Despite the industrial implications and worldwide abundance of gas hydrates, the formation mechanism of these compounds remains poorly understood. We report direct molecular dynamics simulations of the spontaneous nucleation and growth of methane hydrate. The multiple-microsecond trajectories offer detailed insight into the process of hydrate nucleation. Cooperative organization is observed to lead to methane adsorption onto planar faces of water and the fluctuating formation and dissociation of early hydrate cages. The early cages are mostly face-sharing partial small cages, favoring structure II; however, larger cages subsequently appear as a result of steric constraints and thermodynamic preference for the structure I phase. The resulting structure after nucleation and growth is a combination of the two dominant types of hydrate crystals (structure I and structure II), which are linked by uncommon 5(12)6(3) cages that facilitate structure coexistence without an energetically unfavorable interface.

Some overharvested fish populations fail to recover even after considerable reductions in fishing pressure. The reasons are unclear but may involve genetic changes in life history traits that are detrimental to population growth when natural environmental factors prevail. We empirically modelled this process by subjecting populations of a harvested marine fish, the Atlantic silverside, to experimental size-biased fishing regimes over five generations and then measured correlated responses across multiple traits. Populations where large fish were selectively harvested (as in most fisheries) displayed substantial declines in fecundity, egg volume, larval size at hatch, larval viability, larval growth rates, food consumption rate and conversion efficiency, vertebral number, and willingness to forage. These genetically based changes in numerous traits generally reduce the capacity for population recovery.

Methane hydrate nucleation rates are reported from over 200 μs of molecular dynamics simulations across a range of thermodynamic conditions and varying degrees of methaneÀwater interfacial curvature. Calculated nucleation rates increase with aqueous phase methane concentration (X CH 4 ), consistent with experimental results. The effect of interfacial curvature on X CH 4 is quantified, with dissolved methane concentration increasing with the degree of curvature (i.e., the number of dimensions in which curvature exists). Nucleation rates are reported for system sizes of 3456 and 13 824 molecules (H 2 O + CH 4 ). Among the smaller simulation systems (which comprise the majority of the data), the calculated hydrate nucleation rates follow the same trend when plotted against X CH 4 regardless of whether the predominant contribution to the effective system pressure is the simulation barostat or the methaneÀwater interfacial curvature (YoungÀLaplace pressure). The incipient hydrate nuclei are destabilized in the immediate vicinity (∼ 1 nm) of the methaneÀwater interface, and the calculated nucleation rates for the larger simulation systems (in which the incipient hydrate solids are less affected by interfacial destabilization) fall above the trend observed in the smaller systems.

The ecological impacts of the indirect effects of predators are well established, but the evolutionary consequences are unknown. Predators often decrease prey density, which indirectly increases the resources available to surviving prey. This ecological effect could provide a link to evolution because it is generally assumed that resource availability influences life history evolution. Yet, predictions from theory that consider food availability are inconsistent, and evidence for an important role of resources in shaping life history evolution is absent. We compared life history traits in a Trinidadian killifish, Rivulus hartii, from fish communities that differ in predation intensity; predators are associated with lower population density and faster growth rates. To determine whether the indirect effects of predators influence evolutionary change, we reared second-generation-born fish under two food levels that approximated natural differences in resources between communities. Rivulus from sites with predators are younger and smaller at maturity. They have increased reproductive investment and produce many small eggs and smaller hatchlings. Such divergence is predicted as a direct effect of predation. We also found significant interactions between predator community and food level for age and size at maturity, fecundity, and egg size. These interactions, whereby the differences between communities were more pronounced at high-food levels, argue that evolution in Rivulus has been influenced by the indirect effects of predators mediated through resource availability. Rivulus from sites with predators better exploit the higher resources in those habitats. Therefore, both direct and indirect effects of predators have evolutionary consequences.density ͉ guppy ͉ resource availability ͉ trophic cascade R esource availability has historically been considered an important selective pressure on the evolution of life histories. For instance, Lack (1, 2) proposed that clutch size variation is driven by food limitation, although differences in resource availability were implicitly assumed in r-and K-selection (3, 4). Despite this interest, evidence demonstrating how resource availability molds the evolution of general life history strategies is severely lacking. One reason is that life history theory has failed to yield consistent predictions as to how evolution should proceed when resources are limited (5-7). Furthermore, differences in resource availability often covary with additional ecological selective pressures such as population density and predators. For example, although predators have the direct effect of increasing mortality rates, they also have the indirect effect of reducing the population density of prey and possibly increasing resource availability to surviving prey (8-10). Therefore, the indirect effects of predators, which are a prevalent feature of ecological systems (8), potentially have evolutionary consequences that are mediated through resource availability. However, theory suggests that evolutionary...