📚 Natural Selection | 自然选择
Natural selection is the central mechanism of evolution and a core topic in your WJEC IGCSE Biology course. It explains how populations of living organisms adapt and change over time, leading to the great diversity of life on Earth. Understanding natural selection means grasping how variation, competition and inheritance work together to shape species.
自然选择是进化的核心机制，也是 WJEC IGCSE 生物课程的重要考点。它解释了生物种群如何随时间推移发生适应和变化，造就了地球上丰富的生命多样性。理解自然选择意味着要把握变异、竞争和遗传如何共同塑造物种。
1. What is Natural Selection? | 什么是自然选择？
Natural selection is a process in which organisms better adapted to their environment tend to survive and produce more offspring. The theory, first proposed by Charles Darwin and Alfred Russel Wallace, states that individuals with advantageous traits are more likely to pass those traits to the next generation. Over many generations, these beneficial traits become more common in the population.
自然选择是一个过程，在这个过程中，更能适应环境的生物往往能够存活并繁殖更多后代。这一理论最初由查尔斯·达尔文和阿尔弗雷德·拉塞尔·华莱士提出，它指出具有有利性状的个体更有可能将这些性状传递给下一代。经过许多代之后，这些有利性状在种群中会变得更加普遍。
In nature, there is no intentional goal or direction; natural selection simply acts on existing variation. The environment 'selects' which phenotypes are favourable. This is often described as 'survival of the fittest', where fitness refers to reproductive success, not just physical strength.
在自然界中，自然选择没有预设的目标或方向，它只是作用于现有的变异。环境会“选择”哪些表型是有利的。这常被描述为“适者生存”，其中的“适”指的是繁殖的成功，而非仅仅身体强壮。
2. Variation within Populations | 种群内的变异
Every population contains genetic variation. No two individuals are exactly alike, even within the same species. Variation arises from mutations, the random mixing of alleles during sexual reproduction (meiosis and fertilisation) and other genetic mechanisms.
每个种群都存在遗传变异。即便在同一物种内，也没有两个个体是完全相同的。变异来源于基因突变、有性生殖过程中（减数分裂和受精）等位基因的随机组合以及其他遗传机制。
Phenotypic differences might include size, colour, speed, disease resistance or ability to use certain food sources. Without variation, natural selection could not occur because there would be no differences on which selection can act.
表型差异可能包括体型、颜色、速度、抗病能力或利用特定食物来源的能力。没有变异，自然选择就无法发生，因为没有可供选择的差异。
In WJEC IGCSE exams, you should be able to explain that variation within a species is the raw material for natural selection. Mutations are the original source of new alleles, although most mutations are neutral or harmful.
在 WJEC IGCSE 考试中，你需要能够说明物种内的变异是自然选择的原材料。突变是新等位基因的最初来源，尽管大多数突变是中性的或有害的。
3. Overproduction of Offspring | 后代的过程繁殖
All species tend to produce more offspring than can survive to maturity. A single bacterial cell can divide every 20 minutes, a frog can lay thousands of eggs, and an oak tree produces thousands of acorns each year. Yet most populations remain relatively stable in size.
所有物种都会产生远远超过能够存活至成熟的后代数量。一个细菌细胞每20分钟就能分裂一次，一只青蛙能产下数千枚卵，一棵橡树每年产生数千颗橡子。然而，大多数种群的规模保持相对稳定。
This overproduction leads to a struggle for resources such as food, water, territory and mates. Many offspring die before reproducing, and only a fraction survive to pass on their genes.
这种过度繁殖导致了对食物、水、领地和配偶等资源的竞争。许多后代在繁殖前就死亡了，只有一小部分存活下来并将基因传递下去。
Darwin recognised that overproduction is a key observation. It creates the conditions under which individuals with a slight advantage are more likely to survive.
达尔文认识到过度繁殖是一个关键的观察结果。它创造了这样的条件，让那些具有微小优势的个体更有可能存活下来。
4. Struggle for Existence | 生存竞争
Because resources are finite, organisms compete within and between species. This struggle for existence includes predation, disease, competition for food and space, and the challenge of finding a mate. Organisms also face environmental pressures such as temperature changes and natural disasters.
因为资源是有限的，生物在种内和种间展开竞争。这种生存竞争包括捕食、疾病、对食物和空间的争夺，以及寻找配偶的挑战。生物还面临着温度变化和自然灾害等环境压力。
Individuals with traits that give them an edge in this struggle are more likely to become part of the surviving group. For example, a rabbit that can run slightly faster may escape predators more often, increasing its chance of reproducing.
在这场斗争中，那些性状提供优势的个体更有可能成为存活群体的一部分。例如，跑得稍快一点的兔子可能更频繁地逃脱捕食者，增加其繁殖机会。
In your WJEC IGCSE answers, link competition directly to the selective pressure that drives natural selection. Avoid vague statements; specify what organisms compete for.
在你的 WJEC IGCSE 答案中，要直接将竞争与驱动自然选择的选择压力联系起来。避免模糊的表述，要具体说明生物在竞争什么。
5. Survival of the Fittest | 适者生存
'Survival of the fittest' is a phrase often used to summarise natural selection, but in biology it refers specifically to differential reproductive success. Fitness is measured by the number of viable, fertile offspring an organism produces relative to others in the population.
“适者生存”是常用来概括自然选择的短语，但在生物学中，它特指差异性的繁殖成功率。适应度是根据一个生物相对于种群中其他个体所产生的可存活、可育后代的数量来衡量的。
An organism that lives for many years but produces few offspring may have lower fitness than one that lives only briefly but leaves many descendants. Natural selection therefore favours traits that enhance lifetime reproductive output.
一个活了很多年但后代很少的生物，其适应度可能低于一个寿命短暂但留下大量后代的生物。因此，自然选择倾向于那些能提高终生繁殖产量的性状。
You may be asked to apply this concept to data, such as comparing the survival rates of two colour morphs of a moth, or beak sizes in finches after a drought. Always link survival back to reproduction.
你可能会被要求将此概念应用于数据，比如比较一种蛾子两种颜色形态的存活率，或干旱后雀类喙的大小。答案一定要把存活与繁殖联系起来。
6. Inheritance of Favourable Traits | 有利性状的遗传
For natural selection to lead to evolutionary change, advantageous traits must be heritable. That means alleles for the successful phenotype must be passed from parents to offspring through DNA. If a trait were purely environmental and not genetic, it would not be subject to natural selection in the same way.
要使自然选择导致进化改变，有利性状必须是可遗传的。这意味着成功表型的等位基因必须通过 DNA 从亲代传递给子代。如果某个性状纯粹是环境因素造成的，并非遗传，它就不会以同样的方式受到自然选择的影响。
Over generations, the frequency of beneficial alleles increases in the gene pool, while harmful alleles become rarer. This shift in allele frequencies is the very definition of evolution.
经过许多代后，有利等位基因在基因库中的频率上升，而有害等位基因变得稀有。这种等位基因频率的变化正是进化的定义。
In WJEC IGCSE, you should be able to define evolution as a change in the inherited characteristics of a population over time. Emphasise that individuals do not evolve; populations evolve.
在 WJEC IGCSE 中，你需要能够将进化定义为种群遗传特征随时间的改变。要强调个体不会进化，进化的是种群。
7. Antibiotic Resistance in Bacteria | 细菌的抗生素耐药性
Antibiotic resistance is a clear, real‑world example of natural selection that you must know for your exam. When a bacterial population is exposed to an antibiotic, most bacteria are killed. However, a few may carry a mutation that gives them resistance.
抗生素耐药性是自然选择一个清晰且真实的例子，你必须为考试掌握。当细菌种群接触抗生素时，大多数细菌会被杀死。然而，少数细菌可能携带一个赋予它们耐药性的突变。
These resistant bacteria survive and reproduce, passing the resistance allele to their offspring. Soon, the entire population consists mainly of resistant bacteria, and the antibiotic becomes ineffective. This is why doctors are careful to prescribe antibiotics only when necessary and why patients must complete the full course.
这些耐药细菌存活下来并进行繁殖，将耐药等位基因传递给后代。很快，整个种群主要由耐药细菌组成，抗生素因而失效。这就是为什么医生仅在必要时才仔细开具抗生素，以及为什么患者必须完成整个疗程。
Horizontal gene transfer can also spread resistance between different species of bacteria, but the fundamental mechanism is still natural selection acting on variation.
水平基因转移也能在不同种类的细菌之间传播耐药性，但其基本机制仍然是自然选择作用于变异。
pressure → resistant bacteria survive → reproduce → resistance allele frequency increases.
在考试问题中，你需要描述这一顺序：变异（一些细菌天然耐药）→ 抗生素充当选择压力 → 耐药细菌存活 → 繁殖 → 耐药等位基因频率上升。
8. Evidence for Evolution | 进化的证据
Several lines of evidence support the theory of evolution by natural selection. Fossil records show gradual changes in organisms over millions of years, with simpler forms appearing in older rocks and more complex forms in younger rocks.
有多方面的证据支持自然选择进化论。化石记录显示生物在数百万年间发生了逐渐的变化，较简单的形态出现在较古老的岩石中，而较复杂的形态出现在较年轻的岩石中。
Comparative anatomy reveals homologous structures – body parts that share a common underlying anatomy because they evolved from a common ancestor. For instance, the pentadactyl limb in vertebrates, seen in humans, whales, bats and frogs, indicates shared ancestry despite different functions.
比较解剖学揭示了同源结构——这些身体部位共享共同的深层解剖结构，因为它们是从共同祖先进化而来的。例如，脊椎动物的五指肢，见于人类、鲸鱼、蝙蝠和青蛙，尽管功能不同，却表明了共同的祖先。
DNA and protein sequence comparisons provide the most powerful evidence. Species that are closely related share a greater percentage of similar sequences. For WJEC IGCSE, you can mention that humans and chimpanzees share around 98 % of their DNA, pointing to a recent common ancestor.
DNA 和蛋白质序列比较提供了最有力的证据。亲缘关系近的物种共享更高比例的相似序列。对于 WJEC IGCSE，你可以提到人类和黑猩猩共享约 98% 的 DNA，表明它们有一个较近期的共同祖先。
9. Speciation | 物种形成
Over long periods of time, natural selection can lead to the formation of new species, a process called speciation. A species is defined as a group of organisms that can interbreed to produce fertile offspring. Speciation often occurs when populations of the same species become geographically isolated.
经过漫长的时期，自然选择能导致新物种的形成，这一过程称为物种形成。物种被定义为能够互相交配并产生可育后代的一组生物。当同一物种的不同种群发生地理隔离时，物种形成经常发生。
Imagine a population of birds living on a mainland. Some are blown to an isolated island. Over many generations, the separated population experiences different environmental conditions and selection pressures. Mutations and natural selection cause the two populations to diverge genetically.
想象一个生活在陆地上的鸟类种群。其中一些被风吹到了一个孤立的岛屿上。经过许多代，被隔离的种群经历了不同的环境条件和选择压力。突变和自然选择使这两个种群发生遗传上的分化。
Eventually, even if they meet again, they can no longer interbreed successfully. The two groups have become separate species. In your WJEC IGCSE, you might be asked to explain how a single ancestral species can give rise to two or more species using this model.
最终，即使它们再次相遇，也无法成功交配。这两个群体已经变成了不同的物种。在 WJEC IGCSE 中，你可能会被要求使用这个模型解释单一祖先物种如何产生两个或更多物种。
10. Natural Selection vs Selective Breeding | 自然选择与选择育种
It is essential to distinguish natural selection from selective breeding (artificial selection). In natural selection, the environment determines which individuals survive and reproduce; humans play no role. In selective breeding, humans deliberately choose organisms with desirable traits to be the parents of the next generation.
务必区分自然选择和选择育种（人工选择）。在自然选择中，环境决定哪些个体存活并繁殖，人类不参与。在人工选择中，人类有意识地选择具有所需性状的生物作为下一代的亲本。
| Feature / 特征 | Natural Selection / 自然选择 | Selective Breeding / 选择育种 |
|---|---|---|
| Selective agent / 选择因素 | Environment / 环境 | Humans / 人类 |
| Speed / 速度 | Slow, over many generations / 缓慢，历经多代 | Relatively rapid / 相对较快 |
| Outcome / 结果 | Adaptation to environment / 对环境的适应 | Breeds with desired traits / 具有所需性状的品种 |
| Example / 例子 | Peppered moth / 桦尺蛾 | Dog breeds, crop plants / 犬种、作物 |
In selective breeding, reduced genetic variation and health problems can arise. In natural selection, heritable traits that decrease fitness tend to be eliminated from the population.
在人工选择中，可能会出现遗传变异减少和健康问题。在自然选择中，降低适应度的可遗传性状往往会从种群中被淘汰。
11. Key Terminology Summary | 关键术语总结
- Natural selection – Process by which favourable traits become more common in a population over generations. / 自然选择：有利性状在种群中代代变得更常见的过程。
- Variation – Differences between individuals within a species. / 变异：同一物种内个体间的差异。
- Mutation – A random change in DNA; source of new alleles. / 突变：DNA的随机变化，新等位基因的来源。
- Adaptation – A feature that enhances an organism's survival in its environment. / 适应：增强生物在环境中存活力的特征。
- Fitness – Reproductive success relative to others. / 适应度：相对于其他个体的繁殖成功率。
- Evolution – Change in the inherited characteristics of a population over time. / 进化：种群遗传特征随时间的改变。
- Speciation – Formation of a new species through natural selection and isolation. / 物种形成：通过自然选择和隔离形成新物种。
- Selective pressure – Environmental factor that causes some phenotypes to be more successful. / 选择压力：导致某些表型更成功的环境因素。
12. Exam Tips for WJEC IGCSE | WJEC IGCSE 考试技巧
e advantageous allele increases in frequency over generations. Avoid saying that organisms 'adapt themselves' or 'choose' to change.
回答自然选择问题时，始终使用清晰的顺序结构：陈述存在的变异 → 确定选择压力 → 解释哪种表型存活及其原因 → 将存活与繁殖增加联系起来 → 说明有利等位基因的频率在代际间上升。避免说生物“自我适应”或“选择”改变。
Practice interpreting graphs showing antibiotic resistance data, peppered moth frequencies or beak depth changes in finches. Be ready to calculate changes in allele frequency if given figures. For the peppered moth example, link the darker form to increased survival during the Industrial Revolution when tree bark became soot-covered, and the lighter form when pollution decreased.
练习解读显示抗生素耐药性数据、桦尺蛾频率或雀类喙深度变化的图表。如有数据，要能计算等位基因频率的变化。对于桦尺蛾的例子，将深色类型与工业革命期间树皮被煤烟覆盖时存活率增加联系起来，当污染减少时浅色类型数量又回升。
Remember that WJEC often awards marks for using precise terminology. Words like ‘allele’, ‘gene pool’, ‘reproductive success’ and ‘differential survival’ can earn you top marks if used correctly. Finally, always read the question to see if they ask for an example; if so, integrate one as evidence.
请记住，WJEC 常因使用精确术语而给分。像“等位基因”“基因库”“繁殖成功率”和“差异性存活”这类词语，如果使用得当，可以为你赢得高分。最后，一定要仔细读题，看他们是否要求举例；如果要求，就把例子作为证据整合进去。
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