The Dispositionalist Conception of Laws

This paper sketches a dispositionalist conception of laws and shows how the dispositionalist should respond to certain objections. The view that properties are essentially dispositional is able to provide an account of laws that avoids the problems that face the two views of laws (the regularity and the contingent nomic necessitation views) that regard properties as categorical and laws as contingent. I discuss and reject the objections that (i) this view makes laws necessary whereas they are contingent; (ii) this view cannot account for certain kinds of laws of nature and their properties.     

Young chondrules in CB chondrites from a giant impact in the early Solar System

Chondrules, which are the major constituent of chondritic meteorites, are believed to have formed during brief, localized, repetitive melting of dust (probably caused by shock waves) in the protoplanetary disk around the early Sun. The ages of primitive chondrules in chondritic meteorites indicate that their formation started shortly after that of the calcium-aluminium-rich inclusions (4,567.2 +/- 0.7 Myr ago) and lasted for about 3 Myr, which is consistent with the dissipation timescale for protoplanetary disks around young solar-mass stars. Here we report the 207Pb-206Pb ages of chondrules in the metal-rich CB (Bencubbin-like) carbonaceous chondrites Gujba (4,562.7 +/- 0.5 Myr) and Hammadah al Hamra 237 (4,562.8 +/- 0.9 Myr), which formed during a single-stage, highly energetic event. Both the relatively young ages and the single-stage formation of the CB chondrules are inconsistent with formation during a nebular shock wave. We conclude that chondrules and metal grains in the CB chondrites formed from a vapour-melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated. These findings therefore provide evidence for planet-sized objects in the earliest asteroid belt, as required by current numerical simulations of planet formation in the inner Solar System.     

Doping semiconductor nanocrystals

Doping--the intentional introduction of impurities into a material--is fundamental to controlling the properties of bulk semiconductors. This has stimulated similar efforts to dope semiconductor nanocrystals. Despite some successes, many of these efforts have failed, for reasons that remain unclear. For example, Mn can be incorporated into nanocrystals of CdS and ZnSe (refs 7-9), but not into CdSe (ref. 12)--despite comparable bulk solubilities of near 50 per cent. These difficulties, which have hindered development of new nanocrystalline materials, are often attributed to 'self-purification', an allegedly intrinsic mechanism whereby impurities are expelled. Here we show instead that the underlying mechanism that controls doping is the initial adsorption of impurities on the nanocrystal surface during growth. We find that adsorption--and therefore doping efficiency--is determined by three main factors: surface morphology, nanocrystal shape, and surfactants in the growth solution. Calculated Mn adsorption energies and equilibrium shapes for several nanocrystals lead to specific doping predictions. These are confirmed by measuring how the Mn concentration in ZnSe varies with nanocrystal size and shape. Finally, we use our predictions to incorporate Mn into previously undopable CdSe nanocrystals. This success establishes that earlier difficulties with doping are not intrinsic, and suggests that a variety of doped nanocrystals--for applications from solar cells to spintronics--can be anticipated.     

Diatom carbon export enhanced by silicate upwelling in the northeast Atlantic

Diatoms are unicellular or chain-forming phytoplankton that use silicon (Si) in cell wall construction. Their survival during periods of apparent nutrient exhaustion enhances carbon sequestration in frontal regions of the northern North Atlantic. These regions may therefore have a more important role in the 'biological pump' than they have previously been attributed, but how this is achieved is unknown. Diatom growth depends on silicate availability, in addition to nitrate and phosphate, but northern Atlantic waters are richer in nitrate than silicate. Following the spring stratification, diatoms are the first phytoplankton to bloom. Once silicate is exhausted, diatom blooms subside in a major export event. Here we show that, with nitrate still available for new production, the diatom bloom is prolonged where there is a periodic supply of new silicate: specifically, diatoms thrive by 'mining' deep-water silicate brought to the surface by an unstable ocean front. The mechanism we present here is not limited to silicate fertilization; similar mechanisms could support nitrate-, phosphate- or iron-limited frontal regions in oceans elsewhere.     

Gene flow maintains a large genetic difference in clutch size at a small spatial scale

Understanding the capacity of natural populations to adapt to their local environment is a central topic in evolutionary biology. Phenotypic differences between populations may have a genetic basis, but showing that they reflect different adaptive optima requires the quantification of both gene flow and selection. Good empirical data are rare. Using data on a spatially structured island population of great tits (Parus major), we show here that a persistent difference in mean clutch size between two subpopulations only a few kilometres apart has a major genetic component. We also show that immigrants from outside the island carry genes for large clutches. But gene flow into one subpopulation is low, as a result of a low immigration rate together with strong selection against immigrant genes. This has allowed for adaptation to the island environment and the maintenance of small clutches. In the other area, however, higher gene flow prevents local adaptation and maintains larger clutches. We show that the observed small-scale genetic difference in clutch size is not due to divergent selection on the island, but to different levels of gene flow from outside the island. Our findings illustrate the large effect of immigration on the evolution of local adaptations and on genetic population structure.     

Direct observation of steps in rotation of the bacterial flagellar motor

The bacterial flagellar motor is a rotary molecular machine that rotates the helical filaments that propel many species of swimming bacteria. The rotor is a set of rings up to 45 nm in diameter in the cytoplasmic membrane; the stator contains about ten torque-generating units anchored to the cell wall at the perimeter of the rotor. The free-energy source for the motor is an inward-directed electrochemical gradient of ions across the cytoplasmic membrane, the protonmotive force or sodium-motive force for H+-driven and Na+-driven motors, respectively. Here we demonstrate a stepping motion of a Na+-driven chimaeric flagellar motor in Escherichia coli at low sodium-motive force and with controlled expression of a small number of torque-generating units. We observe 26 steps per revolution, which is consistent with the periodicity of the ring of FliG protein, the proposed site of torque generation on the rotor. Backwards steps despite the absence of the flagellar switching protein CheY indicate a small change in free energy per step, similar to that of a single ion transit.     

Pterosaur diversity and faunal turnover in Cretaceous terrestrial ecosystems in China

New specimens and an analysis of the Jehol pterosaur faunae of northeastern China show an unexpected diversity of flying reptile groups in terrestrial Cretaceous ecosystems. Here we report two new pterosaurs that are referred to European groups previously unknown in deposits of northeastern China. Feilongus youngi, from the Yixian Formation, is closely related to the Gallodactylidae and is distinguished by the presence of two independent sagittal crests and a protruding upper jaw. Nurhachius ignaciobritoi, from the Jiufotang Formation, has teeth formed by labiolingually compressed triangular crowns, only previously reported in Istiodactylus latidens from England. With these new discoveries, the Jehol pterosaurs show a wide range of groups including both primitive and derived forms that are not matched by any other deposit in the world. The discoveries also document the turnover of pterosaur faunae, with the primitive Anurognathidae and early archaeopterodactyloids being replaced by derived pterodactyloids. Furthermore, these deposits offer an opportunity to examine the interaction and competition between birds and pterosaurs--it indicates that the avian fauna during the Lower Cretaceous (and possibly most of the Mesozoic) dominated terrestrial, inland regions, whereas pterosaurs were more abundant in coastal areas.     

Circadian organization in reindeer

The light/dark cycle of day and night synchronizes an internal 'biological clock' that governs daily rhythms in behaviour, but this form of regulation is denied to polar animals for most of the year. Here we demonstrate that the continuous lighting conditions of summer and of winter at high latitudes cause a loss in daily rhythmic activity in reindeer living far above the Arctic Circle. This seasonal absence of circadian rhythmicity may be a ubiquitous trait among resident polar vertebrates.     

An expression profile for diagnosis of lymph node metastases from primary head and neck squamous cell carcinomas

Metastasis is the process by which cancers spread to distinct sites in the body. It is the principal cause of death in individuals suffering from cancer. For some types of cancer, early detection of metastasis at lymph nodes close to the site of the primary tumor is pivotal for appropriate treatment. Because it can be difficult to detect lymph node metastases reliably, many individuals currently receive inappropriate treatment. We show here that DNA microarray gene-expression profiling can detect lymph node metastases for primary head and neck squamous cell carcinomas that arise in the oral cavity and oropharynx. The predictor, established with an 82-tumor training set, outperforms current clinical diagnosis when independently validated. The 102 predictor genes offer unique insights into the processes underlying metastasis. The results show that the metastatic state can be deciphered from the primary tumor gene-expression pattern and that treatment can be substantially improved.