SEARCH · PubMed Health
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.
Africa has lost only very few big mammals at the end of the pleistocene. Evidence is put forward that the reason for the present richness of the African mammal fauna is a very early coevolution between evolving man and the megafauna of this area. Areas without such an early coevolution suffered severe losses (up to 100%) of their native megafauna, when early hunters invaded them.
Sexual reproduction occurs in many small eukaryotes by fusion of similar gametes (isogamy). In the absence of distinguishable sperm and eggs, male and female mating types are missing. However, species with distinct males and females have so prospered that almost all familiar plants and animals have these mating types. Why has sexual reproduction involving sperm and eggs been so successful? An answer is obtained by considering physical limitations on encounter rates between gametes. A biophysical model based on well-established relationships produces fitness landscapes for the evolution of gamete size and energy allocation between motility and pheromone production. These landscapes demonstrate that selection for high gamete encounter rates favors large, pheromone-producing eggs and small, motile sperm. Thus, broadcast-spawning populations with males and females can reproduce at lower population densities and survive under conditions where populations lacking males and females go extinct. It appears that physical constraints on gamete encounter rates are sufficient to explain the first two steps in the isogamy-->anisogamy-->oogamy-->internal fertilization evolutionary sequence observed in several lineages of the eukaryotes. Unlike previous models, assumptions concerning zygote fitness or decreasing speed of swimming with increasing gamete size are not required.
We study the optimal conservation effort for a population in a fluctuating environment. The survivorship of a population is affected by unpredictable environmental fluctuation (noise) and can be improved by conservation effort accompanied by a cost. The optimal effort level is the one that minimizes the total cost, defined as the weighted sum of the population extinction risk and the economic cost of conservation effort. The optimal effort depends on the variance and the probability distribution of the noise, the relative importance of the population's survival vs. the economic cost, the effectiveness of conservation effort, and the time scope over which we optimize. The analysis of dynamic programming illustrates that the choice of extinction risk function greatly affects the optimal effort level. The conservation effort level that is the best solution of a multiple-year optimization may be higher than that for the corresponding single-year optimization, if the population is relatively safe. However, the conservation level for the multiple-year optimization becomes lower than for the single-year optimization if the population is endangered. In a similar manner, the optimal conservation effort level for the problem with a short time scope is either higher or lower than that for the problem with a long time scope, depending on the extinction risk of the population. Next, for each parameter of the model, we define five different sensitivities of extinction probability or of the total cost. We then study the mean increase in the total cost caused by the uncertainty of parameters. To achieve the best conservation result, we need to invest the limited research effort to the parameter with the largest effect to the optimal effort level, rather than to those with large impacts on the extinction probability or on the total cost. The recommended policy should depend critically on the choice of the criterion to optimize, which shows the importance of theoretical study of the relationship in performing proper decision making in conservation practice.
Only five of fourteen species of Fucales reported at the end of the XIXth century are currently present in the Albères Coast (France, NW Mediterranean). According to historical data there has been a steady decrease of all the populations since the 1940s. Seven taxa now extinct (Cystoseira crinita, Cystoseira barbata, Cystoseira foeniculacea f. tenuiramosa, Cystoseira spinosa, Cystoseira spinosa var. compressa, Sargassum hornschuchii and Sargassum vulgare) were considered frequent and some of them were the dominant and engineering species in several phytobenthic assemblages. Moreover, only one of the five species left, shows no signs of regression (Cystoseira compressa), two are considered as rare (Cystoseira caespitosa, Cystoseira zosteroides), and one is very rare (Cystoseira elegans). Cystoseira mediterranea, a species that was reported to make a continuous belt along the shores of the Albères coast, has almost disappeared from some areas. Overgrazing by sea urchins, outcompetition by mussels, habitat destruction, scientific research sampling and, probably, human trampling and chemical pollution are to be blamed for the decline of populations thriving in shallow waters. Deep-water species have been affected by an increase in water turbidity and, probably, chemical pollution and direct plant destruction attributed to net fishing. If degradation of the environmental conditions continues, the remaining Cystoseira species will face a most unwelcome prospect. Even after the removal of the causes that led to its die-off, natural restoration of extinct species seems not to be possible because the decline has also affected populations from nearby areas and zygotes are unable to disperse over long distances. Urgent management actions have to be designed in order to improve the current situation of the populations of Fucales in the Albères coast.
Cytoplasmic incompatibility between arthropods infected with different strains of Wolbachia has been proposed as an important mechanism for speciation. However, a basic requirement for this mechanism is the coexistence of different strains in neighbouring populations. Here we test whether this required coexistence is possible in a spatial context. Continuous-time models for the behaviour of one and two strains of Wolbachia within a single well-mixed population demonstrate the Allee effect and founder control, such that one strain is always driven extinct. In contrast, discretised spatial models show patchy persistence of the two strains although coexistence within the same habitat is rare. A simplified model of such founder control suggests that it is fragmentation of (or barriers within) the habitat rather than space itself that leads to persistence.
Determining patterns of change in species richness and the processes underlying the dynamics of biodiversity are of key interest within the field of ecology, but few studies have investigated the dynamics of vertebrate communities at a decadal temporal scale. Here, we report findings on the spatio-temporal variability in the richness and composition of fish communities along the Norwegian Skagerrak coast having been surveyed for more than half a century. Using statistical models incorporating non-detection and associated sampling variance, we estimate local species richness and changes in species composition allowing us to compute temporal variability in species richness. We tested whether temporal variation could be related to distance to the open sea and to local levels of pollution. Clear differences in mean species richness and temporal variability are observed between fjords that were and were not exposed to the effects of pollution. Altogether this indicates that the fjord is an appropriate scale for studying changes in coastal fish communities in space and time. The year-to-year rates of local extinction and turnover were found to be smaller than spatial differences in community composition. At the regional level, exposure to the open sea plays a homogenizing role, possibly due to coastal currents and advection.
A method for inferring the ancestral genotypes for the founders of a population is developed. This method uses the algorithms for the computation of probabilities on pedigrees of arbitrary complexity, developed by Cannings et al. (1978) and implemented by Thompson (1977b). When characteristics are simply determined by underlying genotypes the inference problem is simplified, and larger and more complex pedigrees may therefore be analysed. The problem of estimating the allele frequencies to be used in computing prior genotype probabilities for those founders on whom a likelihood function is not required is discussed. The same method allows us to compute extinction probabilities for any combination of original founder genes; these probabilities are interesting parameters of pedigree structure, which, since they relate to the actual genes present in a population, help to provide a clearer understanding of observed distributions of autosomal traits.
The ecology of the multimammate mouse ,Mastomys natalensis, is reviewed and approximations are derived for the parameters governing population growth. By means of computer simulation, the relative importance of the timing of reproduction, the age class distribution or age structure of the population, the competition between Mastomys and Rattus, and the interaction with a predator are evaluated. Although each of these demographic or ecological factors modifies the fate of the Mastomys population, the greatest single impact results from a reproductive season that is divided into two parts rather than a single continuous reproductive season. Division into two parts, correlated with a similar distribution of rainfall, allows time for maturing of the young born early in the season and for production of young by them, thus adding to the momentum of population increase. The interaction of density-dependent factors controlling population growth, competition with another rodent, or predation by a Viverrid predator, may increase the growth rate of the Mastomys population or may depress population growth rate, even to the point of extinction. These simulation studies demonstrate the necessity for critical evaluation of the demographic parameters and ecological characteristics of a particular Mastomys population before an effective control programme can be designed. They also demonstrate, however, that if the programme is based upon sound ecological theory control can be effected.
Johnson and Cicero (2004) claimed that inspection of a distribution of uncorrected mitochondrial DNA avian sister-taxon distances illustrated that the late Pleistocene was an important time for avian speciation. They believed this finding to be at odds with conclusions of Klicka and Zink (1997). However, both studies document recent speciation events. More germane to the discussion is what is meant by an "important" time for speciation, which we take to mean above some baseline diversification rate. We constructed a null distribution of sister-taxon distances based on a model of constant speciation and extinction rates. The empirical distribution of sister-taxon distances in Johnson and Cicero (2004) did not differ from the null model. Therefore, our analysis of Johnson and Cicero's data suggests that the late Pleistocene was no more important for avian speciation than any other time during this time period.
One of the goals of the present Martian exploration is to search for evidence of extinct (or even extant) life. This could be redefined as a search for carbon. The carbon cycle (or, more properly, cycles) on Earth is a complex interaction among three reservoirs: the atmosphere; the hydrosphere; and the lithosphere. Superimposed on this is the biosphere, and its presence influences the fixing and release of carbon in these reservoirs over different time-scales. The overall carbon balance is kept at equilibrium on the surface by a combination of tectonic processes (which bury carbon), volcanism (which releases it) and biology (which mediates it). In contrast to Earth, Mars presently has no active tectonic system; neither does it possess a significant biosphere. However, these observations might not necessarily have held in the past. By looking at how Earth's carbon cycles have changed with time, as both the Earth's tectonic structure and a more sophisticated biology have evolved, and also by constructing a carbon cycle for Mars based on the carbon chemistry of Martian meteorites, we investigate whether or not there is evidence for a Martian biosphere.
A synthesis of developmental genetics with evolutionary genetics is now making possible to understand significant evolutionary changes in multicellular organisms. The key concept for unifying the two must be heterochrony. Heterochrony causes evolutionary modifications due to changes in timing and/or rate of development. The heterochrony is conventionally categorized into three patterns as neoteny (retardation in somatic development), progenesis (acceleration in gonadal development), and direct development (acceleration in somatic development, resulting in lack of larval or tadpole stages). A lot of species showing neoteny are known in urodeles, but not in anurans. Neotenic urodeles are also divided into three categories; permanent or obligate, "inducible" obligate and facultative neotenies. Hynobius retardatus, a specific population of which had been reported to show neoteny but is believed to be extinct at present, has become to be used for experimental analysis of heterochronic expression of several adult characters during its ontogeny. Gonadal maturation and a transition of globin subunits from larval to adult types have been shown to occur independently on the morphological metamorphosis in H. retardatus. Mechanisms underlying the heterochrony, including morphogenetic clock, heterochronic genes in Drosophila and C. elegans, temporal colinearity in Hox gene complex in mice, and atavistic transformation induced by altered expression of Hox genes are discussed in terms of current molecular biology.
The explicit consideration of space in ecological research is of paramount importance to understand the structure and functioning of ecological systems. In this paper we develop a simple spatially explicit metapopulation model in which colonization is constant and independent of the number of occupied patches (i.e. propagule-rain effect, Gotelli, 1991). Extinction, on the other hand, is modelled as a stochastic process whose intensity depends on the number of occupied patches in the neighborhood of each focal patch. Our model is the CA counterpart of two classical patch occupancy metapopulation models. We analytically prove this by showing that our CA converges to the differential equation in the mean-field approximation. The asymptotic behaviour of the system, expressed as the proportion of occupied patches, agrees with the equilibrium proportion of patches derived by using ODEs. In both models, the existence of a rescue-effect increases the range of extinction and colonization parameters over which the system attains complete occupancy of patches. However, in our model this result is strongly influenced by the degree of coupling among patches and is apparent only for local interactions. With local interactions and particular parameter values of colonization and extinction, self-organized spatio-temporal patterns emerge with a fractal-like clustering, even though the environment is spatially homogeneous. Our results point out that the importance of being spatial and discrete (Durrett & Levin, 1994a) in our model is a result of local interactions.
Here we review the phenomenon of ecomorph evolution and the hypothesis of iterative climatic cycles. Although a widely known phenomenon, convergent evolution has been underappreciated in both its scope and commonality. The power of natural selection to override genealogy to create similar morphologies (even among distantly related organisms) supports classical Darwinian evolution. That this occurs repeatedly in stratigraphically closely spaced intervals is one of the most striking features of Earth history. Periodic extinctions followed by re-evolution of adaptive types (ecomorphs) are not isolated occurrences but are embedded within complex ecological systems that evolve, become extinct, and repeat themselves in temporal synchrony. These complexes of radiation and extinction bundle the biostratigraphic record and provide the basis for a global stratigraphy. At this scale, climatic change is the only mechanism adequate to explain the observed record of repeating faunas and floras. Understanding of the underlying causes may lead to predictive theories of global biostratigraphy, evolutionary processes, and climatic change.
Biological changes due to the menstrual cycle may account for the fact that fears are not equally distributed between the sexes. In a differential, classical conditioning paradigm, women in premenstrual phase of their menstrual cycles were compared with a control group of women at other points in their cycles except within seven days before menstruation. Electric shock and pictures of natural scenes were used as UCS and CS respectively. Premenstrual women showed an enhanced susceptibility to the acquisition of a conditioned skin conductance response and to delayed extinction, while control women did not. The possible role of an altered physiological state during the premenstrual phase in the acquisition of fear responses is discussed.
Thomas et al. have carried out a useful analysis of the extinction risk from climate warming. Their overall conclusion, that a large fraction of extant species could be driven to extinction by expected climate trends over the next 50 years, is compelling: it adds to the many other reasons why new energy policies are needed to reduce the pace of warming.
Climate change and habitat destruction are two of the greatest threats to global biodiversity. Lattice models have been used to investigate how hypothetical species with different characteristics respond to habitat loss. The main result shows that a sharp threshold in habitat availability exists below which a species rapidly becomes extinct. Here, a similar modelling approach is taken to establish what determines how species respond to climate change. A similar threshold exists for the rate of climate change as has been observed for habitat loss-patch occupancy remains high up to a critical rate of climate change, beyond which species extinction becomes likely. Habitat specialists, especially those of relatively poor colonizing ability are least able to keep pace with climate change. The interaction between climate change and habitat loss might be disastrous. During climate change, the habitat threshold occurs sooner. Similarly, species suffer more from climate change in a fragmented habitat.
The autocatalytic reaction system with a small number of molecules is studied numerically by stochastic particle simulations. A novel state due to fluctuation and discreteness in molecular numbers is found, characterized as an extinction of molecule species alternately in the autocatalytic reaction loop. Phase transition to this state with changes of the system size and flow is studied, while a single-molecule switch of the molecule distributions is reported. The relevance of the results to intracellular processes is briefly discussed.
1. Lysine vasopressin induces resistance to extinction of active avoidance behaviour (De Wied, 1971).2. Digestion of lysine vasopressin with trypsin almost completely destroys the pressor-, antidiuretic-, oxytocic- and corticotrophin-releasing factor activities of lysine vasopressin, but does not materially influence its effect on the maintenance of an avoidance response.