Species and Speciation | 物种与物种形成

📚 Species and Speciation | 物种与物种形成

In A-Level Biology, a species is more than just a name given to a type of organism. Speciation is the evolutionary process by which one species splits into two or more distinct species, and it depends crucially on the development of reproductive isolation. Understanding species definitions and the mechanisms of speciation helps explain the diversity of life on Earth.

在A-Level生物中，物种不仅仅是一种生物的名称。物种形成是一个物种分裂为两个或多个不同物种的进化过程，它关键取决于生殖隔离的形成。理解物种定义和物种形成机制有助于解释地球上生命的多样性。

1. Defining a Species | 物种的定义

Biologists need a practical way to group organisms. A species is often described as a group of organisms that can interbreed under natural conditions to produce fertile offspring. This definition emphasises reproductive compatibility rather than physical appearance alone.

生物学家需要一种实用的方法来对生物进行分组。物种通常被描述为在自然条件下能够相互交配并产生可育后代的一组生物。该定义强调生殖相容性，而不仅仅是外形相似性。

For example, horses and donkeys look similar and can mate, but their offspring, the mule, is almost always sterile. Therefore, horses and donkeys remain separate species because they do not produce fertile offspring together.

例如，马和驴看起来相似且可以交配，但它们的后代骡子几乎总是不育的。因此，马和驴仍然是不同的物种，因为它们不能共同产生可育后代。

2. The Biological Species Concept | 生物学物种概念

The biological species concept, proposed by Ernst Mayr, defines a species as a group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. Gene flow within the species holds its gene pool together, while reproductive barriers prevent gene flow between species.

生物学物种概念由恩斯特·迈尔提出，将一个物种定义为实际或潜在能够相互交配的自然种群，并且与其他此类种群存在生殖隔离。物种内部的基因流使基因库保持统一，而生殖屏障阻止物种之间的基因流。

This concept is widely used for sexually reproducing animals and plants. It highlights that the key criterion is not how similar organisms look, but whether they can exchange genes successfully.

这一概念广泛用于有性生殖的动物和植物。它强调关键标准不是生物看起来有多相似，而是它们能否成功地交换基因。

3. Limitations of the Biological Species Concept | 生物学物种概念的局限性

The biological species concept cannot be applied to fossils, because mating behaviour cannot be observed in extinct organisms. It also cannot be used for organisms that reproduce asexually, such as many bacteria, or for species that self-fertilise.

生物学物种概念不能用于化石，因为已灭绝生物的繁殖行为无法观察。它也不能用于无性繁殖的生物，如许多细菌，或用于自花受精的物种。

In addition, some ring species present a problem. In a ring species, neighbouring populations can interbreed, but populations at the two ends of the ring cannot. This means there is gene flow around the ring, yet the endpoints behave as separate species.

此外，一些环状物种也带来问题。在环状物种中，相邻种群可以交配，但环两端的种群不能交配。这意味着环周围存在基因流，但两端表现得像不同物种。

4. Other Species Concepts | 其他物种概念

Because the biological species concept has limitations, biologists also use other definitions. The morphological species concept classifies organisms mainly by body form and structure. It is useful for fossils and asexual organisms but can group genetically distant look-alikes together.

由于生物学物种概念存在局限，生物学家还使用其他定义。形态学物种概念主要根据身体形态和结构对生物进行分类。它适用于化石和无性生物，但可能将遗传上相距很远但外形相似的生物归为一类。

The ecological species concept defines a species by its ecological niche, meaning the resources it uses and its role in the environment. The phylogenetic species concept defines a species as the smallest group of individuals that shares a common ancestor and has unique evolutionary features.

生态学物种概念根据生态位来定义物种，即它所利用的资源及在环境中的作用。系统发育物种概念将物种定义为共享共同祖先并具有独特进化特征的最小个体群。

Each concept has strengths and weaknesses, and the choice depends on the type of organism and the data available. Cambridge exams often expect students to discuss the biological species concept and its limitations.

每个概念都有优点和缺点，选择取决于生物类型和可用数据。剑桥考试通常期望学生讨论生物学物种概念及其局限性。

5. Reproductive Isolation: The Key Barrier | 生殖隔离：关键屏障

For speciation to occur, gene flow between populations must be reduced or stopped. Reproductive isolating mechanisms are barriers that prevent different species from interbreeding or producing fertile offspring. They are divided into prezygotic and postzygotic barriers.

要发生物种形成，种群之间的基因流必须减少或停止。生殖隔离机制是阻止不同物种交配或产生可育后代的屏障。它们分为合子前屏障和合子后屏障。

Prezygotic barriers act before fertilisation, while postzygotic barriers act after fertilisation. Together they maintain species boundaries even when ranges overlap.

合子前屏障在受精前起作用，合子后屏障在受精后起作用。它们共同维持物种边界，即使分布区重叠也是如此。

6. Prezygotic Isolation Mechanisms | 合子前隔离机制

Habitat isolation occurs when two species live in the same region but occupy different habitats, so they rarely meet. For example, two species of snake may live in the same area but one is aquatic and the other is terrestrial.

生境隔离指两个物种生活在同一地区但占据不同生境，因此很少相遇。例如，两种蛇可能生活在同一地区，但一种是水生的，另一种是陆生的。

Temporal isolation occurs when species breed at different times of day, season or year. One species of frog may breed in early spring, while a related species breeds in late summer.

时间隔离指物种在一天、一季或一年中的不同时间繁殖。一种蛙可能在早春繁殖，而近缘种在夏末繁殖。

Behavioural isolation occurs when courtship rituals, songs, pheromones or other signals differ, preventing attraction between members of different species. Many bird songs are species-specific.

行为隔离指求偶仪式、鸣叫、信息素或其他信号不同，从而阻止不同物种个体之间的吸引。许多鸟类的鸣叫具有物种特异性。

Mechanical isolation occurs when reproductive structures are incompatible. In flowering plants, differences in flower shape can prevent pollination by the wrong pollinator or species.

机械隔离指生殖结构不兼容。在开花植物中，花形的差异可以阻止错误授粉者或错误物种完成传粉。

Gametic isolation occurs when male and female gametes meet but cannot fuse because of molecular incompatibility. In many marine invertebrates, sperm and egg surface proteins do not recognise each other between species.

配子隔离指雌雄配子相遇但不能融合，因为分子不兼容。在许多海洋无脊椎动物中，不同物种的精子和卵子表面蛋白不能相互识别。

7. Postzygotic Isolation Mechanisms | 合子后隔离机制

Hybrid inviability occurs when a zygote forms, but the hybrid embryo does not survive to birth or adulthood. In some crosses between sheep and goats, the hybrid embryos die during early development.

杂种不活指合子形成，但杂种胚胎不能存活到出生或成年。在绵羊和山羊的一些杂交中，杂种胚胎在早期发育中死亡。

Hybrid sterility occurs when hybrids survive but are sterile, so they cannot pass on their genes. The mule, a hybrid between a horse and a donkey, is a classic example of hybrid sterility.

杂种不育指杂种能存活但不育，因此不能传递基因。骡子是马和驴的杂种，是杂种不育的典型例子。

Hybrid breakdown occurs when first-generation hybrids are fertile, but their offspring are weak, infertile or have reduced fitness. This reduces gene flow over subsequent generations and reinforces species separation.

杂种衰败指第一代杂种可育，但其后代虚弱、不育或适合度降低。这会在后续世代中减少基因流并加强物种分离。

8. Allopatric Speciation | 异域物种形成

Allopatric speciation occurs when a geographical barrier physically separates a population. Barriers can include mountain ranges, rivers, seas, deserts or roads. Once separated, natural selection, mutation and genetic drift act independently on each group.

异域物种形成发生在地理屏障将种群物理隔离时。屏障可包括山脉、河流、海洋、沙漠或道路。一旦隔离，自然选择、突变和遗传漂变会独立作用于每个群体。

Over many generations, the two populations accumulate genetic differences. If they later meet, they may no longer interbreed successfully because reproductive isolating mechanisms have developed. At this point, they are considered separate species.

经过许多世代，两个种群积累了遗传差异。如果它们后来相遇，可能不再成功交配，因为生殖隔离机制已经形成。此时，它们被视为不同的物种。

A classic example is the Kaibab squirrel and Abert’s squirrel in the Grand Canyon. The canyon acts as a geographical barrier that has separated the two populations, leading to the evolution of distinct species.

一个经典例子是大峡谷中的凯巴布松鼠和艾伯特松鼠。大峡谷作为地理屏障将两个种群隔开，导致它们进化为不同物种。

9. Sympatric Speciation | 同域物种形成

Sympatric speciation occurs without geographical isolation. Reproductive isolation must arise within a single interbreeding population that shares the same habitat. This process is rarer in animals but important in plants and some insects.

同域物种形成发生在没有地理隔离的情况下。生殖隔离必须在共享同一生境的单一种群内部出现。这一过程在动物中较罕见，但在植物和一些昆虫中很重要。

In sympatric speciation, a genetic change can suddenly produce reproductive isolation. Polyploidy in plants is the best-known example, but shifts in host preference or feeding site can also drive sympatric speciation in insects.

在同域物种形成中，遗传变化可以突然产生生殖隔离。植物中的多倍体是最著名的例子，但寄主偏好或取食地点的改变也可以驱动昆虫的同域物种形成。

For example, apple maggot flies in North America have begun to diverge into groups that feed and mate on apples instead of native hawthorn. This host shift reduces gene flow between the two groups.

例如，北美的苹果实蝇已开始分化为取食并在苹果上交配的群体，而不是在原生山楂上。这种寄主转移减少了两个群体之间的基因流。

10. Polyploidy and Instant Speciation | 多倍体与瞬时物种形成

Polyploidy is a sudden increase in the number of complete sets of chromosomes. A diploid organism has two sets of chromosomes (2n), while a tetraploid has four sets (4n). Polyploidy can create a new species in a single generation because polyploid individuals are often reproductively isolated from their diploid relatives.

多倍体是完整染色体组数的突然增加。二倍体生物有两套染色体（2n），四倍体有四套染色体（4n）。多倍体可以在一个世代内产生新物种，因为多倍体个体通常与其二倍体亲属存在生殖隔离。

Autopolyploidy occurs within one species when a diploid individual produces diploid gametes due to non-disjunction. If two diploid gametes fuse, the result is a tetraploid offspring. When a tetraploid crosses with a diploid, the triploid hybrid is usually sterile because meiosis is unbalanced.

同源多倍体发生在同一物种内，二倍体个体由于不分离产生二倍体配子。如果两个二倍体配子融合，就会产生四倍体后代。当四倍体与二倍体杂交时，三倍体杂种通常不育，因为减数分裂不平衡。

Allopolyploidy occurs between two different species. A hybrid may form, and if its chromosome number doubles, it can become fertile. Bread wheat is an allopolyploid formed from the hybridisation of different grass species, with six sets of chromosomes (6n).

异源多倍体发生在两个不同物种之间。可能形成杂种，如果其染色体数目加倍，就可以变得可育。普通小麦是由不同草种杂交形成的异源多倍体，具有六套染色体（6n）。

11. Adaptive Radiation and Speciation Patterns | 适应辐射与物种形成模式

Adaptive radiation is the rapid evolution of many diverse species from a single ancestral species when new habitats or ecological niches become available. It often happens after mass extinctions or when organisms colonise remote islands or lakes.

适应辐射是当一个祖先物种进入新的栖息地或生态位时，迅速演化出许多不同物种的过程。它通常发生在大灭绝之后或生物迁入偏远岛屿或湖泊时。

Different populations adapt to different food sources, microhabitats or feeding strategies. Natural selection favours different traits in each environment, leading to reproductive isolation and the formation of several species from one ancestor.

不同种群适应不同的食物来源、微生境或取食策略。自然选择在每个环境中青睐不同的性状，导致生殖隔离，并从一个祖先形成多个物种。

Darwin’s finches on the Galápagos Islands show adaptive radiation. From a common ancestor, they evolved different beak shapes and sizes suited to eating seeds, insects, cactus flowers or even blood.

加拉帕戈斯群岛上的达尔文雀展示了适应辐射。它们从一个共同祖先演化出不同的喙形状和大小，适应取食种子、昆虫、仙人掌花甚至血液。

12. Speciation as a Continuous Process | 物种形成的连续性

Speciation is usually a gradual process rather than an instant event, except in cases such as polyploidy. Populations may become partially reproductively isolated before full speciation is complete. Hybrid zones and ring species show intermediate stages of this continuum.

除多倍体等情况外，物种形成通常是一个渐进过程，而不是瞬间事件。种群在完全形成物种之前可能已部分生殖隔离。杂交区和环状物种展示了这一连续过程的中间阶段。

Understanding that species boundaries can be fuzzy is important for studying evolution. It also explains why different species concepts sometimes disagree. The formation of a new species requires the accumulation of reproductive barriers, and these barriers can strengthen over time.

理解物种边界可能模糊这一点对于研究进化很重要。它也解释了为什么不同的物种概念有时不一致。新物种的形成需要生殖屏障的积累，而这些屏障会随着时间增强。

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Species and Speciation | 物种与物种形成