New Releases by Jen Wang

A central goal of evolutionary biology is to understand the origin and maintenance of genetic and phenotypic variation in natural populations. Examining cases of adaptive variation, in particular, across different environments provides an opportunity to understand the essential evolutionary forces underlying biological diversification. Color polymorphism is a particularly promising system for the study of adaptive variation, because color patterns provide quantifiable and often dramatic differences within species that are conspicuously affected by natural selection. For my dissertation research, I examined color variation in two species of poison-dart frogs, Dendrobates pumilio and D. granuliferus, to investigate several fundamental questions in phenotypic evolution, including whether color traits are highly constrained in aposematic species, whether genetic isolation allows for the evolution of color polymorphism, and whether variation in conspicuousness evolves to match variation in toxicity. Contrary to prevailing theories, my results demonstrate that color traits are actually highly labile in D. pumilio, that divergence in coloration in D. pumilio led to genetic isolation rather than vice versa, and that conspicuousness is actually negatively associated with toxicity in D. granuliferus. Together, these results challenge some of the major tenets of aposematic theory and support new theories that may prove more appropriate for understanding aposematic evolution in species with environmentally derived toxicity.

The current class of amorphous Fe-based alloys, also referred to as amorphous steels, possess some outstanding physical properties, including high specific strength, elastic modulus, and excellent corrosion resistance, as well as maintaining fairly low cost. However, thermally-induced embrittlement and insufficient overall ductility are detrimental factors that prohibit amorphous steels from being widely used as structural materials for civilian and military applications. Also, the desire to continuously enlarge the physical size of amorphous alloys, which requires significant improvement of the glass formability, is another hurdle that requires more scientific understanding in order to promote their future application. Consequently, obtaining systematic and detailed characterizations of amorphous alloys from structural and electronic aspects are important for future improvements in their development. In this research, two directions are focused on in characterizing these effects. First, studying the crystallization processes can help gain insight as to the alloy''s overall stability. Measuring formation kinetics of crystalline phases that result and the atomic segregation mechanisms associated with devitrification can be employed to interpret the factors contributing to the glass formability. Second, examining the electronic and atomic structures in the vitrified state can assist in determining effects associated with bonding strength and electronic interactions among the various elements, which can be correlated with glass formability and the alloy''s fracture behavior when key elements are added to bring the alloy through the ductile-to-brittle transition. The characterization methods utilized in this research involve the complementary studies of crystallization sequences, phase identification, growth kinetics, chemical and structural analyses, as well as the atomic and electronic structure examination in the vitrified state of the alloys. Differential thermal analysis (DTA), X-ray diffraction (XRD), three-dimensional atom probe (3DAP), and conventional and analytical transmission electron microscopy (TEM) are employed to carry out these studies. Detailed investigation on the atomic and electronic structure in the undercooled amorphous region of the alloys is implemented with XRD, and electron energy loss spectroscopy (EELS).

We propose a method of moment estimator for a stochastic frontier model in which one of the independent variables is measured with errors. The estimator corrects for the measurement errors, and it requires only minimal assumption on the error distribution, has no need for additional data, and is computationally inexpensive. A Monte Carlo study shows favorable statistical properties of this estimator. We apply this estimator to an investment model with financing constraint, where a major explanatory variable, Tobin''s Q, is known to prone to measurement problems. We find that the Q''s explanatory powers increase substantially upon correcting for the measurement errors.

Abstract: In 1984, Patterson and Piatetski-Shapiro constructed cuspidal distinguished representations on the three-fold covers of GL(3). Their construction is based on the work of Kazhdan and Patterson on metaplectic forms and the method of the converse theorems given by Jacquet, Piatetski-Shapiro, and Shalika. These distinguished representations can be viewed as generalizations of classical theta functions. This dissertation consists of two parts. In the first part, we apply the approach of Patterson and Piatetski-Shapiro to the case of the two fold cover of GL(2). We show that each Hecke character corresponds to a distinguished representation on the two-fold cover of GL(2). If the Hecke character is odd, then the corresponding distinguished representation is cuspidal. The same result has been obtained by Gelbart and Piatetski-Shapiro with a different approach using Weil representations. In the second part, we make progress toward generalizing the work of Patterson and Piatetski-Shapiro to the case of the four-fold cover of GL(4). The goal is to develop the relevant local Rankin-Selberg theory, adapt the converse theorem of Cogdell and Piatetski-Shapiro to the metaplectic setting and verify the analytic information needed to construct cuspidal distinguished representations by the converse theorem.

We show that investment under financing constraints can be modeled as a one-sided deviation from a frictionless investment level, and that effects of financing constraints can be identified and quantified by imposing a distributional assumption on the effects. Panel data on Taiwanese manufacturing firms between 1989 and 1996 are used in the estimation. This period chronicles important financial reforms in the Taiwanese markets, thus providing a unique opportunity to test the financing constraint hypothesis. Estimation results show that cash flow not only have the first order effect of reducing the level of constraints, but also has the second order effect of reducing the variance of the constraints. The use of the quantitative measures of financing constraints in the post-estimation analysis yields two interesting findings: (1) Some of the sorting criteria used in the literature do not have significant and monotonic relationships with the degrees of financing constraint, resulting in problematic sample separations. (2) The effects of financial liberalization in Taiwan are such that the investment efficiency improved over time for a typical firm, and the improvement was particularly large for smaller firms.

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