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451.
Speciation—the origin of new species—is the source of the diversity of life. A theory of speciation is essential to link poorly
understood macro-evolutionary processes, such as the origin of biodiversity and adaptive radiation, to well understood micro-evolutionary
processes, such as allele frequency change due to natural or sexual selection. An important question is whether, and to what
extent, the process of speciation is ‘adaptive’, i.e., driven by natural and/or sexual selection. Here, we discuss two main
modelling approaches in adaptive speciation theory. Ecological models of speciation focus on the evolution of ecological differentiation
through divergent natural selection. These models can explain the stable coexistence of the resulting daughter species in
the face of interspecific competition, but they are often vague about the evolution of reproductive isolation. Most sexual
selection models of speciation focus on the diversification of mating strategies through divergent sexual selection. These
models can explain the evolution of prezygotic reproductive isolation, but they are typically vague on questions like ecological
coexistence. By means of an integrated model, incorporating both ecological interactions and sexual selection, we demonstrate
that disruptive selection on both ecological and mating strategies is necessary, but not sufficient, for speciation to occur.
To achieve speciation, mating must at least partly reflect ecological characteristics. The interaction of natural and sexual
selection is also pivotal in a model where sexual selection facilitates ecological speciation even in the absence of diverging
female preferences. In view of these results, it is counterproductive to consider ecological and sexual selection models as
contrasting and incompatible views on speciation, one being dominant over the other. Instead, an integrative perspective is
needed to achieve a thorough and coherent understanding of adaptive speciation. 相似文献
452.
On the choice of farm management practices after the reform of the Common Agricultural Policy in 2003 总被引:1,自引:0,他引:1
The Common Agricultural Policy (CAP) was fundamentally reformed in 2003. From 2005, farmers will receive decoupled income support payments instead of production premiums if basic standards for environment, food safety, animal health and welfare are met. Farmers are likely to adjust production and management practices to the new policy framework. We describe how this reform fits into the EU strategy of making agricultural production more environmentally friendly by concentrating on the financial aspects of the reforms. Using an agricultural sector model for Austria, we show that the reform will further decrease agricultural outputs, reduce farm inputs, lessen nitrogen surpluses and make environmentally friendly management practices more attractive for farmers. 相似文献
453.
Here we report N2O emission results for freshwater marshes isolated from human activities at the Sanjiang Experimental Station of Marsh Wetland
Ecology in northeastern China. These results are important for us to understand N2O emission in natural processes in undisturbed freshwater marsh. Two adjacent plots of Deyeuxia angustifolia freshwater marsh with different water regimes, i.e., seasonally waterlogged (SW) and not- waterlogged (NW), were chosen for
gas sampling, and soil and biomass studies. Emissions of N2O from NW plots were obviously higher than from the SW plots. Daily maximum N2O flux was observed at 13 o′clock and the seasonal maximum occurred in end July to early August. The annual average N2O emissions from the NW marsh were 4.45 μg m−2 h−1 in 2002 and 6.85 μg m−2 h−1 in 2003 during growing season. The SW marsh was overall a sink for N2O with corresponding annual emissions of −1.00 μg m−2 h−1 for 2002 and −0.76 μg m−2 h−1 for 2003. There were significant correlations between N2O fluxes and temperatures of both air and 5-cm-depth soil. The range of soil redox potential 200–400 mV appeared to be optimum
for N2O flux. Besides temperature and plant biomass, the freeze–thaw process is also an important factor for N2O emission burst. Our results show that the freshwater marsh isolated from human activity in northeastern China is not a major
source of N2O. 相似文献