物种

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生物分类
科学分类法
阶元分类
域 | 界 | 门
纲 | 目 | 科
属 | 种 | 亚种
双名法
非细胞生物
原核生物
真核生物
•动物
•真菌
•植物
•原生生物
五界法
原核生物界
原生生物界
真菌界
植物界
动物界
四界法
菌界
原生生物界
植物界
动物界
三域法
古细菌域
细菌域
真核生物域

种，或稱物种，是生物分类法中最後一级，位于属之下。

在生物学中,笼统的说,一个种就是一群或多或少与其它这样的群体形态不同的、能够交配繁殖的相关的有机体。用美爾(Ernst Mayr)的定义来说, 物种就是「能够(或可能)相互配育的自然种群的类群，这些类群与其它这样的类群在生殖上相互隔离着。」

中国昆虫学家陈世骧(1978)对物种所下定义为：「物种是繁殖单元，由又连续又间断的局群所组成；物种是进化单元，是生物系统线上的基本环节，是分类的基本单元。」（關於物種的定義，可详细阅读下文）。

在科学分类学中, 一个物种被赋予一个雙名法名称。该名称使用斜体印刷，属名首字母大写，紧跟一个唯一的形容词。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, 我们的种名叫「智人」（拉丁文：Homo sapiens，即有智慧的人)。

目录 1 物种的定义 2 在生物学分类上的重要性 3 "指定物种状态"的含义 4 更详细的隔离种的概念 4.1 隔离的问题 4.2 The difference question 5 物種分類的歷史 6 See also 7 External links

[编辑] 物种的定义

上面对于物种的定义来自于美爾，但多少上不太完善。原因是它假设了物种都进行有性生殖, 它把很多进行无性生殖的物种丢到了一边。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视角:

• 形态学种 (morphological species)是一群在外观上不同的生物: 例如, 我们可以分辨出鸡和鸭，因为它们长着不同形状的喙，而且鸭子的足有蹼。 在有记录的历史之前，物种就已经像这样定义了。尽管受到不少批评, 形态学种的概念仍然是日常生活中最常使用的物种概念，而且在生物学中依然扮演着重要角色，尤其是在植物学中。

• 生物学种 或 隔离种概念把物种定义为一组能够或潜在能够相互培育的生物。对于研究现存高等生物，如哺乳类、鱼类和鸟类的科学家来说，这不失为一种简洁的表达方式，但对于不进行有性生殖的生物来说这个定义是毫无意义的。它把互相配育在理论上的可能性和种群间基因流动的可能性区分开来。例如, 马和驴杂交是可行的，并且能够产生后代，然而它们仍然属于两个种;这有两个原因: 第一，因为在野生状态下，马和驴通常不会杂交。第二，因为它们结合产生的后代很少具有繁殖能力。定义生物学种的关键在于两个种群间没有显著的基因物质交流。

• 配偶识别种定义了一群能够把种群中的其它个体识别为潜在配偶的生物。正如上述隔离种的概念那样，它仅适用于进行有性生殖的生物。

• 系统发生种或者进化种或者达尔文种是一组具有相同祖先的生物; 在时间和空间上，这个世系与其它世系保持分离。在这样一组生物在的某个时候，其成员可能分道扬镳: 当其不同越来越显著的时候，这两个种群就被认为是两个不同的种了。

• 另请参阅孤雌生殖中的小种, 涵盖了那些没有经过减数分裂或有丝分裂导致每代都在基因上与上代完全相同的物种。

• 对属于原核生物的细菌和古菌来说，因为不存在有性生殖，一个种除形态和生物化学特征不能有较大差别外，还必须满足rRNA序列的相似性要大于97%。

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:

• 它是普通人区分不同生物的基础- 狗是一种，猫是另一种。
• 科学家通常使用标准的双名制命名法 (或 三名制命名法)来指代生物。
• 这是唯一的具有经验成分的分类阶元，比如断言两只动物是两个物种就不单单是对它们进行分类那么简单了。

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

• 初定的两个种分布区重叠的—它们占有相同的栖息地。 经过多年的观察，都无法在重叠区分辨出有两个以上的种群，这些种群可能在随后时间又合为一个大种群。没有生殖隔离,种群就不得以分化, 有了生殖隔离, 种群间的基因无法交流，也就无法抹掉差异。这不是说杂交完全不会发生, 只是说它可以忽略不计。通常，杂种个体无法像其亲本纯种个体那样成功的繁殖。

• 初定的两个种地理隔离—它们占有不同的地理区域。很明显, 无法直接观察到分隔两地的两个种群之间的生殖隔离情况。通常情况下也不可能通过试验来完成: 即使在豢养情况下两个初定的种能配育, 这不代表在野外它们也会自由交配, 这也不总是可以对杂种后代在进化上的适应性提供多少信息。通过其它一些试验可能会得出更多信息: 例如, 种群A的个体是否对种群B发出的求偶鸣叫录音有正确的反应呢? 有时候, 试验能够提供确切的答案。例如, 巴拿马海峡两边居住着七组看上去几乎一样的海虾(Altheus)种群, 它们在约3百万年前出现。直到不久前, 它们还被认为是相同的7个种。但是当把来自海峡两边的雄性和雌性放在一起时，它们没有交配反而大打出手。甚至如果海峡再次下沉, 这些种群在基因上还会保持隔离: 所以，它们现在被划分为不同的种。但在更多情况下, 观察和试验都无法提供确定的答案, 这是就要通过对邻近物种的经验来猜测它们之间的关系了。

• 初定的两个种地理比邻—它们的繁殖区域比邻但不重叠。这种情况很少有, 尤其在温带地区。分界线通常都是栖息地突然改变的地方(群落交错区)，比如像森林边缘或者山上的雪线等, 但有时却不大起眼。地理上的比邻指出这两个种群占有很相似的生态位导致它们无法在同一个地区共存。又因为它们不进行杂交, 可以肯定地说有一种机制（通常是行为上的）来防止种群间的基因交流，因此它们应该属于不同的种。

• 在两个种群混合区有杂交区域。典型情况下, 杂交区域会存在'纯种'种群一方或双方的代表, 加上第一代杂交种和反交得到的后代。通过对杂交区域的广度与存在疑问的两个种群典型散布区广度的比较，可以评估两个'纯种'种群之间基因交流阻力的大小。例如橡树的分布距离是鸟或松鼠携带橡子的距离; 袋食蚁兽的散布距离约为15公里，因为这是小袋食蚁兽在离巢后寻找空闲领地时通常会经过的距离。 如果杂交区域相对于散布区域越窄, 则两个种群间的基因交流就越少，它们之间就更倾向于在进化到路上继续分化。然而, 很难去预测未来杂交区域的变化; 无论把具有杂交的种群定为两个种还是一个种，都具有潜在的争议性。

• 种群的变化是渐变式的—在种群地理分布的两极, 其典型个体间差异十分明显, 但是在它们间的过渡是逐渐进行的。例如, 北澳大利亚的树袋熊比南方的亲戚来得要小而且颜色也较浅, 但是它们之间没有绝然的分界线: 越向南部，树袋熊的个体就越大而且颜色越深; 在中间地带的个体则体型和色泽都是中等。相反, 在相同的地理范围内, 黑背(北方)和白背(南方)钟鹊不从一种形态向另一种过渡;北方种群黑背, 南方种群白背, 在杂交区域“纯种”与杂合类型同样常见。树袋熊的变化是渐变式的(从北到南逐渐过渡，在小范围内的种群具有统一的外观), 但钟鹊的变化不是渐变的。在这两个例子中, 对其分类都存在一些不肯定, 但是大多数的看法是两者的南北双方都不足以构成种。由于认为南北的钟鹊基因交流受到了限制，足以称它们为亚种(而非种); 但树袋熊之间存在南北间实质的基因交流，所以专家甚至拒绝认定它们为两个亚种。

[编辑] The difference question

Obviously, 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:

• It is possible to reliably measure a quantitative difference between the two groups that does not overlap. A population has, for example, thicker

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.

• It is possible to distinguish a qualitative difference between the populations; a feature that does not vary continuously but is either entirely

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:

molecular phylogeny).

A species name can be:

• A noun in apposition with the genus: Panthera leo. The words agree in case but not necessarily in gender.
• An adjective, agreeing in case and gender with the genus: Allium sativum.
• A noun or adjective in the genitive. This is common in parasites: Xenos vesparum, Anaticola phoenicopteri. Also,

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

• Speciation

[编辑] External links

• http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html
• [http://www.sciencedaily.com/releases/2003/12/031231082553.htm 2003-12-31, ScienceDaily: Working On The 'Porsche Of Its Time': New Model For Species

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..."

• [http://www.sciencedaily.com/releases/2003/08/030808081854.htm 2003-08-08, ScienceDaily: Cross-species Mating May Be Evolutionarily Important And Lead To

Rapid Change, Say Indiana University Researchers] Quote: "...the sudden mixing of closely related species may occasionally provide the energy to impel rapid

evolutionary change..."

• [http://www.sciencedaily.com/releases/2004/01/040109064407.htm 2004-01-09 ScienceDaily: Mayo Researchers Observe Genetic Fusion Of Human, Animal Cells; May

Help Explain Origin Of AIDS] Quote: "...The researchers have discovere