared to similar studies that have explored the massed vs distributed dimension, both in CER and avoidance-extinction (using response prevention or flooding). The relation of animal studies to parallel human studies using exposure therapy was also discussed.

Animals↗