首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
Plants often suffer reductions in fecundity due to fragmentation, degradation and destruction of populations and their sites. Whether this decrease in seed production has population-level consequences is generally unknown. Here, we aimed to determine the current status of remnant populations in the perennial herb Primula veris in Belgium. Furthermore, we investigated the effects of reduced population size and morph bias on reproductive success and explored if changes in demographic structure could be associated with population fecundity. We studied 69 populations that differed in population size from three to nearly 1500 flowering plants. Three different population types could be distinguished: (a) “dynamic” populations, characterized by high densities of 1 year old juveniles, (b) “normal” populations with adult age-stages prevailing, but still a considerable number of juveniles, and (c) “regressive” populations, in which only flowering adults dominate and rejuvenation hardly occurs. The three population types differed with respect to population size and morph frequency. Dynamic populations were significantly larger and showed a weaker morph bias compared to the intermediate normal and the small regressive populations. Reproductive success, studied the previous year in 26 populations, decreased significantly with decreasing population size and was significantly associated with the demographic structure of the populations. Coefficients of variation for the proportion of flowers setting fruit, the number of seeds per fruit and the total number of seeds per plant decreased significantly with increasing population size. Hence, the observed variability in seed set may be one of the causal factors affecting the observed types of population demographic structure.  相似文献   

2.
I evaluated demographic parameters as indicators of fitness by calculating the net reproductive rate (R0), exponential rate of change (r), lifetime reproductive success (LRS), and Malthusian parameter (m) for nine genotypes and four phenotypes (two alleles at each of two independent loci) of an age-structured population. The given starting conditions included age-specific survival rates of males and females and age-specific fecundity of females for each genotype (to simplify the problem I presumed no differences in survivorship or fecundity of genotypes with the same phenotype) and the same age structure for each genotype. The prevailing genotype had the greatestm, but it did not have the greatestr,R0, or LRS, or even the greatest survivorship of either juveniles or adults, or the greatest fecundity. This result indicates thatmis the only correct measure of fitness (i.e., as a predictor of which genotype should prevail from among a group of genotypes) and that comparisons ofr,R0, LRS, juvenile or adult survival rates, or fecundity may be misleading indicators of which genotype should prevail (i.e., be most “fit”) over time (i.e., be selected for).  相似文献   

3.
Concentrations of cadmium in the grain of durum wheat (Triticum turgidum L. var durum) are often above the internationally acceptable limit of 0.2 mg kg−1. Cultivars that vary in concentrations of cadmium in the grain have been identified but the physiology behind differential accumulation has not been determined. Three pairs of near-isogenic lines (isolines) of durum wheat that vary in aboveground cadmium accumulation (8982-TL ‘high’ and ‘low’, W9260-BC ‘high’ and ‘low’, and W9261-BG ‘high’ and ‘low’) were used to test the hypothesis that the greater amounts of cadmium in shoots of the ‘high’ isolines are correlated with greater volumes of water transpired. In general, cadmium content was positively correlated with transpiration only in the ‘low’ isolines. Although shoots of the ‘high’ isolines of W9260-BC and W9261-BG contained higher concentrations of cadmium than did their corresponding ‘low’ isolines, they did not transpire larger volumes of water. In addition, isolines of 8982-TL transpired less water than did the other pairs of isolines yet both ‘high’ and ‘low’ isolines of 8982-TL contained higher amounts of cadmium than did the other pairs. The difference between ‘high’ and ‘low’ isolines appears to be related to the relative contribution of transpiration to cadmium translocation to the shoot. Increased transpiration was associated with increased cadmium content in the ‘low’ isolines but in the ‘high’ isolines increased cadmium in the shoot occurred independently of the volume of water transpired.  相似文献   

4.
Mathematical models are developed in order to analyze whether or not social factors, such as, for example, the “social fence” (J. B. Hestbeck, 1982, Oikos 39, 157–163) will stabilize population density: the dynamic interaction between social factors and (dynamic) trophic factors is analyzed. It is concluded that social factors such as the “social fence” tend to stabilize population density; hence, if density cycles (as, e.g., seen in many microtine rodents) are observed in nature, it seems reasonable to conclude that density cycles are driven by, for example, trophic interactions and not by social factors. It is suggested that the “social fence” may explain why so many populations including several microtine populations have fairly stable densities despite the ever-existing destabilizing trophic interactions. Contrary to what is implied by J. B. Hestbeck (1983, “A Mathematical Model of Population Regulation in Cyclic Mammals,” Lecture notes in biomathematics, Vol. 52, Springer-Verlag, Berlin/New York), the analysis presented in this paper demonstrates that seasonal environmental changes are not essential for the generation of regular density cycles. Seasonal changes may, however, be necessary for generating a microtine-like density cycle. Empirical information on microtine rodents relating to the “social fence hypothesis” is discussed.  相似文献   

5.
Mammalian birth is accompanied by profound changes in metabolic rate that can be described in terms of body size relationship (Kleiber's rule). Whereas the fetus, probably as an adaptation to the low intrauterine pO2, exhibits an “inappropriately” low, adult-like specific metabolic rate, the term neonate undergoes a rapid metabolic increase up to the level to be expected from body size. A similar, albeit slowed, “switching-on” of metabolic size allometry is found in human preterm neonates whereas animals that are normally born in a very immature state are able to retard or even suppress the postnatal metabolic increase in favor of weight gain and O2 supply. Moreover, small immature mammalian neonates exhibit a temporary oxyconforming behavior which enhances their hypoxia tolerance, yet is lost to the extent by which the size-adjusted metabolic rate is “locked” by increasing mitochondrial density. Beyond the perinatal period, there are no other deviations from metabolic size allometry among mammals except in hibernation where the temporary “switching-off” of Kleiber's rule is accompanied by a deep reduction in tissue pO2. This gives support to the hypothesis that the postnatal metabolic increase represents an “escape from oxygen” similar to the evolutionary roots of mitochondrial respiration, and that the overall increase in specific metabolic rate with decreasing size might contribute to prevent tissues from O2 toxicity.  相似文献   

6.
The fast–slow continuum hypothesis has been proposed to explain the diversity of life-history patterns exhibited by biological populations, but the quantification and population-dynamic consequences of the continuum has remained unclear. I used the ratio of fertility rate to age at first reproduction (F/α ratio) to quantify the tempo of life-history of 138 populations of mammals, and investigated the life-history and population-dynamic consequences of being “fast” or “slow”. “Fast” mammals (F/α>0.60) were characterized by early maturity, short lifespans, low survival rates, and high fertility and projected population growth rate (λ) compared to “slow” (F/α<0.15) mammals. In “fast” populations, λ was overwhelmingly most sensitive to changes in reproductive parameters (age at first reproduction and fertility rates) and relatively insensitive to changes in survival rates. In “slow” populations, λ was very sensitive to changes in juvenile or adult survival rates, and relatively insensitive to changes in reproductive parameters. The pattern of relationships between the F/α ratio and life-history variables, λ, and elasticity of λ to changes in life-history variables persisted even after the effects of body size and phylogeny were statistically removed. These results suggest that fast–slow continuum in mammalian life-history is independent of body size or phylogeny, that the F/α ratio adequately quantifies the position of a population along a fast–slow continuum, and that the tempo of life- histories has substantial population-dynamic consequences.

Zusammenfassung

Die r-K-Kontinuum-Hypothese wurde aufgestellt, um die Diversität von ,,life-history“-Mustern biologischer Populationen zu erklären, aber die Quantifizierung und die Kosnsequenzen für die Populationsdynamik des Kontinuums blieben unklar. Ich benutze das verhältnis der Fortpflanzungsrate zum Fortpflanzungsalter (F/α-Verhältnis) um die Geschwindigkeit der ,,life-history“ von 138 Populationen von Säugetieren zu quantifizieren und untersuchte die Konsequenzen fur die Lebensweise sowie die Populationsdynamik des,,schnell“oder,,langsam “-Seins. ,,Schnelle“Säugetiere (F/α>0.60) waren durch eine frühe Reife, kurze Lebenszeiten, geringe Überlebensraten sowie durch eine große Fertilität und hochgerechnete Populationswachstumsrate (λ) im Vergleich zu ,,langsamen“(F/α<0.15) Säugetieren charakterisiert. In ,,schnellen“ Population reagierte (λ) überwältigend sensibel auf Änderungen in den Fortpflanzungsparametern (Fortpflanzunsalter und Fertilitätsrate) und relativ gering auf Veräanderungen in der Überlebensrate. In ,,langsamen“ Populationen reagierte (λ) sehr sensibel auf Veräanderungen in den reproduktiven Parametern. Das Muster der Beziehung zwischen dem (F/α-Verhältnis) und den Variablen der ,,life-history“,λ, und die Elastizität von λ gegenüber Veränderungen in den variablen der Lebensweise bliev sogar bestehen, nachdem die Effekte von Körpergröße und Phylogenese statistisch eliminiert wurden. Diese Ergebnisse lassen vermuten, dass das r-K-Kontinuum in der ,,life-history“der Säugetiere unabhängig von der Körpergröße und Phylogenie ist, dass das F/α-Verhältnis die Position einer Population im r-K-Kontinuum quantifiziert und dass die Geschwindigkeit der,,life-history“beachtliche konsequenzen fur die Populationsdynamik hat.  相似文献   

7.
The Framingham Risk Score (FRS) was developed to predict coronary heart disease in various populations, and it tended to under-estimate the risk in chronic kidney disease (CKD) patients. Our objectives were to determine whether FRS was associated with cardiovascular events, and to evaluate the role of new risk markers and echocardiographic parameters when they were added to a FRS model. This study enrolled 439 CKD patients. The FRS is used to identify individuals categorically as “low” (<10% of 10-year risk), “intermediate” (10–20% risk) or “high” risk (≧ 20% risk). A significant improvement in model prediction was based on the −2 log likelihood ratio statistic and c-statistic. “High” risk (v.s. “low” risk) predicts cardiovascular events either without (hazard ratios [HR] 2.090, 95% confidence interval [CI] 1.144 to 3.818) or with adjustment for clinical, biochemical and echocardiographic parameters (HR 1.924, 95% CI 1.008 to 3.673). Besides, the addition of albumin, hemoglobin, estimated glomerular filtration rate, proteinuria, left atrial diameter >4.7 cm, left ventricular hypertrophy or left ventricular ejection fraction<50% to the FRS model significantly improves the predictive values for cardiovascular events. In CKD patients, “high” risk categorized by FRS predicts cardiovascular events. Novel biomarkers and echocardiographic parameters provide additional predictive values for cardiovascular events. Future study is needed to assess whether risk assessment enhanced by using these biomarkers and echocardiographic parameters might contribute to more effective prediction and better care for patients.  相似文献   

8.
Daniela Rder  Kathrin Kiehl 《Flora》2006,201(6):499-507
In 2003, the last existing population of Pulsatilla patens in Germany was studied in the nature reserve “Garchinger Heide”, a calcareous grassland north of Munich. The aim of our study was to evaluate the endangerment of the species in ancient and young grasslands of the nature reserve. Therefore, the size, structure and generative reproduction of the population in relation to vegetation characteristics were investigated. Total population size and mean density of individual plants in different zones of the nature reserve were determined by a GPS-supported counting and compared to a mapping of 1991. The population structure was analyzed by counting the number of individuals of different age–state classes on 4 m2 plots in ancient and young grasslands of the nature reserve. The cover of phanerogams, cryptogams, litter and bare soil was studied in order to characterize habitat quality.In 2003, about 9700 individuals of P. patens were counted which means that population size strongly decreased in comparison to 1991 (about 27 000). In the ancient grasslands, the number of individuals of all age–state classes of P. patens correlated negatively with the cover of phanerogams and litter. In contrast, phanerogam and cryptogam cover had a positive influence on the number of individuals of all age–state classes on a topsoil-removal site due to protection from desiccation. This site was the only one in the nature reserve, where population size increased between 1991 and 2003. The large population size indicates that short-term endangerment or extinction of the species in the nature reserve is not very likely. Nevertheless, the management of the nature reserve has to be optimized in order to prevent a further long-term decrease of the population size of P. patens.  相似文献   

9.
Aspergillus flavus is a common filamentous fungus that produces aflatoxins and presents a major threat to agriculture and human health. Previous phylogenetic studies of A. flavus have shown that it consists of two subgroups, called groups I and II, and morphological studies indicated that it consists of two morphological groups based on sclerotium size, called “S” and “L.” The industrially important non-aflatoxin-producing fungus A. oryzae is nested within group I. Three different gene regions, including part of a gene involved in aflatoxin biosynthesis (omt12), were sequenced in 33 S and L strains of A. flavus collected from various regions around the world, along with three isolates of A. oryzae and two isolates of A. parasiticus that were used as outgroups. The production of B and G aflatoxins and cyclopiazonic acid was analyzed in the A. flavus isolates, and each isolate was identified as “S” or “L” based on sclerotium size. Phylogenetic analysis of all three genes confirmed the inference that group I and group II represent a deep divergence within A. flavus. Most group I strains produced B aflatoxins to some degree, and none produced G aflatoxins. Four of six group II strains produced both B and G aflatoxins. All group II isolates were of the “S” sclerotium phenotype, whereas group I strains consisted of both “S” and “L” isolates. Based on the omt12 gene region, phylogenetic structure in sclerotium phenotype and aflatoxin production was evident within group I. Some non-aflatoxin-producing isolates of group I had an omt12 allele that was identical to that found in isolates of A. oryzae.  相似文献   

10.
A sample of 84 female subjects (students) was taken and split up on the basis of the IST-ratings (“Intelligence Structure Test” by Amthauer) into two extreme groups of “more intelligent” (N = 14) and “less intelligent” (N = 11) subjects with an aim to testing Kozeny's findings. Using his photographic-statistical method, Kozeny had observed that “more intelligent” people have a more open eye, a smaller and straighter mouth, a stronger chin, and—above all in its side dimensions—a better developed forehead. These physiognomic indicators of differences in intelligence were confirmed to a large extent by the measurements taken from the portraits of the subjects.  相似文献   

11.
The purpose of the current experiment was to investigate the impacts of grinding and pelleting procedures applied to wheat in a wheat–rapeseed meal diet on the coefficients of standardized ileal digestibility, i.e., apparent digestibility corrected for basal endogenous losses (CSID), and true ileal digestibility, i.e., apparent digestibility corrected for total endogenous losses (CTID), of nitrogen (N) and amino acids (AA) in pigs. Ileal digestibility was measured by collecting digesta from pigs fitted with ileorectal anastomoses. Four diets, involving four technological treatments applied to wheat, were compared in vivo according to a 4 × 4 Latin square design (four pigs each fed four diets during four successive periods of 1 week). The technological treatments of wheat were two grinding procedures and two pelleting processes. Wheat was ground to obtain mean flour particle sizes of 1000 and 500 μm, leading after mixing with rapeseed meal and minerals-vitamins premix to the first and second diets named “coarse” and “fine”, respectively. Part of the 500 μm wheat flour was pelleted through dies of same screen diameter (4 mm) but different thicknesses, 16 and 20 mm, inducing a low and high compression ratio, leading after mixing with rapeseed meal and premix to the third and fourth diets named “LCR” and “HCR”, respectively. Basal endogenous losses were determined by feeding a protein-free diet during the 5th week of the experiment. Total endogenous losses were measured by way of the isotopic dilution method using 15N-labeled wheat and rapeseed meal. Decreasing wheat particle size from 1000 to 500 μm improved (P<0.05) the coefficient of ileal digestibility of dietary energy (0.707 versus 0.665), organic matter (0.718 versus 0.677) and dry matter (0.681 versus 0.645), but neither AA CSID nor N retention. The pelleting processes did not further increase (P>0.10) energy or organic matter digestibility but improved (P<0.05) N and AA CSID (0.785 versus 0.759 for N and 0.725 versus 0.679 for lysine, with HCR versus fine diet, respectively). Pelleting wheat flour at higher compression ratio (HCR versus LCR diet) was more efficient to improve dietary N and AA digestibility values due to a significant decrease in ileal specific, i.e., total minus basal, N and AA endogenous losses (P<0.05) associated with an increase in CTID. It is concluded that pelleting wheat fine flour at high compression ratio allows maximizing AA digestibility and availability of a wheat–rapeseed meal diet.  相似文献   

12.
The effectiveness of a rehabilitation program for reducing inter-dog aggression was evaluated at the municipal animal shelter. Sixteen dogs (of 60 examined) met the study criteria of medium inter-dog aggression as determined by an inter-dog aggression test. These dogs received a 10-day treatment of daily rehabilitation for 30 min (rehabilitation group, n = 9) or daily release into an outdoor enclosure for 30 min (control group, n = 7). Rehabilitation consisted of desensitising and counter-conditioning dogs to the approach of other “stimulus” dogs. Most dogs in the rehabilitation group showed a decline in aggression scores when re-tested after the last treatment (day 11), and differed significantly from the control dogs which showed either an increase or no change in aggression scores (U = 8.5, P < 0.01). Rehabilitation dogs also showed lower frequencies of aggressive body postures (“facing the stimulus dog”, P < 0.05, and “stiff posture”, P < 0.10) and higher frequencies of less assertive postures (“ears back”, P < 0.05, and “lowered neck”, P < 0.10) on day 11. The differences between groups were no longer significant when a reduced sample of dogs was tested 1 week after rehabilitation ended (day 18). The study shows short-term reduction of inter-dog aggression through rehabilitation, but further work is needed on effective ways of maintaining the behavioural change.  相似文献   

13.
The concept of the effective population size is discussed. It is shown that the “eigenvalue” and the “inbreeding” effective population sizes are in principle different, even though they have been sometimes identified in the literature. On the other hand the “eigenvalue” and “variance” effective sizes are usually both close when the latter exists. Since, however, there are many models for which a variance effective size cannot in principle exist, it seems useful to introduce the eigenvalue effective size and to examine some of its properties.  相似文献   

14.
A review of species in the genus Praon Haliday, 1833 is presented. Twenty described species are keyed and illustrated with scanning electron micrographs and line drawings. The Praon species presented in this work have been identified from 67 aphid taxa occurring on 120 plant taxa. Furthermore, 87 original parasitoid–aphid–plant associations of the species mentioned in the key are presented. Phylogenetic relationships among Praon species are reconstructed using parsimony and cladistic distance methods. Praon abjectum is the sister taxon to the remaining Praon species. We recognized three species group: “Parapraon”, “dorsale-yomenae” and “rosaecola”. Monophyly is suggested for “Parapraon” species group and paraphyly for “dorsale-yomenae” group. Finally, by phylogenetic reconstruction, a close phylogenetic relationship between “Parapraon” and “dorsale-yomenae” species group was found.  相似文献   

15.
The Effective Size of a Subdivided Population   总被引:22,自引:4,他引:18       下载免费PDF全文
This paper derives the long-term effective size, N(e), for a general model of population subdivision, allowing for differential deme fitness, variable emigration and immigration rates, extinction, colonization, and correlations across generations in these processes. We show that various long-term measures of N(e) are equivalent. The effective size of a metapopulation can be expressed in a variety of ways. At a demographic equilibrium, N(e) can be derived from the demography by combining information about the ultimate contribution of each deme to the future genetic make-up of the population and Wright's F(ST)'s. The effective size is given by N(e) = 1/(1 + var ( &))<(1 - f(STi))/N(i)n>, where n is the number of demes, &(i) is the eventual contribution of individuals in deme i to the whole population (scaled such that σ(i) &(i) = n), and < > denotes an average weighted by &(i)(2). This formula is applied to a catastrophic extinction model (where sites are either empty or at carrying capacity) and to a metapopulation model with explicit dynamics, where extinction is caused by demographic stochasticity and by chaos. Contrary to the expectation from the standard island model, the usual effect of population subdivision is to decrease the effective size relative to a panmictic population living on the same resource.  相似文献   

16.
This paper examines the effect of inbreeding level of population on the magnitude of inbreeding depression expressed by comparing them between two cultured populations (A and B) in the hermaphroditic animal of the bay scallop Argopecten irradians irradians. Population A is expected to have less genetic variations and higher inbreeding level due to longer cultured history (20 generations) and less “ancestral” individuals (26 individuals) than population B due to shorter cultured history (4 generations) and more “ancestral” individuals (406 individuals). Two groups within each population were produced, one using self-fertilization and one using mass-mating within the same population. Selfed offspring (AS and BS) from two populations both had lower fitness components than their mass-mated counterparts (AM and BM) and exhibited inbreeding depression for all examined traits, e.g. lower hatching, less viability and slower growth, indicating that inbreeding depression is a common feature in this animal. Fitness components in all traits of offspring from population A significantly differed those from population B and the magnitude of inbreeding depression for all traits in population A with higher inbreeding level was significantly smaller than that in population B with lower inbreeding level, indicating that both fitness components and magnitude of inbreeding depression were significantly affected by inbreeding level of populations and genetic load harbored in population A may be partially purged through inbreeding. Moreover, the magnitude of inbreeding depression in the two populations both varied among traits and life history stages. The present results support the partial-dominance hypothesis of inbreeding depression.  相似文献   

17.
New measurements have been made of fluorescence lifetime (τ) of chlorophyll a in the algae Chlorella pyrenoidosa, Porphyridium cruentum, Anacystis nidulans, and in spinach chloroplast. τ-values of 0.6 and 0.7 nsec were obtained with green plants. Anacystis and Porphyridium gave a τ of 0.5 nsec. The previously described two stage decay of fluorescence in vivo in these organisms could not be confirmed. This observation could have been caused by a second wave of light emission from the exciting hydrogen lamp (not detected in earlier work). The lifetimes found in this study (calculated, as before, by the method of convolution integrals) were close to those found by other observers for “low” excitation intensities; the value first reported from this laboratory (1.0-1.7 nsec) may have corresponded to “high” excitation intensity.  相似文献   

18.
Increasingly, land managers have attempted to use extreme prescribed fire as a method to address woody plant encroachment in savanna ecosystems. The effect that these fires have on herbaceous vegetation is poorly understood. We experimentally examined immediate (<24 hr) bud response of two dominant graminoids, a C3 caespitose grass, Nassella leucotricha, and a C4 stoloniferous grass, Hilaria belangeri, following fires of varying energy (J/m2) in a semiarid savanna in the Edwards Plateau ecoregion of Texas. Treatments included high‐ and low‐energy fires determined by contrasting fuel loading and a no burn (control) treatment. Belowground axillary buds were counted and their activities classified to determine immediate effects of fire energy on bud activity, dormancy, and mortality. High‐energy burns resulted in immediate mortality of N. leucotricha and H. belangeri buds (p < .05). Active buds decreased following high‐energy and low‐energy burns for both species (p < .05). In contrast, bud activity, dormancy, and mortality remained constant in the control. In the high‐energy treatment, 100% (n = 24) of N. leucotricha individuals resprouted while only 25% (n = 24) of H. belangeri individuals resprouted (p < .0001) 3 weeks following treatment application. Bud depths differed between species and may account for this divergence, with average bud depths for N. leucotricha 1.3 cm deeper than H. belangeri (p < .0001). Synthesis and applications: Our results suggest that fire energy directly affects bud activity and mortality through soil heating for these two species. It is imperative to understand how fire energy impacts the bud banks of grasses to better predict grass response to increased use of extreme prescribed fire in land management.  相似文献   

19.

Objective

To evaluate quality of routine and emergency intrapartum and postnatal care using a health facility assessment, and to estimate “effective coverage” of skilled attendance in Brong Ahafo, Ghana.

Methods

We conducted an assessment of all 86 health facilities in seven districts in Brong Ahafo. Using performance of key signal functions and the availability of relevant drugs, equipment and trained health professionals, we created composite quality categories in four dimensions: routine delivery care, emergency obstetric care (EmOC), emergency newborn care (EmNC) and non-medical quality. Linking the health facility assessment to surveillance data we estimated “effective coverage” of skilled attendance as the proportion of births in facilities of high quality.

Findings

Delivery care was offered in 64/86 facilities; only 3-13% fulfilled our requirements for the highest quality category in any dimension. Quality was lowest in the emergency care dimensions, with 63% and 58% of facilities categorized as “low” or “substandard” for EmOC and EmNC, respectively. This implies performing less than four EmOC or three EmNC signal functions, and/or employing less than two skilled health professionals, and/or that no health professionals were present during our visit. Routine delivery care was “low” or “substandard” in 39% of facilities, meaning 25/64 facilities performed less than six routine signal functions and/or had less than two skilled health professionals and/or less than one midwife. While 68% of births were in health facilities, only 18% were in facilities with “high” or “highest” quality in all dimensions.

Conclusion

Our comprehensive facility assessment showed that quality of routine and emergency intrapartum and postnatal care was generally low in the study region. While coverage with facility delivery was 68%, we estimated “effective coverage” of skilled attendance at 18%, thus revealing a large “quality gap.” Effective coverage could be a meaningful indicator of progress towards reducing maternal and newborn mortality.  相似文献   

20.
Shifts between adaptive peaks, caused by sampling drift, are involved in both speciation and adaptation via Wright's “shifting balance.” We use techniques from statistical mechanics to calculate the rate of such transitions for a population in a single panmictic deme and for a population which is continuously distributed over one- and two-dimensional regions. This calculation applies in the limit where transitions are rare. Our results indicate that stochastic divergence is feasible despite free gene flow, provided that neighbourhood size is low enough. In two dimensions, the rate of transition depends primarily on neighbourhood size N and only weakly on selection pressure (≈sk exp(− cN)), where k is a number determined by the local population structure, in contrast with the exponential dependence on selection pressure in one dimension (≈exp(− cNs)) or in a single deme (≈exp(− cNs)). Our calculations agree with simulations of a single deme and a one-dimensional population.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号