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

The Capital Structure Decisions of New Firms

Based on data from the Kauffman Firm Survey, examines the funding sources of firms in their first year of operations. Highlights start-ups' reliance on external debt financing such as bank loans and credit cards, regardless of their credit scores

Patterns of Financing: A Comparison Between White- and African-American Young Firms

Based on Kauffman Firm Survey data, examines differences in start-up and follow-on capital injections into and capital use by firms with African-American and white owners. Explores how access to capital affects the racial gap in new business formation

Synthesis of Silver Colloids: Experiment and Computational Model

We summarize our recent results that model the formation of uniform spherical silver colloids prepared by mixing iso-ascorbic acid and silver-amine complex solutions in the absence of dispersants. We found that the experimental results can be modeled effectively by the two-stage formation mechanism used previously to model the preparation of colloidal gold spheres. The equilibrium concentration of silver atoms and the surface tension of silver precursor nanocrystals are both treated as free parameters, and the experimental reaction time scale is fit by a narrow region of this two-parameter space. The kinetic parameter required to match the final particle size is found to be very close to that used previously in modeling the formation of uniform gold particles, suggesting that similar kinetics governs the aggregation process. The model also reproduces semi quantitatively the effects of temperature and solvent viscosity on particle synthesis.Comment: 6 pages, 5 figure

Mitochondrial variation in Bolivian populations of the Variable Antshrike (Thamnophilus caerulescens)

The Variable Antshrike (Thamnophilus caerulescens) is arguably the most polytypic thamnophilid, with males ranging from almost entirely jet black to nearly white. The four subspecies that occur in Bolivia are strikingly divergent in male plumage: T. c. aspersiventer (black with white-barred belly), T. c. connectens (black back and bib with white belly), T. c. dinellii (gray throat and back with rufous belly), and T. c. paraguayensis (light gray with white belly). To assess the genetic structure of those taxa in Bolivia, sequence variation at the mitochondrial cytochrome-b gene was screened in 126 individuals collected across transects spanning the plumage and vocal variation in the four forms. A 95-km-wide cline in haplotype frequencies from T. c. aspersiventer to T. c. dinellii was centered in the Serrania Cochabamba across an ecotone from humid to dry Andean foothill habitats. Thamnophilus caerulescens connectens is not a valid taxon, instead representing an introgressed population near the dinellii tail of the T. c. aspersiventer-T. c. dinellii hybrid zone. Although direct contact between T. c. dinellii and T. c. paraguayensis remains undocumented, the mitochondrial data were consistent with introgression along a broad cline extending across most of southern Bolivia. Overall, the transitions in mitochondrial frequencies were remarkably concordant with clinal changes in vocalizations among those same populations (Isler et al. 2005). Both studies highlight the need for increased sampling, in both the geographic extent and number of individuals per population, to address adequately the potential for clinal variation between populations that are not isolated geographically. A more restricted sampling design in the present study might have led to the erroneous conclusion that T. c. aspersiventer, T. c. dinellii, and T. c. paraguayensis have reciprocally monphyletic mitochondrial lineages, making them full species according to some species concepts. © The American Ornithologists\u27 Union, 2005