Speciation in IGCSE Edexcel Biology | IGCSE Edexcel 生物：物种形成 考点精讲

📚 Speciation in IGCSE Edexcel Biology | IGCSE Edexcel 生物：物种形成 考点精讲

Speciation is the evolutionary process by which new biological species arise. In IGCSE Edexcel Biology, understanding how one species can split into two or more distinct species involves concepts of variation, natural selection, isolation, and genetic divergence. This article covers key points, definitions, mechanisms, and examples to help you master the topic for your exam.

物种形成是新物种产生的进化过程。在 IGCSE Edexcel 生物学中，理解一个物种如何分裂成两个或多个不同物种，涉及变异、自然选择、隔离和遗传分化等概念。本文涵盖关键要点、定义、机制和实例，助你掌握考点，从容应考。

1. What Is a Species? | 什么是物种？

A species is defined as a group of organisms that can interbreed to produce fertile offspring. This means members of the same species share similar characteristics, genes, and can reproduce naturally, giving rise to viable, fertile young.

物种被定义为能相互交配并产生可育后代的一群生物。这意味着同一物种的成员具有相似的特征和基因，能够自然繁殖，并产下能存活且可育的后代。

This biological species concept is the cornerstone for understanding speciation: if two populations can no longer interbreed successfully, they are considered separate species.

这种生物学物种概念是理解物种形成的基础：如果两个种群不能再成功交配，它们就被视为不同的物种。

2. The Process of Speciation | 物种形成的过程

Speciation occurs when populations of the same species become reproductively isolated. This isolation prevents gene flow between them. Over time, each population accumulates different mutations and undergoes natural selection under different environmental pressures. Eventually, they become so genetically distinct that they can no longer interbreed to produce fertile offspring, even if the barrier is removed.

当同一物种的种群变得生殖隔离时，物种形成就会发生。这种隔离阻止了它们之间的基因流动。随着时间推移，每个种群积累不同的突变，并在不同的环境压力下经历自然选择。最终，它们遗传上差异如此之大，以至于即使屏障消失，它们也不能再交配产生可育后代。

3. Geographic Isolation and Allopatric Speciation | 地理隔离与异域物种形成

Allopatric speciation is the most common form of speciation in nature. It begins with a physical barrier such as a mountain range, river, or ocean that splits a population into two or more isolated groups. This is called geographic isolation.

异域物种形成是自然界中最常见的物种形成方式。它始于物理屏障，如山脉、河流或海洋，将一个种群分成两个或多个隔离的群体，这称为地理隔离。

Once separated, the two groups experience different environmental conditions. Mutations that arise in one group cannot spread to the other. Different selection pressures lead to different adaptations. Over many generations, the populations diverge genetically. If they later come into contact, they are reproductively incompatible.

一旦分开，两个群体经历不同的环境条件。在一个群体中出现的突变无法传播到另一个群体。不同的选择压力导致不同的适应性。经过许多世代，种群在遗传上分化。如果它们后来再次接触，它们已产生生殖不兼容。

Example: Darwin’s finches on the Galápagos Islands. Ancestral finches blown from the mainland colonised different islands. Geographic isolation and different food sources led to the evolution of distinct beak shapes and ultimately separate species.

示例：加拉帕戈斯群岛上的达尔文雀。从大陆吹来的祖先雀在不同岛屿定居。地理隔离和不同的食物来源导致了不同喙形的进化，最终形成独立物种。

4. Sympatric Speciation | 同域物种形成

Sympatric speciation occurs without a physical barrier. Populations live in the same area but become reproductively isolated through other mechanisms, such as changes in chromosome number (polyploidy), behavioural differences, or habitat preferences within the same location.

同域物种形成没有物理屏障也能发生。种群生活在同一地区，但通过其他机制出现生殖隔离，如染色体数目变化（多倍体）、行为差异或在同一地点的栖息地偏好差异。

Polyploidy is common in plants and is a key mechanism of sympatric speciation. An error during meiosis can produce diploid gametes, which fuse to form a tetraploid individual. This tetraploid can self-pollinate or reproduce with other tetraploids but is reproductively isolated from the original diploid population.

多倍体在植物中很常见，是同域物种形成的关键机制。减数分裂期间的错误可产生二倍体配子，这些配子融合形成四倍体个体。该四倍体可以自花授粉或与其他四倍体繁殖，但与原始二倍体种群生殖隔离。

5. Reproductive Isolation Mechanisms | 生殖隔离机制

Reproductive isolation is the key to speciation. There are prezygotic barriers (before fertilisation) and postzygotic barriers (after fertilisation).

生殖隔离是物种形成的关键。存在合子前隔离（受精前）和合子后隔离（受精后）。

• Prezygotic barriers: temporal isolation (different breeding times), ecological isolation (different habitats), behavioural isolation (different courtship rituals), mechanical isolation (incompatible reproductive organs), and gametic isolation (sperm and egg cannot fuse).
• 合子前隔离：时间隔离（不同繁殖期）、生态隔离（不同栖息地）、行为隔离（不同求偶仪式）、机械隔离（生殖器官不匹配）和配子隔离（精卵无法融合）。
• Postzygotic barriers: hybrid inviability (hybrid zygote does not develop), hybrid sterility (offspring is sterile, e.g., mule), and hybrid breakdown (F1 fertile but F2 weak or sterile).
• 合子后隔离：杂种不活（杂种合子不发育）、杂种不育（后代不育，如骡子）和杂种衰败（F1 可育但 F2 弱小或不育）。

These barriers ensure that gene pools remain separate, completing the speciation process.

这些屏障确保基因库保持分离，完成物种形成过程。

6. Role of Natural Selection and Mutation | 自然选择与突变的作用

Genetic variation within a population, caused by mutation and sexual reproduction, provides the raw material for natural selection. In isolated populations, different alleles are favoured depending on local environmental conditions. Over time, allele frequencies change, leading to divergence.

由突变和有性生殖引起的种群内遗传变异为自然选择提供了原材料。在隔离的种群中，根据当地环境条件，不同等位基因被青睐。随着时间推移，等位基因频率发生变化，导致分化。

If a geographical barrier separates a population, each subpopulation adapts to its own niche. For example, a plant species might evolve thicker leaves in a dry area and thinner leaves in a wet area. Accumulation of many small changes can eventually lead to reproductive isolation.

如果地理屏障分隔种群，每个亚种群适应其自身生态位。例如，一种植物在干旱地区可能进化出更厚的叶片，在潮湿地区进化出更薄的叶片。许多微小变化的积累最终可导致生殖隔离。

7. Evidence for Speciation | 物种形成的证据

Fossil records show gradual changes in organisms over time, with transitional forms linking ancestral and modern species. These demonstrate how populations diverged.

化石记录显示了生物随时间的逐渐变化，过渡形态连接了祖先与现代物种。这表明种群如何分化。

DNA sequencing and molecular phylogenetics provide strong evidence. Closely related species have more similar DNA sequences than distantly related ones. By comparing genetic sequences, scientists can estimate when two species diverged from a common ancestor.

DNA 测序和分子系统发育提供了强有力的证据。亲缘关系近的物种比关系远的物种具有更相似的 DNA 序列。通过比较遗传序列，科学家可以估算两个物种何时从共同祖先分化。

Ring species, such as the greenish warbler around the Himalayas, show a continuum of populations. Neighbouring populations can interbreed, but the populations at the two ends of the ring are reproductively isolated, showing speciation in action.

环物种，如围绕喜马拉雅山的暗绿柳莺，展示了种群的连续体。相邻种群可以交配，但环两端的种群是生殖隔离的，展示了进行中的物种形成。

8. Speciation and Extinction | 物种形成与灭绝

Speciation increases biodiversity by generating new species. However, extinction decreases it. Species may become extinct due to catastrophic events, climate change, habitat loss, or competition. The rate of speciation versus extinction determines overall biodiversity trends.

物种形成通过产生新物种增加生物多样性。然而，灭绝则减少多样性。物种可能因灾难性事件、气候变化、栖息地丧失或竞争而灭绝。物种形成速率与灭绝速率的对比决定了整体生物多样性趋势。

Mass extinctions have occurred several times in Earth’s history. After mass extinctions, adaptive radiation often follows, where many new species evolve rapidly to fill empty ecological niches. This demonstrates the dynamic nature of speciation.

地球历史上发生过数次大灭绝。大灭绝之后，常发生适应辐射，许多新物种快速进化以填充空缺的生态位。这体现了物种形成的动态本质。

9. Exam Tips for IGCSE Edexcel | IGCSE Edexcel 考试技巧

When answering speciation questions, always explain the sequence logically: start with a single interbreeding population, introduce an isolating barrier (geographical or reproductive), describe variation and different selection pressures, mention accumulation of genetic differences, and conclude with the inability to produce fertile offspring.

回答物种形成问题时，务必逻辑清晰地解释顺序：从单一可交配种群开始，引入隔离屏障（地理或生殖），描述变异和不同选择压力，提及遗传差异的积累，并以无法产生可育后代作结。

Use precise terminology: ‘reproductive isolation’, ‘gene pool’, ‘allele frequency’, ‘geographical isolation’, ‘natural selection’. Diagrams of Darwin’s finches or ring species can support your answer if well annotated.

使用精确术语：“生殖隔离”、“基因库”、“等位基因频率”、“地理隔离”、“自然选择”。达尔文雀或环物种的图示若注释得当，可为你的答案增色。

Remember that sympatric speciation via polyploidy is especially relevant in plants and occurs without a physical barrier. Be ready to give examples like wheat or primrose.

记住通过多倍体的同域物种形成在植物中尤其相关，且无需物理屏障。准备好给出小麦或报春花等例子。

10. Key Definitions Summary Table | 关键定义总结表

Term	Definition	术语	定义
Species	Group of organisms that can interbreed to produce fertile offspring	物种	能够相互交配并产生可育后代的一群生物
Speciation	Formation of new species through reproductive isolation	物种形成	通过生殖隔离形成新物种
Allopatric speciation	Speciation due to a physical barrier separating populations	异域物种形成	因物理屏障分隔种群而发生的物种形成
Sympatric speciation	Speciation without a physical barrier, often via polyploidy	同域物种形成	无物理屏障的物种形成，常通过多倍体
Reproductive isolation	Barriers preventing gene flow between populations	生殖隔离	阻止种群之间基因交流的屏障
Gene pool	All alleles present in a population	基因库	一个种群中所有等位基因的总和

11. Common Misconceptions | 常见误区

‘Evolution always leads to perfection.’ In reality, evolution is driven by natural selection acting on existing variation; it does not produce ideal organisms, only those suited enough to survive and reproduce in their current environment.

“进化总是导致完美。”实际上，进化是由自然选择作用于现有变异驱动的；它不会产生理想的生物体，只会产生足以在当前环境中存活和繁殖的生物体。

‘Individuals evolve.’ This is incorrect. Populations evolve over generations. An individual cannot change its genetic makeup in response to the environment (except for epigenetic changes, not covered at IGCSE).

“个体进化。”这是错误的。种群经过多代进化。个体不能根据环境改变其遗传组成（除表观遗传变化外，IGCSE 不涉及）。

‘Speciation happens quickly.’ Speciation usually takes thousands or millions of years, although polyploidy can cause instant reproductive isolation in a single generation in plants.

“物种形成发生得很快。”物种形成通常需要数千年或数百万年，尽管多倍体可在植物的一代中造成即时生殖隔离。

12. Practice Question | 练习题

Explain how a population of rabbits separated by a new river might evolve into two distinct species. (6 marks)

解释被一条新河流分隔的兔子种群如何可能进化成两个不同物种。(6 分)

Model Answer: 1) Originally, rabbits form a single interbreeding population with a shared gene pool. 2) A new river forms, causing geographical isolation, preventing gene flow between groups. 3) The two groups experience different environmental conditions (e.g., predators, food, climate). 4) Different mutations and natural selection lead to different advantageous alleles being selected in each group. 5) Over many generations, allele frequencies change, and the populations become genetically distinct. 6) Eventually, even if the river dries up, the two populations cannot interbreed to produce fertile offspring; they are now separate species due to reproductive isolation.

参考答案：1) 最初，兔子形成一个单一可交配种群，拥有共同基因库。2) 一条新河流形成，造成地理隔离，阻止群体间基因交流。3) 两组兔子经历不同的环境条件（如捕食者、食物、气候）。4) 不同的突变和自然选择导致各组中不同的有利等位基因被选择。5) 经过许多代，等位基因频率改变，种群遗传上变得不同。6) 最终，即使河流干涸，两个种群也不能交配产生可育后代；由于生殖隔离，它们现在是不同的物种。

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