灾变论与恐龙绝灭

今年第一期发表了题为《恐龙绝灭与彗星陨落》的文章,现在又发表这篇观点相反的报道.恐龙是如何绝灭的,这是一个没有定论的问题.我们了解不同的观点,可以获得更多的知识.     

恐龙绝灭原因的种种解释

有关恐龙绝灭的种种解释归纳起来主要为灾变论和渐变论。灾变论认为恐龙和其它生物的大规模绝灭是一个短期的突变的过程,是由来自地球以外的突发事件,外星体对地球的撞     

一类竞争——扩散组的爆破

本文讨论了人口动力学中一类竞争——扩散方程组的混合初边值问题解的爆破性质,并由此导出了关于恐龙绝灭的一个新解释.     

恐龙绝灭与银河旋臂

自从1822年英国乡村医生曼特尔首次发现恐龙化石以来,这种在中生代不可一世,称霸地球1亿多年的庞然大物突然绝灭之谜一直困扰着无数科学家。有人认为是地球大规模的火山爆发和造山运动引起的,有人认为是太阳黑子大爆发造成的,有人认为是尚未观测到的太阳伴星闯的祸,还有人认为是哺乳动物的竞争使恐龙败下阵来。笔者认为恐龙绝灭与银河系旋臂有关。     

鲸类从陆生到水生的证据

哺乳动物从中生代三叠纪开始出现,至今已度过了大约两亿年的漫长岁月.随着白垩纪末期恐龙的绝灭,开始了被称为哺乳动物时代的新生代.在这个时期,哺乳动物大发展,大分化,成为地球上的     

博物馆里的“活恐龙”

看到这个题目,人们不禁要问:恐龙不是早已绝灭了的远古动物吗,怎么又活了呢?当我在英国进修期间看到大英自然博物馆这则广告时也莫名其妙.展览开幕的当天我就迫不及待地来到了展览大厅.说来也巧,招贴标志是个恐龙足迹.这个足迹的原型正是北京自然博物馆甄朔南研究员和我本人共同研究命名的、发现于我国四川省的岳池嘉陵足迹.现在这个广告招贴画贴遍了伦敦城,仿佛恐龙走遍了伦敦的每个角落,到处都留下了它们的足迹.     

“复制”恐龙可能吗?

美国电影《侏罗纪公园》不仅使人们领略了早已绝灭了的恐龙家族多姿多彩生活,也极大地激发了人们的想象力。国内颇有名气的一位科幻小说作者称“将在天安门广场检阅恐龙大军”;两年多以前,北京大学生命科学院的几位科学家也正式宣布,他们成功地从一枚特殊的恐龙蛋中获得了恐龙基因——     

鳄类种种

鳄类是古老而特化的爬行动物,已经有两亿年的发展历史.它们和恐龙有共同的祖先,是恐龙的近亲.但恐龙在7000万年之前就已经绝灭,而鳄类则经历了大陆的变迁和冰川时期的严峻考验,侥幸生存下来.鳄类从侏罗纪起,除了荐前椎改变成前凹型以外,在头后骨骼上很少变化,头骨上的变化是次生腭发达,便于咀嚼时正常呼吸.心脏分为两个心耳、两个心室.牙齿为多换性的槽生     

恐龙蛋的秘密

对于恐龙蛋化石,大家都不陌生,但是关于它的故事,恐怕没人能说得清.无论是形态各异的蛋形,还是构造多样的蛋壳,以及蛋内胚胎、蛋周围环境和微量元素含量等都是科学家研究恐龙繁殖行为、生殖生理特性,以及古地理、古环境和恐龙灭绝等重要信息. 恐龙是地球上曾经生存过的最具神秘色彩的爬行动物,无论是在博物馆中,还是在科幻影视作品中,它们都是吸引大众目光的绝对主角.在恐龙骨骼、恐龙蛋和恐龙足迹等与恐龙有关的化石中,恐龙蛋有其特殊的意义,在恐龙的繁殖行为、生殖生理特性、羊膜卵的起源与演化、古地理对比、古环境恢复以及恐龙绝灭等研究领域中,往往能提供其他材料所不具备的重要信息.     

贵州龙

贵州龙是早已绝灭的古脊椎动物,它与蛇、鳄、蜥蜴等同属爬行类。虽然冠以“龙”的名字,但它并不是恐龙家族的成员,而是生活在大约2.15亿年前的海相水生爬行动物。这个时期,恐龙还没有出现呢。贵州     

论消弧线圈在城乡配电网中的应用

通过对城乡配电网中电容电流的分析,阐述了消弧线圈在配电网中应用的必要性,并从消弧线圈的工作原理、容量选择以及接地变压器的选择等方面进行了探讨。     

侏罗纪双壳类绝灭事件

侏罗纪原始大西洋的双壳类数据反映了３次区域性海生无脊椎动物绝灭事件,分别出现于Ｐｌｉｅｎｓｂａｃｈｉａｎ末期,Ｃａｌｌｏｖｉａｎ初期以及Ｔｉｔｈｏｎｉａｎ期,其中以Ｔｉｔｈｏｎｉａｎ事件规模最大,影响范围最广,海平面变化以及所伴随的环境恶化是导致绝灭主要原因。     

Adaptive radiation of multituberculate mammals before the extinction of dinosaurs

The Cretaceous-Paleogene mass extinction approximately 66 million years ago is conventionally thought to have been a turning point in mammalian evolution. Prior to that event and for the first two-thirds of their evolutionary history, mammals were mostly confined to roles as generalized, small-bodied, nocturnal insectivores, presumably under selection pressures from dinosaurs. Release from these pressures, by extinction of non-avian dinosaurs at the Cretaceous-Paleogene boundary, triggered ecological diversification of mammals. Although recent individual fossil discoveries have shown that some mammalian lineages diversified ecologically during the Mesozoic era, comprehensive ecological analyses of mammalian groups crossing the Cretaceous-Paleogene boundary are lacking. Such analyses are needed because diversification analyses of living taxa allow only indirect inferences of past ecosystems. Here we show that in arguably the most evolutionarily successful clade of Mesozoic mammals, the Multituberculata, an adaptive radiation began at least 20 million years before the extinction of non-avian dinosaurs and continued across the Cretaceous-Paleogene boundary. Disparity in dental complexity, which relates to the range of diets, rose sharply in step with generic richness and disparity in body size. Moreover, maximum dental complexity and body size demonstrate an adaptive shift towards increased herbivory. This dietary expansion tracked the ecological rise of angiosperms and suggests that the resources that were available to multituberculates were relatively unaffected by the Cretaceous-Paleogene mass extinction. Taken together, our results indicate that mammals were able to take advantage of new ecological opportunities in the Mesozoic and that at least some of these opportunities persisted through the Cretaceous-Paleogene mass extinction. Similar broad-scale ecomorphological inventories of other radiations may help to constrain the possible causes of mass extinctions.     

Rapid spread of an inherited incompatibility factor in California Drosophila

In Drosophila simulans in California, an inherited cytoplasmic incompatibility factor reduces egg hatch when infected males mate with uninfected females. The infection is spreading at a rate of more than 100 km per year; populations in which the infection was rare have become almost completely infected within three years. Analyses of the spread using estimates of selection in the field suggest dispersal distances far higher than those found by direct observation of flies. Hence, occasional long-distance dispersal, possibly coupled with local extinction and recolonization, may be important to the dynamics. Incompatibility factors that can readily spread through natural populations may be useful for population manipulation and important as a post-mating isolating mechanism.     

昆虫行为进化中的熵初探

昆虫的行为进化是昆虫学和进化论研究的重要内容。昆虫行为进化与熵有着密切的关系,昆虫行为进化是遗传物质变异基础上,长期自然选择作用下负熵的不断增加过程。自然选择是对昆虫各种行为的综合的、复杂的选择。     

Scanning for sisnatures of geosraphically restricted selection based on population genomics analysis

Natural selection, as the driving force of human evolution, has direct impact on population differentiation. However, it is still unclear to what extent the genetic differentiation has been caused by natural selection. To explore this question, we performed a genome-wide scan with single nucleotide polymorphism （SNP） data from the International HapMap Project. Single locus FsTanalysis was applied to assess the frequency difference among populations in autosomes. Based on the empirical distribution of FsT, we identified 12669 SNPs correlating to population differentiation and 1853 candidate genes subjected to geographic restricted natural selection. Further interpretation of gene ontogeny revealed 121 categories of biological process with the enrichments of candidate genes. Our results suggest that natural selection may play an important role in human population differentiation. In addition, our analysis provides new clues as well as research methods for our understanding of population differentiation and natural selection.     

以地球上古生物的灭绝周期论宇宙年对地球生命影响的假说

几十亿年来地球上的生物是否存在周期性的繁盛与衰亡,可以说是全人类都非常关注的问题。寻找发现生物大灭绝的规律可以说是人类的梦想。从已发现的化石来看,地球上确实发生过若干次生物大灭绝,而且距我们今天都十分遥远。我们仅仅从现在地球的某些变化似乎很难发现古代生物大灭绝的规律。所以,如果我们从一个宇宙年的角度来看待地球上的生物大灭绝现象,可能会使我们对问题的认识更接近于正确答案。     

四川野生荞麦种质资源的保护研究

四川省有着极为丰富的野生荞麦资源。但近年来,由于自然和人为的原因,野生荞麦栖息地和生态环境遭受严重破坏,致使野生荞麦分布区域萎缩,种群和植株数量减少,面临着濒危甚至灭绝的危险。本文在对野生荞麦资源野外实地调查的基础上,研究了四川野生荞麦的资源种类、地理分布、生态环境、目前状况、濒危原因等,并提出了保护措施和对策。     

Genetic segregation and the maintenance of sexual reproduction

Sexual reproduction confronts evolutionary biology with a paradox: other things being equal, an asexual (all-female) population will have twice the reproductive potential of a competing sexual population and therefore should rapidly drive the sexual population to extinction. Thus, the persistence of sexual reproduction in most life forms implies a compensatory advantage to sexual reproduction. Work on this problem has emphasized the evolutionary advantages produced by the genetic recombination that accompanies sexual reproduction. Here we show that genetic segregation produces an advantage to sexual reproduction even in the absence of an advantage from recombination. Segregation in a diploid sexual population allows selection to carry a single advantageous mutation to a homozygous state, whereas two separate mutations are required in a parthenogenetic population. The complete fixation of advantageous mutations is thus delayed in a heterozygous state in asexual populations. Calculation of the selective load incurred suggests that it may offset the intrinsic twofold reproductive advantage of asexual reproduction and maintain sexual reproduction in diploid populations.