共查询到20条相似文献,搜索用时 15 毫秒
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A recent study of environmental effects on rates of molecular evolution in the plant subgenus Mearnsia shows that species occurring in more equatorial latitudes have higher rates of substitution in rDNA sequences as compared to their more southerly congeners (Wright et al. 2003). However, we believe that the statistical approach employed by Wright et al. (2003) insufficiently accounts for the phylogenetic nonindependence of the species examined, given that all six equatorial species of Mearnsia form a clade. To distinguish between the effect of latitude and that of phylogenetic nonindependence, we have employed a variety of comparative approaches that use independent contrasts to test for an effect of environment across this entire subgenus. We find very little evidence for an effect of latitude on rate of molecular evolution using these approaches and believe that the shared evolutionary history of the clade is a plausible explanation of the apparent rate difference between equatorial and subequatorial Mearnsia species. 相似文献
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Gavin Miller 《CMAJ》1933,29(6):596-598
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J. C. Houston 《CMAJ》1939,40(2):143-146
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SIRKKU K. HELLSTEN 《Developing world bioethics》2008,8(2):70-81
This article discusses what 'global bioethics' means today and what features make bioethical research 'global'. The article provides a historical view of the development of the field of 'bioethics', from medical ethics to the wider study of bioethics in a global context. It critically examines the particular problems that 'global bioethics' research faces across cultural and political borders and suggests some solutions on how to move towards a more balanced and culturally less biased dialogue in the issues of bioethics. The main thesis is that we need to bring global and local aspects closer together, when looking for international guidelines, by paying more attention to particular cultures and local economic and social circumstances in reaching a shared understanding of the main values and principles of bioethics, and in building 'biodemocracy'. 相似文献
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A. C. Kemp 《Ostrich》2013,84(1-3):147-150
Summary Kemp, A. C. 1985. Life-history traits: adaptations or effects? Ostrich 56:147-150. Attention is drawn to the epigeneticist viewpoint concerning life-history traits, whereby differences between organisms are considered as effects of, among other things, scaling relationships. The recent papers of Western &; Ssemakula (1982) and Calder (1983) have shown the extent to which variation in life-history traits of endotherms can be explained by scaling, mainly to body size, brain size and body temperature. Collection of comparative data and the search for scaling relationships in birds is encouraged. 相似文献
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Purdy LM 《Bioethics》1989,3(1):18-34
The morality of surrogate mothering is analyzed from a "consequentialist" framework which attempts to separate those consequences that invariably accompany a given act from those that accompany it only in particular circumstances. Critics of surrogacy argue that it transfers the burden and risk of pregnancy onto another woman, separates sex and reproduction, and separates reproduction and childrearing; none of these acts is necessarily wrong, either morally or for women's or society's basic interests. While surrogate mothering can be rendered immoral if women are coerced into the practice or become victims of subordinating or penalizing contracts, it has the potential to empower women and increase their status in society by providing a job that is less risky and more enjoyable than other jobs women are forced to take and by achieving greater social recognition for reproductive labor. 相似文献
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Burt A 《Evolution; international journal of organic evolution》2000,54(2):337-351
The idea that sex functions to provide variation for natural selection to act upon was first advocated by August Weismann and it has dominated much discussion on the evolution of sex and recombination since then. The goal of this paper is to further extend this hypothesis and to assess its place in a larger body of theory on the evolution of sex and recombination. A simple generic model is developed to show how fitness variation and covariation interact with selection for recombination and illustrate some important implications of the hypothesis: (1) the advantage of sex and recombination can accrue both to reproductively isolated populations and to modifiers segregating within populations, but the former will be much larger than the latter; (2) forces of degradation that are correlated across loci within an individual can reduce or reverse selection for increased recombination; and (3) crossing-over (which can occur at different places in different meioses) will create more variability than having multiple chromosomes and so will have more influence on the efficacy of selection. Several long-term selection experiments support Weismann's hypothesis, including those showing a greater response to selection in populations with higher rates of recombination and higher rates of recombination evolving as a correlated response to selection for some other character. Weismann's hypothesis is also consistent with the sporadic distribution of obligate asexuality, which indicates that clones have a higher rate of extinction than sexuals. Weismann's hypothesis is then discussed in light of other patterns in the distribution of sexuality versus asexuality. To account for variation in the frequency of obligate asexuality in different taxa, a simple model is developed in which this frequency is a function of three parameters: the rate of clonal origin, the initial fitness of clones when they arise, and the rate at which that fitness declines over time. Variation in all three parameters is likely to be important in explaining the distribution of obligate asexuality. Facultative asexuality also exists, and for this to be stable it seems there must be ecological differences between the sexual and asexual propagules as well as genetic differences. Finally, the timing of sex in cyclical parthenogens is most likely set to minimize the opportunity costs of sex. None of these patterns contradict Weismann's hypothesis, but they do show that many additional principles unrelated to the function of sex are required to fully explain its distribution. Weismann's hypothesis is also consistent with what we know about the mechanics and molecular genetics of recombination, in particular the tendency for chromatids to recombine with a homolog rather than a sister chromatid at meiosis, which is opposite to what they do during mitosis. However, molecular genetic studies have shown that cis-acting sites at which recombination is initiated are lost by gene conversion as a result, a factor that can be expected to affect many fine details in the evolution of recombination. In summary, although Weismann's hypothesis must be considered the leading candidate for the function of sex and recombination, nevertheless, many additional principles are needed to fully account for their evolution. 相似文献
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W. J. Greer 《BMJ (Clinical research ed.)》1914,2(2793):95-96