物种
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在分类学中，一个物种被赋予一个拉丁化的雙名法名称。该名称使用斜体或是加上底線（現在較少見）印刷；属名首字母大写，屬名之後紧跟一个唯一的形容词，這個詞稱為種小名或種加詞，其首字母不可大寫。只有完整的双名制名称才称为「种名」，而非仅仅是双名制名称的第二个部分。例如人的种名叫Homo sapiens（有智慧的人），而不是sapiens。
[编辑] 物种的定义恩斯特·麥爾对于物种的定义不太完善。因為這個定義假设只適用於进行有性生殖的物种，它把很多进行无性生殖的物种丢到了一边。此外生物学家很多时候都不知道两组形态学上类似的生物是否具有“潜在的”配育能力。况且利用不同的繁殖方法，自然状态或人工状态的杂交可能会获得成功，甚至在有性生殖情况下两个个体交配也可以。因此，出现了下面的这些对于物种定义的觀點:
在实际操作中，这些定义通常都相一致，这些定义的不同点主要是侧重点不同而非相互否定。然而，还没有提出一个物种概念可以在无需具体分析情况下，就能完全否定、或应用于所有情况的。 [编辑] 在生物学分类上的重要性物种的概念历史悠久。这之所以是最重要的分类阶元之一, 有下面的原因:
在使用了几千年后，这个概念依然集中在生物学和相关学科上，也仍然被错误的定义和争论不休。 [编辑] "指定物种状态"的含义对于某个物种的命名应该看作是对于该群生物的进化关系及其独特性的一种假定。更新的资料会证实或推翻这个假设。有时候, 尤其是旧时通讯不发达的时候，分居异地的分类学家会为后来定为同一种的生物命两个不同的名字。 当两个经过命名的种发现其实是同一个种时, 先发表的那个种名通常会保留，后发表的被废弃, 这一过程称为 synonymization, 或 lumping。将一个分类阶元分开成多个通常是新的阶元称为splitting。分类学家常被他们的同时戏称为"lumpers" 或 "splitters", 视乎他们找到了物种间的不同点还是相同点。(参阅 en:lumpers and splitters). 传统上, 研究人员依赖于解剖学上的不同, 和通过观察不同种群间是否有成功混交来区分物种; 解剖结构和生殖行为依然是指定物种状态的重要手段 得益于上几个十年里微观生物学研究技术的日新月异, 包括 DNA 分析等, 得出了一大批关于物种间相似与区别的知识。许多从前认为是不同物种的类群现在归入了单一的分类阶元, 许多过去归在一起的类群被分开了。任何的分类阶元(种,属,科 等)都可以合并和分开。对于一个较高的阶元，这样的修订则更显得意义深远。 从分类学的角度看, 种下的类群定义为低于种的分类阶元。在动物学中，仅使用亚种概念, 而植物学还会使用诸如变种, 亚变种, 和 变型等单位。 [编辑] 更详细的隔离种的概念总的说来, 对于那些大型复杂的有性生殖生物(例如 哺乳类 和 鸟类), 生物学种或隔离种概念的定义有几种变体。通常，区分两个物种，甚至是亲缘关系很近的两个物种都是很简单的。例如，马 (Equus caballus) 和驴 (Equus asinus)不需要经过专门训练或学习的人就能够把它们分开。然而它们相近得可以进行杂交，并产生不育的骡。它们明显就是不同的种。 但是许多情形则让人难以决定。这就是隔离种和进化种的分歧之处。两个概念都同意一个物种在时间上保持着其血统的独立性, 它与其他这样的血统可以被我们区分(否则我们就认不出它们来了), 是生殖隔离的(否则一个种群如果一有机会就和另外一个合并起来了), 并有有效的种内识别 机制(没有这个，该血统不可能延续)。在实际上, 两种观点都同意一个种必须有其自身的独立进化历史;否则刚才提到的特征都不适用了。 其不同在于：进化种概念不对种群的未来作出预测，而仅仅是记录已知的类群。相反, 如果经过研究人员的研究，某两个种群在可能情况下能够再次合并，隔离种概念就不认为这两个种群为两个种。 [编辑] 隔离的问题有两个关键的问题需要解决。第一，初定的这个种是不是能持续的、可靠的与其它物种区分呢? 第二, 这在未来也会持续下去吗? 先来讨论第二个问题, 下面有几个地理分布上的可能性
[编辑] The difference questionObviously, when defining a species, the geographic circumstances become meaningful only if the populations groups in question are clearly different: if they are not consistently and reliably distinguishable from one another, then we have no grounds for believing that they might be different species. The key question in this context, is "how different is different?" and the answer is usually "it all depends". In theory, it would be possible to recognise even the tiniest of differences as sufficient to delineate a separate species, provided only that the difference is clear and consistent (and that other criteria are met). There is no universal rule to state the smallest allowable difference between two species, but in general, very trivial differences are ignored on the twin grounds of simple practicality, and genetic similarity: if two population groups are so close that the distinction between them rests on an obscure and microscopic difference in morphology, or a single base substitution in a DNA sequence, then a demonstration of restricted gene flow between the populations will probably be difficult in any case. More typically, one or other of the following requirements must be met:
fur, rougher bark, longer ears, or larger seeds than another population, and although this characteristic may vary within each population, the two do not grade into one another, and given a reasonably large sample size, there is a definite discontinuity between them. Note that this applies to populations, not individual organisms, and that a small number of exceptional individuals within a population may 'break the rule' without invalidating it. The less a quantitative difference varies within a population and the more it varies between populations, the better the case for making a distinction. Nevertheless, borderline situations can only be resolved by making a 'best-guess' judgement.
present or entirely absent. This might be a distinctively shaped seed pod, an extra primary feather, a particular courting behaviour, or a clearly different DNA sequence. Sometimes it is not possible to isolate a single difference between species, and several factors must be taken in combination. This is often the case with plants in particular. In eucalypts, for example, Corymbia ficifolia cannot be reliably distinguished from its close relative Corymbia calophylla by any single measure (and sometimes individual trees cannot be definitely assigned to either species), but populations of Corymbia can be clearly told apart by comparing the colour of flowers, bark, and buds, number of flowers for a given size of tree, and the shape of the leaves and fruit. When using a combination of characteristics to distinguish between populations, it is necessary to use a reasonably small number of factors (if more than a handful are needed, the genetic difference between the populations is likely to be insignificant and is unlikely to endure into the future), and to choose factors that are functionally independent (height and weight, for example, should usually be considered as one factor, not two). [编辑] 物種分類的歷史在古時，人只是賦與一群類同的生物一個名詞。希臘哲學家，如亞里士多德，認為生物是可嚴格區分為不同的“物種”，而且物種是永恆不變的。 隨着接觸的生物愈多，人開始嘗試認真把生物分類。最初，人只是以外貌與習性分類，就像中國人把大多數水上生物稱作“魚”。 在十八世紀Carolus Linnaeus首次以生殖器官作生物分類。他似乎只是認為這樣的分類有些意義，而並不是以為有近似生殖器官的物種有可聯繫。畢竟，當時的歐洲人相信創造論，即所有物種由神獨立創造，並無任何其他的聯繫。 可能由於創造論，Linnaeus 對一些差異極微的物種感到困惑。他因而提出一些物種為完美的，樣板的；另一些則是仿造的。 及至十九世紀，多數自然學者開始明白物種是不斷在變，而地球是古老得容許長時間所累積而成的巨變。因而，分類法開始強調物種之間是如何演變。Jean-Baptiste Lamarck認為生物可以把需要的特質遺傳給後代，例如長頸鹿就是因為吹愈來愈高的樹葉而長出愈來愈長的頸項。（這只是簡化的描述，Lamarck的理論當然遠為深邃。） Lamarck最大的貢獻在於提出不同物種之間可以有連續不斷的系諎。他在1809的著作 Zoological Philosophy 是首個以邏輯理由否定創造論。當達爾文的進代論在1860年代被廣為接受後，卻掩蓋了Lamarck的頁獻。及至二十世紀未，Lamarck的理論從新獲得重視，成為adaptive mutation的一個基礎。他另一曾被捨棄的目標為本進化論，亦發展成artificial selection。 達爾文及華箂士提出的進化論是公認最權威和最有說服力的進化理論。Basically, Darwin argued that it is populations that evolve, not individuals. His argument relies on a radical shift in perspective from Linnaeus: rather than defining species in ideal terms (and searching for an ideal representative and rejecting deviations), Darwin considered variation among individuals to be natural. He further argued that variation, far from being problematic, actually provides the explanation for the existence of distinct species. Darwin's work drew on Thomas Malthus' insight that the rate of growth of a biological population will always outpace the rate of growth of the resources in the environment, such as the food supply. As a result, Darwin argued, not all the members of a population will be able to survive and reproduce. Those that did will, on average, be the ones possessing variations—however slight—that make them slightly better adapted to the environment. If these variable traits are heritable, then the offspring of the survivors will also possess them. Thus, over many generations, adaptive variations will accumulate in the population, while counter-adaptive will be eliminated. It should be emphasized that whether a variation is adaptive or non-adaptive depends on the environment: different environments favor different traits. Since the environment effectively selects which organisms live to reproduce, it is the environment (the "fight for existence") that selects the traits to be passed on. This is the theory of evolution by natural selection. In this model, the length of a giraffe's neck would be explained by positing that proto- giraffes with longer necks would have had a significant reproductive advantage to those with shorter necks. Over many generations, the entire population would be a species of long-necked animals. In 1859, when Darwin published his theory of natural selection, the mechanism behind the inheritance of individual traits was unknown. Although Darwin made some speculations on how traits are inherited (pangensis), his theory relies only on the fact that inheritable traits exist, and are variable (which makes his accomplishment even more remarkable.) Although Gregor Mendel's paper on genetics was published in 1866, its significance was not recognized. It was not until 1900 that his work was rediscovered by Hugo de Vries, Carl Correns and Erich von Tschermak, who realised that the "inheritable traits" in Darwin's theory are genes. The theory of the evolution of species through natural selection has two important implications for discussions of species -- consequences that fundamentally challenge the assumptions behind Linnaeus' taxonomy. First, it suggests that species are not just similar, they may actually be related. Some students of Darwin argue that all species are descended from a common ancestor. Second, it supposes that "species" are not homogeneous, fixed, permanent things; members of a species are all different, and over time species change. This suggests that species do not have any clear boundaries but are rather momentary statistical effects of constantly changing gene-frequencies. One may still use Linnaeus' taxonomy to identify individual plants and animals, but one can no longer think of species as independent and immutable. The rise of a new species from a parental line is called speciation. There is no clear line demarcating the ancestral species from the descendant species. Although the current scientific understanding of species suggests there is no rigorous and comprehensive way to distinguish between different species in all cases, biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation; if two creatures cannot reproduce to produce fertile offspring, then they are in different species. This definition captures a number of intuitive species boundaries, but nonetheless has some problems, however. It has nothing to say about species that reproduce asexually, for example, and it is very difficult to apply to extinct species. Moreover, boundaries between species are often fuzzy: there are examples where members of one population can produce fertile offspring with a second population, and members of the second population can produce fertile offspring with members of a third population, but members of the first and third population cannot produces fertile offspring. Consequently, some people reject this notion of species. Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and, for each chromosome, both organisms have the same number of nucleotides (The Blind Watchmaker, p. 118). The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on genetic markers. The results have been nothing short of revolutionary, resulting in the reordering of vast expanses of the phylogenetic tree (see also: A species name can be:
names of people and places are used in the genitive: Latimeria chalumnae. There are several common species names, most of which are adjectives. Linnaean taxonomy discusses how the taxon "species" meshes with other classification categories, such as "kingdom" and "genus". See also: race. [编辑] See also[编辑] External links
Determination Offered] Quote: "...two species of dinosaur that are members of the same genera varied from each other by just 2.2 percent. Translation of the percentage into an actual number results in an average of just three skeletal differences out of the total 338 bones in the body. Amazingly, 58 percent of these differences occurred in the skull alone. "This is a lot less variation than I'd expected", said Novak..."
Rapid Change, Say Indiana University Researchers] Quote: "...the sudden mixing of closely related species may occasionally provide the energy to impel rapid evolutionary change..."
Help Explain Origin Of AIDS] Quote: "...The researchers have discovere |
