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Linear three-dimensional instability is studied in the shock layer and the laminar separation bubble (LSB) induced by shock-wave/boundary-layer interactions in a Mach 7 flow of nitrogen over a double wedge with a $30^{\\circ }\\text {--}55^{\\circ }$ cross-sectional profile. At a free-stream unit Reynolds number $Re=5.2\\times 10^{4}\\,{\\rm m}^{-1}$ this flow exhibits rarefaction effects and has shock thicknesses comparable to the thickness of the boundary layer at separation. Flow features have been fully resolved using a high-fidelity massively parallel implementation of the direct simulation Monte Carlo method that captures the flow evolution from the inception of three-dimensionality, through linear growth of instabilities, to the early stages of nonlinear saturation. It is shown that the LSB sustains self-excited, small-amplitude perturbations that originate past the primary separation line and lead to spanwise-periodic wall striations inside the bubble and downstream of the primary reattachment line, as known from earlier experiments, simulations and instability analyses. A spanwise-periodic instability, synchronised with that in the separation zone, is identified herein for the first time, which exists in the internal structure of the separation and detached shock layers, and manifests itself as spanwise-periodic cats-eyes patterns in the global mode amplitude functions. The growth rate and the spanwise-periodicity length of linear disturbances in the shock layers and the LSB are found to be identical. Linear amplification of the most unstable three-dimensional flow perturbations leads to synchronised low-frequency unsteadiness of the triple point, with a Strouhal number of $St\\approx 0.028$.

The wide distribution of polyploidy among plants has led to a variety of theories for the evolutionary advantages of polyploidy. Here we claim that the abundance of polyploidy may be the result of a simple ratcheting process that does not require evolutionary advantages due to the biological properties of organisms. The evolution of polyploidy is a one-way process in which chromosome number can increase but not decrease. Using a simple mathematical model, we show that average ploidal level within a plant lineage can continually increase to the levels observed today, even if there are ecological or physiological disadvantages to higher ploidy. The model allowed us to estimate the average net speciation and polyploidy rates for ten angiosperm genera. Based on these estimates, the model predicts distributions of ploidal levels statistically similar to those observed in nine of the 10 genera.

Chloroplast capture, the introgression of a chloroplast from one species into another, has been frequently suggested as the explanation for inconsistencies between gene trees based on nuclear and cytoplasmic markers in plants. We use a genetic model to determine the conditions for capture to occur, and we find that they are somewhat more general than those given in earlier verbal arguments. Chloroplast capture can occur if cytoplasm substitution provides an advantage in seed production. This can happen through reallocation to the female function when cytonuclear incompatibilities cause partial male sterility, but also under more general conditions. Capture is promoted by nuclear incompatibilities between the two genomes (or a low heterosis in F1 hybrids) and by partial selfing when hybridization causes a decrease in the selfing rate and inbreeding depression is strong. We discuss empirical predictions that can be used to test this mechanism.

Ecological transitions are at the core of different modes of speciation. These transitions face both genetic and demographic hurdles. This paper focuses on how these hurdles are overcome, allowing ecological speciation and speciation via hybridization and/or polyploidy. Niche shifting is a two-step process. First there is the establishment of ill-adapted populations where ecological opportunity allows. This is followed by the genetic refinement of populations, which allows them to be integrated into novel communities and habitats. These steps are more readily accomplished in unsaturated floras, where competition is less intense. Ecological transitions in saturated floras may be facilitated by disturbance. Invasive species serve as heuristic model systems for understanding the early stages of speciation where niche shifts are involved.

The role of nucleocytoplasmic interactions in the genesis of post-zygotic isolation has been given little attention by plant evolutionists. I present evidence from reciprocal crosses, cytoplasmic substitution lines, and cell fusion lines that hybrid weakness and sterility often arise from interactions between the nuclear genome and the chloroplast and mitochondrial genomes. These interactions are much more important in the origin and isolation of species than we appreciate. The strength of the post-zygotic barriers tends to be a function of cytoplasmic divergence. I also review evidence indicating that the properties and evolutionary potential of allopolyploids and diploid hybrid derivatives may be influenced by cytoplasmic factors.

Much has been written about the role of interspecific competition, disease, herbivory, and the loss of key mutualisms in the extinction of rare plant species. Interspecific hybridization rarely is considered among the biotic interactions that promote extinction. We show how hybridization may contribute to the demise of rare plant species through demographic swamping and genetic assimilation by an abundant congener. We contend that the growth of the hybrid subpopulation is the key to rare species assimilation, and we show how the production of hybrid seed, the fitness of hybrids, and pest pressure affect hybrid proliferation. We also discuss how habitat disturbance, unspecialized pollinators, and weak crossing barriers promote hybridization, and how the negative consequences of hybridization are unlikely to be compensated for by immigration from conspecific populations. We also illustrate stages in the demise of species in island floras. We suggest that hybridization is an increasing threat to rare species because ecological barriers are being disrupted by human activities.

Lithium sulfur (Li–S) batteries have the potential to provide higher energy storage density at lower cost than conventional lithium ion batteries. A key challenge for Li–S batteries is the loss of sulfur to the electrolyte during cycling. This loss can be mitigated by sequestering the sulfur in nanostructured carbon–sulfur composites. The nanoscale characterization of the sulfur distribution within these complex nanostructured electrodes is normally performed by electron microscopy, but sulfur sublimates and redistributes in the high-vacuum conditions of conventional electron microscopes. The resulting sublimation artifacts render characterization of sulfur in conventional electron microscopes problematic and unreliable. Here, we demonstrate two techniques, cryogenic transmission electron microscopy (cryo-TEM) and scanning electron microscopy in air (airSEM), that enable the reliable characterization of sulfur across multiple length scales by suppressing sulfur sublimation. We use cryo-TEM and airSEM to examine carbon–sulfur composites synthesized for use as Li–S battery cathodes, noting several cases where the commonly employed sulfur melt infusion method is highly inefficient at infiltrating sulfur into porous carbon hosts.

The shift from self-sterility to predominant self-fertility has occurred in a multitude of flowering plant lineages. This shift has been attributed to the partial replacement of self-incompatibility alleles at the S locus by alleles conferring self-fertility. I propose that pseudo-self-fertility (i.e., self-fertility dictated by polygenic modifiers of S gene action) may have been pivotal in the evolution of self-fertility. Pseudo-self-fertility is known in numerous species, whereas self-fertility governed by S alleles is known only in a few species, and then only in those with a history of selfing. Pseudo-self-fertility is readily increased by selection, although its expression also is influenced by the environment Pseudo-self-fertility maximizes the dual advantages of outcrossing and selfing.

The process of ecological speciation has not been considered in great detail. We can gain a new perspective on this process by viewing new species as successful invaders, and by using invasive species as a model system for understanding the early stages of speciation. I propose that ecological species may be polyphyletic, and undergo genetic differentiation early in their histories. I also note that there are formidable genetic obstacles to ecological speciation. The rate of speciation is dependent on geographical and ecological variables, as are rates of invasions by exotic species. Ecological opportunity is the key for speciation and invasion; and it is most often found on young islands. In general there are no traits that presage which exotic species will be the best invaders of natural areas. Thus it would be difficult to predict which traits foster ecological speciation. Herbaceous plants are more invasive and have higher rates of diversification than woody plants.

Congeneric species that have evolved on oceanic islands rarely grow with one another. This spatial sorting is thought to be the result of niche pre-emption, where the first species to occupy a given habitat would exclude its relatives through competition. The evidence that competition shapes local species distributions is scant. I propose that hybridization thwarts the invasion of congeners. We know that crossing barriers between products of recent adaptive radiations typically are weak. Colonization of occupied patches would be sporadic, because invaders would be moving from habitats where they are adapted to those where they are not. Nearly all progeny of the scant invaders would be ill-adapted hybrids, because the pollen pool would consist almost exclusively of indigenous pollen. Only if aliens were predominantly inbreeders or apomicts would they have a chance of escaping the deluge of local pollen and becoming established in the occupied patch. The spatial reach of hybridization is much greater than that of competition.