CENTRAL MOMENTS AND PROBABILITY DISTRIBUTION OF COLLESS'S COEFFICIENT OF TREE IMBALANCE

The great increase in the number of phylogenetic studies of a wide variety of organisms in recent decades has focused considerable attention on the balance of phylogenetic trees—the degree to which sister clades within a tree tend to be of equal size—for at least two reasons: (1) the degree of balance of a tree may affect the accuracy of estimates of it; (2) the degree of balance, or imbalance, of a tree may reveal something about the macroevolutionary processes that produced it. In particular, variation among lineages in rates of speciation or extinction is expected to produce trees that are less balanced than those that result from phylogenetic evolution in which each extant species of a group has the same probability of speciation or extinction. Several coefficients for measuring the balance or imbalance of phylogenetic trees have been proposed. I focused on Colless's coefficient of imbalance (7) for its mathematical tractability and ease of interpretation. Earlier work on this statistic produced exact methods only for calculating the expected value. In those studies, the variance and confidence limits, which are necessary for testing the departure of observed values of I from the expected, were estimated by Monte Carlo simulation. I developed recursion equations that allow exact calculation of the mean, variance, skewness, and complete probability distribution of I for two different probability-generating models for bifurcating tree shapes. The Equal-Rates Markov (ERM) model assumes that trees grow by the random speciation and extinction of extant species, with all species that are extant at a given time having the same probability of speciation or extinction. The Equal Probability (EP) model assumes that all possible labeled trees for a given number of terminal taxa have the same probability of occurring. Examples illustrate how these theoretically derived probabilities and parameters may be used to test whether the evolution of a monophyletic group or set of monophyletic groups has proceeded according to a Markov model with equal rates of speciation and extinction among species, that is, whether there has been significant variation among lineages in expected rates of speciation or extinction.     

WHEN MORPHOLOGY MISLEADS: INTERPOPULATION UNIFORMITY IN SEXUAL SELECTION MASKS GENETIC DIVERGENCE IN HARLEQUIN BEETLE-RIDING PSEUDOSCORPION POPULATIONS

Differences in secondary sexual characteristics of males often provide the most conspicuous means of distinguishing between closely related species. Does this therefore imply that the absence of differentiation in exaggerated male traits between allopatric populations provides evidence of a single, genetically cohesive species? We addressed this question with a comprehensive investigation of two populations (French Guiana and Panama) of the harlequin beetle-riding pseudoscorpion, Cordylochernes scorpioides. This highly sexually dimorphic pseudoscorpion is currently described as a single species, ranging throughout the Neotropics. Our morphometric analyses detected minimal differentiation between the two populations in all nine external morphological characters measured, including sexually dimorphic traits in males. Only in traits of the spermatophore was there any appreciable level of differentiation. Behavior differentiation and prezygotic reproductive isolation were also limited: 78.3% of males successfully transferred sperm to “foreign” females, and in 63.9% of these cases, females' eggs were successfully fertilized. By contrast, extensive divergence existed in two of nine electrophoretic loci, including an essentially fixed-allele difference at the Ldh locus. Most significantly, postzygotic reproductive isolation was complete, with heteropopulation zygotes invariably aborting early in development. These results strongly suggest that the two populations are, in fact, sibling species, a conclusion supported by our recently published findings on their marked divergence in minisatellite DNA. How can such interpopulation homogeneity in male sexually dimorphic traits exist in the face of strong genetic divergence? We propose that sexual selection, oscillating between favoring small and then large males, maintains such high levels of male variability within each population that it has obscured a speciation event in which genetic divergence and postzygotic incompatibility have clearly outpaced the evolution of prezygotic reproductive isolation.     

Metabolic shift analysis at high cell densities

Abstract: In high cell density cultures it is virtually inevitable that the environment to which the cells are exposed is heterogeneous. Thus, with suspended cultures, individual cells are subject to temporal changes in their environment whereas with aggregated or immobilized cells, the culture can be considered as being formed by a number of subpopulations, each with its own environmental characteristics. In addition, in a high cell density environment, high concentrations of end products may negatively influence the growth rate. This may result in the selection of organisms with an altered metabolic behaviour or with a decreased sensitivity to the adverse effects of the product. We discuss the consequences of this heterogeneity with regard to carbon source metabolism in view of the ability of many bacterial species to adapt to environmental conditions. Selection of variant organisms was found to occur with Clostridium butyricum when grown for a prolonged time in a medium containing approx. I-50 mM glucose. In contrast to the original strain, these variants could sustain a high maximal growth rate in the presence of butyric acid. In addition, they had acquired the capacity to spontaneously form aggregates and were able to carry out a completely solventogenic fermentation. Heterogeneous metabolic activity in aggregated cells is demonstrated with cultures of Lactobacillus laevolacticus , an aggregateforming lactic acid bacterium that converts glucose completely to o-lactate. By using microelectrodes, we show that the fraction of metabolically active cells decreases with increasing aggregate size: in larger aggregates steep pH gradients occur with the effect that only the outer layer of the aggregate is metabolically active, i.e. contributes to lactic acid formation, whereas with smaller aggregates all cells remain active. As a result, the net specific lactic acid production rate of the population as a whole is not invariably increased with increased aggregate size.     

Wild pearl millet population (Pennisetum glaucum,Poaceae) integrity in agricultural Sahelian areas. An example from Keita (Niger)

Morphometric and isozymic analyses of adjacent cultivated and spontaneous populations of pearl millet in Niger revealed in the field a unique continuous distribution of phenotypes ranging from the most cultivated one to a typical cultivated × wild hybrid. The natural population was subdivided into a major wild group and a hybrid wild × cultivated group. Cultivated millet displayed an equilibrium state between recombined domesticated and wild genes. The natural population, in spite of a high rate of immigration by pollen from cultivated plants, retained its structure by apparently reproducing itself exclusively from the major wild group.     

THE POPULATION DYNAMICS OF NORTHERN SEA LIONS, 1975-1985

Abstract: Populations of northern sea lions ( Eumetopias jubatus ) in the vicinity of Marmot Island, Alaska declined during 1975–1985 at about 5% per year (Merrick et al. 1987). The cause of this decline is not known. A life table for the northern sea lion was calculated assuming that life spans follow a Weibull distribution. Samples of northern sea lions taken in the vicinity of Marmot Island, Alaska during 1975–1978 and 1985–1986 indicate that the average age of females older than 3 yr increased about 1.55 yr (SD = 0.35 yr) while the population was declining at about 5% per year. Fecundity rates decreased by 10% over the same period, but the decrease was not statistically significant (Calkins and Goodwin 1988). Possible causes of the population decline and the change in age structure were examined by writing the Leslie matrix population equation in terms of changes in juvenile and adult survival rates and fecundity, and examining the short–term behavior of the trajectories of the average age of adult females, total number of females, and total number of pups with respect to those changes in the vital parameters. From the observed rate of declines of adults and the changes in average age of adult females and fecundity, estimates of the changes in adult and juvenile survival were calculated; estimates of the standard deviations of these changes were estimated via a bootstrap procedure. One purpose of this exercise is to aid in setting priorities for research for determining the cause of the decline. An explanation for the observed declines in numbers of adult sea lions consistent with the observed fecundity rates, a rate of decrease of 5% in the number of adults, and the corresponding increase in average age (of females age 3 yr and older) was a 10%–20% decrease in the survival of juveniles (age 0-3 yr) coupled with an insignificant change in adult survival (0.03%, SD = 1%).     

Immunological determination of digestive rates in the syntopic scorpions Urodacus armatus Pocock and Urodacus novaehollandiae Peters

Scorpions have lengthy periods of inactivity, which may be caused either by a long physiological digestion time or predator avoidance or both. We used a quantitative immunological assay to monitor the amount of prey antigen remaining in the hepatopancreas of Urodacus armatus and U. novaehollandiae for 32 days after a meal to test the hypothesis of a long digestion time. In both species, prey antigen concentration in the hepatopancreas increased between 0.25 and 0.5 days after feeding, probably because of incomplete transfer of prey proteins from the large preoral cavity to the hepatopancreas at this stage of digestion. This was followed by a rapid decline, and by 3 days after feeding less than 25% of ingested antigen remained in each species. Small prey residues could still be detected up to 32 days. Prey decay curves in both species were best fitted by a log-log transformation, and were not significantly different. Since digestion is essentially complete after 3 days, it appears that longer periods of post-feeding inactivity are for predator avoidance, not digestion. However, the 32-day detection period exceeds those reported for other invertebrates, and suggests an unusual digestive physiology in scorptions.     

Spatial pattern,regeneration and growth rates of Brachystegia spiciformis and Julbernardia globiflora

Past work on tree-to-tree interactions in semi-arid savannas, through the use of nearest neighbour analysis, has shown both a regular dispersion pattern and a positive correlation between the size of a tree and the distance to its nearest neighbour. From these results, the importance of competition has been inferred. In the present study, tree-to-tree interactions in a mesic savanna woodland at Marondera, Zimbabwe, dominated by relatively small-sized individuals of Brachystegia spiciformis and Julbernardia globiflora, were examined. Results show that the regular dispersion pattern is uncommon; that there are few significant positive correlations between size of individual and distance to nearest neighbour; and that growth rate is not consistently correlated with size and distance of nearest neighbour. Results of a study of regeneration relative to canopy cover suggest a possible mechanism for these findings: unlike semi-arid savannas, young plants often grow in under-canopy environments so that early spacing of young trees does not occur. Significant positive correlations between the size of a tree and the distance to its nearest neighbour were found in a stand of larger, presumably older, trees, suggesting that such a relationship only develops in mesic miombo woodland through a thinning process as the trees mature. Familial clumping, which is common in the study area, may mask spatial patterns.Abbreviations RBAI Relative basal area increment     

温度对麦长管蚜种群存活率的特征参数的影响

在测定麦长管蚜种群存活率基础上,本提出反S型曲线模型来描述种群存活率,探讨了麦长管蚜种群存活率过程变化的特征参数,并得到不同的特征值．