9 research outputs found
Ecological Niche Dimensionality and the Evolutionary Diversification of Stick Insects
The degree of phenotypic divergence and reproductive isolation between taxon pairs can vary quantitatively, and often increases as evolutionary divergence proceeds through various stages, from polymorphism to population differentiation, ecotype and race formation, speciation, and post-speciational divergence. Although divergent natural selection promotes divergence, it does not always result in strong differentiation. For example, divergent selection can fail to complete speciation, and distinct species pairs sometimes collapse (‘speciation in reverse’). Widely-discussed explanations for this variability concern genetic architecture, and the geographic arrangement of populations. A less-explored possibility is that the degree of phenotypic and reproductive divergence between taxon pairs is positively related to the number of ecological niche dimensions (i.e., traits) subject to divergent selection. Some data supporting this idea stem from laboratory experimental evolution studies using Drosophila, but tests from nature are lacking. Here we report results from manipulative field experiments in natural populations of herbivorous Timema stick insects that are consistent with this ‘niche dimensionality’ hypothesis. In such insects, divergent selection between host plants might occur for cryptic colouration (camouflage to evade visual predation), physiology (to detoxify plant chemicals), or both of these niche dimensions. We show that divergent selection on the single niche dimension of cryptic colouration can result in ecotype formation and intermediate levels of phenotypic and reproductive divergence between populations feeding on different hosts. However, greater divergence between a species pair involved divergent selection on both niche dimensions. Although further replication of the trends reported here is required, the results suggest that dimensionality of selection may complement genetic and geographic explanations for the degree of diversification in nature
Immunomodulatory role of black tea in the mitigation of cancer induced by inorganic arsenic
Effect of seed mass on emergence and seedling development in Pterocarpus marsupium Roxb.
An Integrated Model of Debt Issuance, Refunding, and Maturity
We integrate previous work in this area and develop a multiperiod model that simultaneously determines bond refunding, bond issuance, maturity structure, cash holdings, and bank borrowing policies. The focus here is on providing the required debt funds in the most cost efficient fashion. A strength of the model is that it allows for time varying interest costs, transaction costs, issuance costs, and refunding costs to be firm specific. The output of the model lays out the optimal financing decisions for each time interval that minimize the total discounted cost of providing the funds that match the requisite funds. By limiting the surplus funds available, the model minimizes the management incentive to over invest and thereby reduces the agency costs. The model has economic implications for the financing decisions and the firm's default risk, growth opportunities, riskiness of cash flows, and firm size. Copyright Springer Science + Business Media, Inc. 2006debt financing, refunding, maturity, simultaneous, multiperiod,