Drift In Relation Research Articles

Gone with the flow: Whitefish egg drift in relation to substrate coarseness under a range of flow velocities.

Most major rivers in Europe have been dammed for hydropower or other purposes. Such river alterations have decimated natural reproduction of many migratory fish species, including that of the anadromous European whitefish, Coregonus lavaretus, which is now maintained by extensive stocking programmes. In addition to stocking, limited natural reproduction may occur downstream of dams, where peak flow bouts from the dams threaten to flush the eggs into unsuitable habitats. Here, we assessed the effects of water flow velocity and substrate coarseness on downstream drift of whitefish eggs under laboratory conditions. The experiment's two different gravel substrates retained eggs better than cobble or sand substrates; the water velocity needed for notable egg drift was higher for the gravel substrates. The results indicate that egg drift is one of the factors that should be considered when evaluating the effects of hydropower plant operations. Moreover, measures mitigating the effects of the artificial flow regimes should incorporate the type and coarseness of the riverbed's substrate.

Compensation for wind drift in the nocturnally migrating Song Thrushes in relation to altitude and wind

Compensation for wind drift in relation to the side-wind velocity and altitude was investigated in Song Thrushes during autumn migration. The birds were recorded at night flying above the prominent leading line of a marine spit which coincided with the general direction of their migration. Among the large size passerine species, Song Thrushes were identified by a combination of five flight characteristics typical only of this species during particular periods of autumn. The thrushes showed different reactions to the crosswinds: complete and partial compensation for the displacement and drift. Under normal visibility, the completeness of compensation depended both on the velocity of the side-wind and altitude. The degree of compensation achieved was reduced with an increase of altitude, regardless of the wind. Under the same wind conditions, the angle of drift (the angle between the track direction and the leading line) increased with altitude, but the number of birds that compensated for drift decreased. On average, at heights below 300 m agl, the thrushes were capable of compensating completely for moderate winds; between 300 and 600 m agl compensation was partial; but above 600 m the birds drifted completely. Birds of the same species flying above the same terrain may demonstrate different reactions to the same crosswind depending on altitude. Meanwhile, flight tracks gradually deviated from the leading line with an increase in altitude, the headings of the birds got closer to the general migratory direction. It is more likely that the birds control displacement using the visual flow regulation principle by the angular velocity of the landmarks below them running aside in relation to their flight direction, which is inversely proportional to the altitude. Low flying thrushes promptly reacted to the shifting of the leading line of the spit with an average angular velocity of more than 0.8°/s perpendicular to the direction of flight and compensated completely for drift. Shifting of the leading line with an angular velocity of less than 0.4°/s, the high flying birds did not seem to notice or did not try to compensate for displacement deliberately.

Spatiotemporal Dynamics of Genetic Variation in the Iberian Lynx along Its Path to Extinction Reconstructed with Ancient DNA.

There is the tendency to assume that endangered species have been both genetically and demographically healthier in the past, so that any genetic erosion observed today was caused by their recent decline. The Iberian lynx (Lynx pardinus) suffered a dramatic and continuous decline during the 20th century, and now shows extremely low genome- and species-wide genetic diversity among other signs of genomic erosion. We analyze ancient (N = 10), historical (N = 245), and contemporary (N = 172) samples with microsatellite and mitogenome data to reconstruct the species' demography and investigate patterns of genetic variation across space and time. Iberian lynx populations transitioned from low but significantly higher genetic diversity than today and shallow geographical differentiation millennia ago, through a structured metapopulation with varying levels of diversity during the last centuries, to two extremely genetically depauperate and differentiated remnant populations by 2002. The historical subpopulations show varying extents of genetic drift in relation to their recent size and time in isolation, but these do not predict whether the populations persisted or went finally extinct. In conclusion, current genetic patterns were mainly shaped by genetic drift, supporting the current admixture of the two genetic pools and calling for a comprehensive genetic management of the ongoing conservation program. This study illustrates how a retrospective analysis of demographic and genetic patterns of endangered species can shed light onto their evolutionary history and this, in turn, can inform conservation actions.

Structure and properties of the egg mass of the ommastrephid squid Todarodes pacificus.

The Japanese flying squid, Todarodes pacificus, is thought to spawn neutrally buoyant egg masses that retain a specific location in the water column by floating at the interface between water layers of slightly different densities. It is important to understand the physical process that determines the vertical distribution of the egg masses to predict their horizontal drift in relation to embryo survival and subsequent recruitment. Here, mesocosm experiments were conducted in a 300 m3 tank by creating a thermally stratified (17–22°C) water column to obtain egg masses. A cage net methodology was developed to sustain egg masses for detailed observation. We measured the density of the egg masses of T. pacificus, and used this information to infer the vertical distribution patterns of the egg masses at the spawning grounds (Tsushima Strait, Japan). When measured separately, the density of the outer jelly of each egg mass was 2.7 σ units higher than that of the surrounding water. The outer jelly and the specific gravity of embedded individual eggs (~1.10) cause the egg masses to have very slight negative buoyancy relative to the water in which they are formed. Analysis of the vertical profile of the spawning ground showed that water density (σθ) increased sharply at ~30 m depth; thus, egg masses might settle above the pycnocline layer. In conclusion, we suggest that T. pacificus egg masses might retain their location in the water column by floating at the interface between water layers of slightly different densities, which happen to be above the pycnocline layer (actual depth varies seasonally/annually) in the Tsushima Strait between Korea and Japan.

An experimental test of sub-hourly changes in macroinvertebrate drift density associated with hydropeaking in a regulated river

ABSTRACTWe investigated the response of the invertebrate drift to the regulated peaking Magpie River under experimental sub-hourly changes in flow conditions. Benthic invertebrates were also quantified upstream of invertebrate drift sampling sites in order to examine the propensity of nearby invertebrates to drift under hydropeaking cycles. To provide an understanding of the natural patterns of drift in relation to changes in discharge, the proximate natural Batchawana River was similarly sampled. Increased discharge was associated with higher drift densities and greater particulate organic matter in the Magpie River. At the family level, multiple drift responses were observed suggesting that ecological traits and behaviors drive drift response to changes in discharge. The drift densities of some invertebrates varied in proportion to discharge; whereas, other invertebrates in the drift decreased to pre-peaking levels shortly after peak discharge was reached, with some invertebrates exhibiting a secondary peak in density as discharge returned to pre-peaking levels. Although average drift densities were comparable between the two rivers, drift density on the natural Batchawana River was much more stable than that of the Magpie River. Changes in drift density associated with flow manipulation likely impact feeding patterns and behavior of drift feeding fish.

An intercomparison of Arctic ice drift products to deduce uncertainty estimates

AbstractAn intercomparison of four low‐resolution remotely sensed ice‐drift products in the Arctic Ocean is presented. The purpose of the study is to examine the uncertainty in space and time of these different drift products. The comparison is based on monthly mean ice drifts from October 2002 to December 2006. The ice drifts were also compared with available buoy data. The result shows that the differences of the drift vectors are not spatially uniform, but are covariant with ice concentration and thickness. In high (low) ice‐concentration areas, the differences are small (large), and in thick (thin) ice‐thickness areas, the differences are small (large). A comparison with the drift deduced from buoys reveals that the error of the drift speed depends on the magnitude of the drift speed: larger drift speeds have larger errors. Based on the intercomparison of the products and comparison with buoy data, uncertainties of the monthly mean drift are estimated. The estimated uncertainty maps reasonably reflect the difference between the products in relation to ice concentration and the bias from the buoy drift in relation to drift speed. Examinations of distinctive features of Arctic sea ice motion demonstrate that the transpolar drift speed differs among the products by 13% (0.32 cm s−1) on average, and ice drift curl in the Amerasian Basin differs by up to 24% (3.3 × 104 m2 s−1). These uncertainties should be taken into account if these products are used, particularly for model validation and data assimilation within the Arctic.

COLLABORATIVE DATA STREAM MINING IN UBIQUITOUS ENVIRONMENTS USING DYNAMIC CLASSIFIER SELECTION

In ubiquitous data stream mining, different devices often aim to learn concepts that are similar to some extent. In many applications, such as spam filtering or news recommendation, the data stream underlying concept (e.g., interesting mail/news) is likely to change over time. Therefore, the resultant model must be continuously adapted to such changes. This paper presents a novel Collaborative Data Stream Mining (Coll-Stream) approach that explores the similarities in the knowledge available from other devices to improve local classification accuracy. Coll-Stream integrates the community knowledge using an ensemble method where the classifiers are selected and weighted based on their local accuracy for different partitions of the feature space. We evaluate Coll-Stream classification accuracy in situations with concept drift, noise, partition granularity and concept similarity in relation to the local underlying concept. The experimental results show that Coll-Stream resultant model achieves stability and accuracy in a variety of situations using both synthetic and real-world datasets.

Deriva Originada por Dos Sistemas de Aplicación de Plaguicidas en Huertos de Cítricos

Drift is the most important factor contributing to environmental pollution during the spraying of pesticides. The objective of this work was to evaluate the drift caused by two pesticide spraying systems in citrus orchards. The experiment was conducted in an orchard of Valencia orange, located at the Sertaozinho farm, Escola Superior de Agricultura Luiz de Queiroz, Piracicaba-SP. An air-assisted sprayer with axial fan was used. One system used a prototype for terrestrial application with a rotary disc atomizer that is widely used in agricultural aviation, and the other system used hollow cone tip hydraulics. For treatment of the trees, the insecticide Metidation was used at the dose of 180 g per hectare. To assess runoff losses, glass slides and water-sensitive papers were placed on the ground below the canopy of the trees, while for the evaluation of drift losses the collection surfaces were placed on wooden towers at different heights above the ground. After spraying, the water-sensitive papers were collected and analyzed using a computerized image analysis system, e-Sprinkle (EMBRAPA, Sao Paulo, Brazil), and the glass slide analyzed by the gas chromatography technique. The application system equipped with rotary disc atomizers resulted in a lower pesticide drift in relation to the system that used hydraulic tips. Factors influencing drift are discussed in the present work.

Geometry and Properties of the Excavation-Induced Fractures at the Meuse/Haute-Marne URL Drifts

The performance and safety assessment and technology demonstration are the main objectives of research programs for feasibility studies for deep geological repository of radioactive waste. In this context, the French national radioactive waste management agency (ANDRA) started to develop the Meuse/Haute-Marne underground research laboratory (URL) at Bure, nearly 300 km East of Paris. The host formation consists of a Callovo-Oxfordian claystone found between 420 and 550 m below ground, overlain and underlain by poorly permeable carbonate formations. One of the major concerns related to performance assessment is the excavation-induced fractures which may provide groundwater preferential pathway for radionuclide migration. The extent of the fractures possibly acting significantly in the radionuclide migration is known as the excavation damaged zone (EDZ). A scientific study on the EDZ characterization is performed at the main level of the URL (−490 m). Observations such as structural analysis on core, overcored resin-filled samples, geological survey of the drift face and sidewalls, were made to better understand the fracture network characteristics, extent and its generation. Pulse and constant head test hydraulic conductivity measurements were performed with multi packer system to estimate the extension of the EDZ hydraulic conductivity. Fractures exhibited high transmissivity near the excavation walls, but farther from the exaction walls, shear fractures showed hydraulic conductivity values reflecting values of undisturbed or slightly disturbed rock mass condition. The major findings in terms of geometry and properties of excavation-induced fractures are discussed in detail in this paper. For example, it is observed that the shape of the fracture network depends on the orientation of the drift in relation to the orientation of the in situ stress field.

Combined effect of applied equipment and formulation of pesticide on spray and dust drift in relation to harmful effects for some non-target organisms

In Dakalt village, Kafrelshikh governorate, Egypt, field and laboratory studies were conducted in summer 2008 to investigate drift of Cyanophos applied on cotton field onto adjacent maize plants. Drift deposits of Cyanophos was determined as μg/kg maize leaves. The determinations were conducted on leaves of maize grown at various distances from the edge of the treated cotton fields (i.e. 5, 8, 14. up to 51 m). Distances traveled by drift in the 1st spray were farther than those of the 2nd spray. This result could be easily explained on the basis that wind speed was higher in 1st spray (wind speeds were 3.8 and 2.6 km/hr during time of application in the 1st and 2nd sprays, respectively). The farthest distance within maize field reached by drift was observed for the dust application followed by micron ULVA and mistblower spraying (the distances were 26, 35 and 44 m in the first spray when using the mistblower, micron ULVA and the mistblower as a duster, respectively). The corresponding values of drift deposits were 18.5, 13.6 and 28.4 μg/kg maize leaves, respectively. The potential drift emitted by micron ULVA compared with that of mistblower may be due to the smaller droplets of the former sprayer. Drift of Cyanophos released by each of the tested equipment caused 100% mortality of fish or honeybees placed at the distances 5 and 8 m from the edge of treated cotton field. For the dust application, 100% mortality was observed at longer distances (14 and 17 m). The study suggests that buffer zones (no spray zones) have to be established downwind of the treated field to avoid environmental contamination due to off-target deposition of pesticide drift. The model and specifications of these zones depend variably on quality of spray, release height, wind speed and other factors. Other measures of drift mitigation have to be considered.

CONNECTIONS BETWEEN LOCAL AND GLOBAL TURBULENCE IN ACCRETION DISKS

We analyze a suite of global magnetohydrodynamic (MHD) accretion disk simulations in order to determine whether scaling laws for turbulence driven by the magnetorotational instability, discovered via local shearing box studies, are globally robust. The simulations model geometrically-thin disks with zero net magnetic flux and no explicit resistivity or viscosity. We show that the local Maxwell stress is correlated with the self-generated local vertical magnetic field in a manner that is similar to that found in local simulations. Moreover, local patches of vertical field are strong enough to stimulate and control the strength of angular momentum transport across much of the disk. We demonstrate the importance of magnetic linkages (through the low-density corona) between different regions of the disk in determining the local field, and suggest a new convergence requirement for global simulations -- the vertical extent of the corona must be fully captured and resolved. Finally, we examine the temporal convergence of the average stress, and show that an initial long-term secular drift in the local flux-stress relation dies away on a time scale that is consistent with turbulent mixing of the initial magnetic field.

Drift benthos relationships in the seasonal colonization dynamics of alpine streams

We examined seasonal patterns in macroinvertebrate benthic abundance and drift in relation to their colonization dynamics in five contrasting streams. The streams, within an alpine glacial flood plain, differed over an annual cycle in flow permanence and surface hydrologic connectivity with the main floodplain channel. Study streams included an upper and lower main channel, a side-channel seasonally connected upstream to the main channel, and an intermittent side-slope tributary and permanent springbrook having no upstream surface connection to the main channel. Flow in the main and side-channels was influenced seasonally by upstream glaciers, whereas the tributary and springbrook had no direct glacial input. Macroinvertebrate assemblages differed markedly among streams. Density, biomass, and taxonomic richness typically were lowest in the upper main channel and highest in the springbrook. Seasonal changes in assemblage composition were more pronounced in the main and side-channels than in the tributary and springbrook. In the upper main channel, larvae colonizing cages filled with natural substrata reflected benthic assemblage structure regardless of season and colonizing taxa exhibited a high propensity to drift. In contrast, larval colonists differed significantly from benthic assemblages in the tributary and springbrook, and colonists were mostly from drift. The lower main and side-channels displayed high seasonality in colonization dynamics, being related to their hydrologic connectivity to other streams that may have provided larval colonists. Benthos to drift abundance ratios suggested that the upper main channel represented a relatively closed system with little interaction among other streams in the flood plain, whereas the springbrook and tributary seasonally subsidized larval colonists to downstream channels.

Plasticity and tuning of the time course of analog persistent firing in a neural integrator.

In a companion paper, we reported that the goldfish oculomotor neural integrator could be trained to instability or leak by rotating the visual surround with a velocity proportional to +/- horizontal eye position, respectively. Here we analyze changes in the firing rate behavior of neurons in area I in the caudal brainstem, a central component of the oculomotor neural integrator. Persistent firing could be detuned to instability and leak, respectively, along with fixation behavior. Prolonged training could reduce the time constant of persistent firing of some cells by more than an order of magnitude, to <1 s. Normal visual feedback gradually retuned persistent firing of integrator neurons toward stability, along with fixation behavior. In animals with unstable fixations, approximately half of the eye position-related cells had upward or unstable firing rate drift. In animals with leaky fixations, two-thirds of the eye position-related cells showed leaky firing drift. The remaining eye position-related cells, generally those with lower eye position thresholds, showed a more complex pattern of history-dependent/predictive firing rate drift in relation to eye drift. These complex drift cells often showed a drop in maximum persistent firing rate after training to leak. Despite this diversity, firing drift and the degree of instability or leak in firing rates were broadly correlated with fixation performance. The presence, strength, and reversibility of this plasticity demonstrate that, in this system, visual feedback plays a vital role in gradually tuning the time course of persistent neural firing.

Avian Sexual Dichromatism in Relation to Phylogeny and Ecology

▪ Abstract The extent and diversity of sexual dichromatism in birds is thought to be due to the intensity of current sexual selection on the plumage ornamentation of males and females. This view leads to an expectation of concordance between ecological conditions and sexual dichromatism. Yet, because expression of dichromatism is the result of not only current selection, but also historical patterns of development, function, and selection, the concordance between ecology and current sexual dichromatism is not straightforward. Recent studies have revealed a number of trends in the evolution of avian sexual ornamentation that seem contrary to what is expected if current sexual selection is the primary force shaping dichromatism. For example, change in sexual dichromatism is often the result of evolutionary changes in female rather than male ornamentation. Moreover, sexual dichromatism is often an ancestral rather than a derived state; current expression of dichromatism is frequently the result of selection for lesser ornamentation in one sex and not for ornament elaboration. Loss and gain of sexual ornamentation sometimes precedes changes in preference for sexual ornamentation, and sexual ornaments can have high evolutionary lability despite their developmental and functional complexity. These findings emphasize that phylogenetic reconstructions must play a central role in attempts to understand the function and evolution of sexual dichromatism. With a historical perspective, one can test the relative importance of direct selection, indirect selection, and drift in relation to changes of sexual dichromatism. If sexual selection is invoked, the mechanisms of sexual selection can be explored by examining the concordance between the elaboration of ornamentation and the preferences for ornamentation across species and by tracing phylogenetic trajectories of sexual ornaments. Finally, placing physiological, genetic, and developmental mechanisms of sexual ornamentation into such a phylogenetic framework will enable greater inference about the past evolution and current function of sexual dichromatism in birds.

Seasonal and diel patterns of invertebrate drift in different alpine stream types

SUMMARY1. We examined the seasonal and diel patterns of invertebrate drift in relation to seston and various habitat characteristics in two each of four different kinds of alpine streams [rhithral (snow‐fed) lake outlets, rhithral streams, kryal (glacial‐fed) lake outlets and kryal streams]. Samples were collected at four times of the day (dawn, midday, dusk and midnight) during three seasons (spring, summer and autumn).2. Habitat characteristics differed mainly between rhithral and kryal sites, with the latter having higher discharge and turbidity, lower water temperature, and higher concentrations of ammonium, and particulate and soluble reactive phosphorus. Seasonality in habitat characteristics was most pronounced for kryal streams with autumn samples being more similar to rhithral sites.3. The concentration of seston was lowest in the glacial‐influenced lake outlets and slightly higher in the stream sites; no seasonal or diel patterns were evident.4. The density of drifting invertebrates averaged less than 100 m−3 and was lowest (&lt;10 m−3) at three of the four kryal sites. Taxon richness and diversity were lowest at rhithral lake outlets. Chironomidae dominated the drift as well as benthic communities and &lt;30% of benthic taxa identified were found in the drift.5. Drifting invertebrates showed no consistent seasonal pattern. However, density tended to be highest in spring at rhithral sites and in autumn at kryal sites. No diel periodicity in drift density was found at any site and the lack of diel pattern may be a general feature of high altitude streams.6. Glacially influenced habitat parameters were a major factor affecting drift in these alpine streams, whereas no clear differences were observed between streams and lake outlets. Our findings indicate that invertebrate drift in alpine streams is primarily influenced by abiotic factors, and therefore, substantially differs from patterns observed at lower altitude.

Macroinvertebrate drift in Amazon streams in relation to riparian forest cover and fish fauna

Macroinvertebrate drift was studied in 12 sites in 9 lowland streams (400m), three mid-altitude streams (2100 m) and three high-altitude streams (3800 m) in Ecuador. The purposes of our study was 1) to examine levels and periodicity of drift in relation to altitude, 2) to examine levels and periodicity of drift in relation to environmental site characteristics and 3) to examine the effect of deforestation and specific fish families on levels and diel periodicity of drift in the lowland streams. At all altitudes, day drift of terrestrial invertebrates was equal to or greater than that of aquatic invertebrates, while aquatic invertebrates always dominated drift at night. Mean day drift density of aquatic invertebrates was the same at the two high altitudes, while it was lower in the lowland streams. In contrast, night drift density was higher in lowland stream compared to the two highland stream groups. In all lowland streams drift of aquatic invertebrates was higher at night than at daytime (night: day drift ratio 2-45, mean: 10), while it was largely aperiodic in both groups of highland streams. In all lowland streams, drift was dominated by insects (mean 99.6 %). No significant correlations were found between drift density, drift % or periodicity and riparian forest cover, benthic macroinvertebrate densities or any other environmental site characteristic in the lowland streams. However, path analyses indicated that riparian forest had a significantly negative indirect effect on night and day drift densities, because an increase in riparian cover was associated with a decline in the abundance of fish and benthos. Species richness, density and biomass of fish were directly correlated with invertebrate drift. These parameters had no effect on total day or night drift densities or drift %, but were positively correlated with the night: day drift ratio. The night: day drift ratios were more closely correlated with the species richness of Loricariidae catfish than to any other fish family. Loricariidae are night active, benthic herbivorous-detritivorous scrapers of biofilm, and their presumed effect on drift periodicity is due to physical disturbance of benthic invertebrates during the night, and not necessarily to a behavioural response from benthic invertebrates. Drift of the dominant invertebrate families (Leptophlebiidae, Baetidae, Tricorythidae, Hydropsychidae and Chironomidae) varied among localities, but night: day drift ratios of all families were positively correlated with species richness of fish, although this pattern was only significant for Leptophlebiidae and Baetidae.

MORPHOMETRIC DIFFERENTIATION AMONG EXPERIMENTAL LINES OF THE HOUSEFLY IN RELATION TO A BOTTLENECK

Differentiation in morphometric traits among experimental populations of the housefly subjected to an experimental bottleneck was examined for replicate lines founded with one, four or 16 pairs of flies. Differentiation among lines within a bottleneck size was significantly greater than predicted by drift in relation to the additive genetic variation for these traits within the founding population. Two models of nonadditive genetic variance were investigated to interpret these results, one involving dominance of allelic effects within loci and another incorporating multiplicative epistasis. Both models generated more variation among lines as a direct result of sampling during the bottleneck than predicted by a model with additive gene action. The pattern of differentiation among our experimental lines in relation to these models conformed more to the model incorporating epistasis. Nevertheless, it may be difficult to distinguish differentiation among lines occurring during a bottleneck as a result of nonadditive gene action from that caused by diversifying selection among lines after the bottleneck.

To spritz or not to spritz: The doubtful value of aimless iontophoresis

Publisher Summary This chapter discusses the different aspects of iontophoresis. The response of a neuron to a drug applied by iontophoresis can be determined by the changes in any parameter of cell activity. For cells with spontaneous electrical activity, changes in discharge pattern can be determined from interspike interval histograms and poststimulus time histograms can be used when test cells can be influenced by defined, isolated, monosynaptic inputs. In each case, accuracy of response measurements requires continuous detection of distinguishable action potentials of a constant spike height. With the recording barrel of a multibarrel electrode used in iontophoresis, extracellular spikes often have low signal to noise ratios and more than one cell can be recorded simultaneously. These signals can be improved electronically with gate or window discriminators provided that spike size remains constant. In iontophoretic tests, spike size can change by drift in the spatial relation between electrode and cell, by interaction between ejection or retention currents and the spike generating mechanisms of the cell, or when the drug to be tested has membrane anesthetizing properties. Noise levels are frequently increased during iontophoretic tests, especially when the drug solution has a high electrical impedance from low solubility or low ionization constants.

Polyallelic random drift in relation to evolution.

The number of alleles that are maintained at a locus in a population of givenl size by a given mutation rate is important in evolutionary theory. The possible number of alleles is enormous. At a locus consisting of only 1000 nucleotide pairs, each with four alternatives, the number differing from the type gene by a single substitution is 3000 and by two substitutions, nearly 4.5 X 106. The total number from substitutions alone is 41000. The actual number present at any given time in a population of size N obviously cannot exceed 2N and may be expected to be very much less. It is instructive to determine the steady-state distribution of frequencies of alleles under the simplifying assumption that all alleles, except perhaps one or more specified ones, have the same properties. The number of such similar alleles present at any time is the reciprocal of the mean frequency, multiplied by the portion of the total frequency which they constitute. The composition of this array is continually changing with turnover 2Nu where u is the mutation rate. The first attempt at such estimates was for the case of self-incompatibility alleles (Wright') in which it was known that rather large numbers actually occur in very small isolated populations.2 Because of the peculiar mode of selection-inhibition of any pollen grain that carries the same allele as either of those in the cells of the style-any novel mutation is subject to maximum favorable selection. There is an equilibrium frequency toward which gene frequency tends to move from either direction. Unfortunately this equilibrium frequency is not constant. It shifts especially with changes in the number of alleles. It was recognized that only an approximate solution could be obtained. Approximate relations between number of alleles (n), effective size of population (N), and rate of mutation from any allele (u) were presented which seemed adequate for most biological purposes. Discussion of the subject3'7 has introduced rather more confusion than clarification, but some improvement of the formulas has emerged, and more is probably possible. The present paper will deal with simpler cases.

Synchronisation of a p.c.m. network using digital techniques

In a network of interconnected digital p.c.m. telephone exchanges, all the signals should be virtually in synchronism to facilitate interconnections between channels on different routes. However, to maintain the security of the system, it is also desirable for each station to have an independent master oscillator. There are several possible methods for effectively synchronising the signals in such a system. Apart from the effect of frequency differences, synchronism is also affected by changes in the propagation delays of the interconnecting junctions, caused by temperature variations. A system is described in which the phase of an incoming signal is allowed to drift in relation to the signal generated locally, until a given limit is reached. The limit may conveniently be related to the amount of storage provided for temperature compensation of the junction delay. When the limit is reached, a digit is either added to, or omitted from, one of the signals during a part of the signal not carrying intelligence, such as a frame-synchronising period, so as to reduce the degree of asynchronism. This system adds some complexity to both the exchange master timing circuits and the received-signal timing circuits, and also complicates the switching of calls through the exchange. The time relationships between the various signals in the network can be deduced mathematically, allowing the behaviour of a network with a given set of initial conditions to be computed. Paralysis of the system can be avoided by a proper choice of the amount of storage.