Genomic variation in a widespread Neotropical bird (Xenops minutus) reveals divergence, population expansion, and gene flow

Elucidating the demographic and phylogeographic histories of species provides insight into the processes responsible for generating biological diversity, and genomic datasets are now permitting the estimation of histories and demographic parameters with unprecedented accuracy. We used a genomic single nucleotide polymorphism (SNP) dataset generated using a RAD-Seq method to investigate the historical demography and phylogeography of a widespread lowland Neotropical bird (Xenops minutus). As expected, we found that prominent landscape features that act as dispersal barriers, such as Amazonian rivers and the Andes Mountains, are associated with the deepest phylogeographic breaks, and also that isolation by distance is limited in areas between these barriers. In addition, we inferred positive population growth for most populations and detected evidence of historical gene flow between populations that are now physically isolated. Even with genomic estimates of historical demographic parameters, we found the prominent diversification hypotheses to be untestable. We conclude that investigations into the multifarious processes shaping species histories, aided by genomic datasets, will provide greater resolution of diversification in the Neotropics, but that future efforts should focus on understanding the processes shaping the histories of lineages rather than trying to reconcile these histories with landscape and climatic events in Earth history.Comment: 61 pages, 4 figures (+3 supplemental), 3 tables (+6 supplemental

Markets, Contracts, or Integration? The Adoption, Diffusion, and Evolution of Organizational Form

The rise of contract farming and vertical integration is one of the most important changes in modern agriculture. Yet the adoption and diffusion of these new forms of organization has varied widely across regions, commodities, or farm types, however. Transaction cost theories and the like are not fully effective at explaining the variation of adoption rates of different organizational forms, in part because of their inherent static nature. In order to explain the adoption, diffusion and evolution of organizational form, a more dynamic framework is required. This paper lays out such a framework for understanding the evolution of organizational practices in U.S. agriculture by drawing on existing theories of economic organization, the diffusion of technological innovation, and organizational complementarities. Using recent trends as stylized facts we argue that the agrifood sector is characterized by strong complementarities among its constituent features and that these complementarities help explain the stylized facts. We also discuss several testable hypotheses concerning changes in organizational form in agriculture.contracting, vertical integration, organizational innovation, diffusion, Institutional and Behavioral Economics, L14, L22, Q13, O33,

Structures and materials technology for hypersonic aerospacecraft

Major considerations in structural design of a transatmospheric aerospacecraft are discussed. The general direction of progress in structures and materials technology is indicated, and technical areas in structures and materials where further research and development is necessary are indicated. Various structural concepts under study and materials which appear to be most applicable are discussed. Structural design criteria are discussed with particular attention to the factor-of-safety approach and the probabilistic approach. Structural certification requirements for the aerospacecraft are discussed. The kinds of analyses and tests which would be required to certify the structural integrity, safety, and durability of the aerospacecraft are discussed, and the type of test facility needed to perform structural certification tests is identified

A phylogeny of birds based on over 1,500 loci collected by target enrichment and high-throughput sequencing

Evolutionary relationships among birds in Neoaves, the clade comprising the vast majority of avian diversity, have vexed systematists due to the ancient, rapid radiation of numerous lineages. We applied a new phylogenomic approach to resolve relationships in Neoaves using target enrichment (sequence capture) and high-throughput sequencing of ultraconserved elements (UCEs) in avian genomes. We collected sequence data from UCE loci for 32 members of Neoaves and one outgroup (chicken) and analyzed data sets that differed in their amount of missing data. An alignment of 1,541 loci that allowed missing data was 87% complete and resulted in a highly resolved phylogeny with broad agreement between the Bayesian and maximum-likelihood (ML) trees. Although results from the 100% complete matrix of 416 UCE loci were similar, the Bayesian and ML trees differed to a greater extent in this analysis, suggesting that increasing from 416 to 1,541 loci led to increased stability and resolution of the tree. Novel results of our study include surprisingly close relationships between phenotypically divergent bird families, such as tropicbirds (Phaethontidae) and the sunbittern (Eurypygidae) as well as between bustards (Otididae) and turacos (Musophagidae). This phylogeny bolsters support for monophyletic waterbird and landbird clades and also strongly supports controversial results from previous studies, including the sister relationship between passerines and parrots and the non-monophyly of raptorial birds in the hawk and falcon families. Although significant challenges remain to fully resolving some of the deep relationships in Neoaves, especially among lineages outside the waterbirds and landbirds, this study suggests that increased data will yield an increasingly resolved avian phylogeny.Comment: 30 pages, 1 table, 4 figures, 1 supplementary table, 3 supplementary figure

Markets, Contracts, or Integration? The Adoption, Diffusion, and Evolution of Organizational Form

The rise of contract farming and vertical integration is one of the most important changes in modern agriculture. Yet the adoption and diffusion of these new forms of organization has varied widely across regions, commodities, and farm types. Transaction cost and other modern theories of the firm help explain the advantages of contracting and integration over reliance on spot markets and commodity brokers. However, these theories do not address the variation in adoption rates of new organizational forms. This paper lays out a more dynamic framework for understanding the evolution of organizational practices in U.S. agriculture, drawing on theories of the diffusion of technology and organizational complementarities. Using recent trends as stylized facts we argue that the agrifood sector is characterized by strong complementarities and that identifying and describing these complementarities more fully sheds considerable light on the organizational structure of agricultural production. We illustrate our arguments with case studies from the oilseed, poultry, and hog industries