📚 Natural Selection: IGCSE CIE Biology Key Points | IGCSE CIE 生物：自然选择 考点精讲
Natural selection is the fundamental mechanism of evolution first proposed by Charles Darwin and Alfred Russel Wallace. It explains how populations become better adapted to their environments over successive generations. In the IGCSE CIE Biology syllabus, you need to understand the principles of variation, overproduction, competition, survival of the fittest and inheritance, as well as being able to apply these concepts to real-world examples such as antibiotic resistance and the peppered moth.
自然选择是查尔斯·达尔文和阿尔弗雷德·拉塞尔·华莱士提出的进化基本机制。它解释了种群如何在连续的世代中变得更能适应其环境。在 IGCSE CIE 生物学大纲中，你需要理解变异、过度繁殖、竞争、适者生存和遗传等原理，并能将这些概念应用于抗生素耐药性和桦尺蛾等现实例子。
1. The Principle of Natural Selection | 自然选择的原理
Charles Darwin, after his voyage on HMS Beagle, developed the theory of evolution by natural selection. He observed that individuals in a population show variation, and those with characteristics best suited to the environment are more likely to survive and reproduce, passing those favourable traits to their offspring. Over many generations, the population evolves.
查尔斯·达尔文在乘坐小猎犬号航行后，发展了自然选择进化理论。他观察到种群中的个体存在变异，那些具有最适应环境特征的个体更有可能生存和繁殖，并将这些有利性状传递给后代。经过许多世代，种群发生进化。
Alfred Russel Wallace independently reached similar conclusions, prompting Darwin to publish ‘On the Origin of Species’ in 1859. The core idea is that nature ‘selects’ individuals that are best adapted, leading to gradual changes in species.
阿尔弗雷德·拉塞尔·华莱士独立得出了相似的结论，促使达尔文于 1859 年发表了《物种起源》。其核心理念是大自然“选择”了最能适应的个体，导致物种逐渐变化。
2. Variation within Species | 物种内的变异
Variation means differences between individuals of the same species. These differences can be structural, functional or behavioural. Variation arises primarily from genetic mutations, meiosis (crossing over and independent assortment) and the random fusion of gametes during sexual reproduction.
变异是指同一物种个体之间的差异。这些差异可以是结构上的、功能上的或行为上的。变异主要来源于基因突变、减数分裂（交叉互换和独立分配）以及有性生殖过程中配子的随机融合。
Most variations are continuous (e.g. height, skin colour) or discontinuous (e.g. blood groups). Only variation that is genetically inherited can be acted upon by natural selection, because environmentally acquired traits are not passed on to offspring.
大多数变异是连续的（如身高、肤色）或不连续的（如血型）。只有遗传的变异才能被自然选择所作用，因为环境获得的性状不会传递给后代。
3. Overproduction of Offspring | 后代过度繁殖
All species tend to produce more offspring than the environment can support. For example, a female rabbit may produce dozens of young in a year, yet the rabbit population does not increase indefinitely. This leads to a struggle for existence, as resources such as food, water and shelter are limited.
所有物种都倾向于产生超过环境承载能力的后代。例如，一只雌兔一年内可能产下几十只幼崽，但兔子的种群数量并不会无限增长。由于食物、水和栖息地等资源有限，这导致了生存斗争。
Overproduction is crucial because it creates competition. If every offspring survived, natural selection would not occur. The fact that many individuals die before reproducing means that only those with advantageous traits are likely to become parents.
过度繁殖至关重要，因为它产生了竞争。如果每个后代都存活，自然选择就不会发生。许多个体在繁殖前死亡的事实意味着，只有那些具有有利性状的个体才有可能成为亲本。
4. Competition and Struggle for Survival | 竞争与生存斗争
Competition occurs between individuals of the same species (intraspecific) and between different species (interspecific). Organisms compete for limited resources: food, water, light, mates and territory. Those that are better adapted to obtain these resources survive longer and have a greater chance of reproducing.
竞争发生在同一物种的个体之间（种内竞争）和不同物种之间（种间竞争）。生物为有限的资源竞争：食物、水、光、配偶和领地。那些能更好地获取这些资源的个体存活时间更长，繁殖机会更大。
Competition can also involve escaping predators or resisting diseases. In any environment, the individuals that can outcompete others leave more offspring, gradually shifting the characteristics of the population.
竞争还涉及逃避捕食者或抵抗疾病。在任何环境中，能在竞争中胜出的个体留下更多的后代，逐渐改变种群的特征。
5. Survival of the Fittest | 适者生存
‘Fitness’ in biological terms means the ability to survive and produce fertile offspring. Individuals with phenotypes that give a survival or reproductive advantage are said to have higher fitness. These advantageous alleles increase in frequency over generations.
生物学上的“适应度”意味着生存并产生可育后代的能力。具有带来生存或繁殖优势的表现型的个体，被称作具有更高的适应度。这些有利等位基因的频率会代代增加。
Note that ‘fittest’ does not necessarily mean the strongest or biggest. It could refer to better camouflage, more efficient metabolism, resistance to disease, or any heritable trait that increases the chance of leaving viable offspring.
注意，“最适者”并不一定意味着最强壮或最大。它可以指更好的伪装、更高效的新陈代谢、对疾病的抵抗力，或任何增加留下可存活后代机会的可遗传性状。
6. Inheritance of Advantageous Traits | 有利性状的遗传
For natural selection to cause evolutionary change, the advantageous traits must be heritable. Organisms with beneficial alleles survive and reproduce, passing those alleles to their offspring. Over time, the proportion of individuals with the favourable trait increases in the population.
为了使自然选择引起进化变化，有利性状必须是可遗传的。带有有利等位基因的生物生存并繁殖，将那些等位基因传递给后代。随着时间的推移，种群中具有有利性状的个体比例增加。
If an environmental change occurs, different traits may become advantageous. The alleles that once had no selective advantage can become beneficial, leading to directional selection and shifts in the population’s genetic makeup.
如果环境发生变化，不同的性状可能变得有利。曾经没有选择优势的等位基因可以变得有利，导致定向选择和种群遗传结构的转变。
7. Darwin’s Finches – A Classic Example | 达尔文雀——经典例子
Darwin observed several species of finches on the Galápagos Islands, each with beak shapes adapted to different food sources. Some had large, strong beaks for cracking seeds, while others had slender beaks for catching insects or probing flowers. These finches all evolved from a common ancestral species.
达尔文在加拉帕戈斯群岛上观察到几种雀类，每种雀的喙形都适应不同的食物来源。有些具有大而强壮的喙用于敲碎种子，而另一些具有细长的喙用于捕虫或探入花朵。这些雀类都从共同的祖先物种进化而来。
Variation in beak size and shape was inherited. During droughts, finches with larger, more powerful beaks could feed on tough seeds and survive, while those with smaller beaks died out. Over generations, the average beak size in the population increased – a clear demonstration of natural selection.
喙的大小和形状的变异是可遗传的。在干旱期间，具有更大、更有力的喙的雀类能够以坚硬的种子为食并存活下来，而喙较小的雀类则死亡。经过几代，种群的平均喙大小增加了——这是自然选择的明确证明。
8. Peppered Moth – Evolution in Action | 桦尺蛾——进化在行动
The peppered moth (Biston betularia) provides a famous example of natural selection in response to environmental pollution. Before the Industrial Revolution, most peppered moths were light-coloured with speckled wings, well camouflaged on lichen-covered tree trunks. Dark-coloured (melanic) moths were rare.
桦尺蛾（Biston betularia）提供了一个响应环境污染的自然选择的著名例子。在工业革命之前，大多数桦尺蛾颜色浅且有斑点，在覆盖着地衣的树干上伪装得很好。深色（黑化）蛾很罕见。
Industrial pollution killed lichens and blackened tree trunks with soot. Light moths now stood out and were eaten by birds, while dark moths were better camouflaged. The frequency of the dark allele increased dramatically, and in some areas over 90% of moths were dark. Clean air regulations later reversed this trend, showing evolution can be bi-directional.
工业污染杀死了地衣，并用煤烟使树干变黑。浅色蛾现在变得显眼并被鸟类捕食，而深色蛾则伪装得更好。深色等位基因的频率急剧增加，在一些地区超过 90% 的蛾是深色的。后来，清洁空气法规逆转了这一趋势，表明进化可以是双向的。
9. Antibiotic Resistance in Bacteria | 细菌的抗生素耐药性
Antibiotic resistance is a clear and medically significant example of natural selection. In a bacterial population, a few individuals may carry a mutation that makes them resistant to a particular antibiotic. When the antibiotic is used, non-resistant bacteria are killed, but the resistant ones survive.
抗生素耐药性是自然选择的一个清晰且具有重要医学意义的例子。在一个细菌种群中，少数个体可能携带某种突变，使它们对特定抗生素具有耐药性。当使用该抗生素时，非耐药细菌被杀死，而耐药细菌存活下来。
These resistant bacteria then reproduce rapidly, passing on the resistance allele to their offspring. Within a short time, the entire population may become resistant, making the antibiotic ineffective. This is why it is crucial to complete prescribed courses of antibiotics and avoid overuse.
这些耐药细菌随后迅速繁殖，将耐药性等位基因传递给后代。在短时间内，整个种群可能变得耐药，使抗生素失效。这就是为什么完成处方抗生素疗程并避免过度使用至关重要。
10. Speciation – Formation of New Species | 物种形成——新物种的产生
When populations of the same species become geographically isolated (e.g. by a river, mountain or sea), they experience different environmental conditions and selective pressures. Over many generations, natural selection operates differently on each group, leading to divergence in their characteristics.
当同一物种的种群被地理隔离（例如被河流、山脉或海洋隔开），它们经历不同的环境条件和选择压力。经过许多代，自然选择对每个群体产生不同的作用，导致其特征分化。
If the differences become so great that individuals from the two populations can no longer interbreed to produce fertile offspring, a new species has formed. This process is called allopatric speciation and highlights how natural selection can ultimately lead to biodiversity.
如果差异变得如此之大，以至于两个种群的个体无法再进行异种交配产生可育后代，新物种就形成了。这一过程被称为异域物种形成，突显了自然选择最终如何导致生物多样性。
11. Comparing Darwin and Lamarck | 比较达尔文与拉马克
Jean-Baptiste Lamarck proposed an earlier theory of evolution based on the inheritance of acquired characteristics. He suggested that if an organism used a body part extensively, it would become more developed, and that this development would be passed on to its offspring. For example, giraffes stretching to reach leaves would pass on longer necks.
让-巴蒂斯特·拉马克提出了一个更早的进化理论，基于获得性性状的遗传。他提出，如果一个生物体大量使用某个身体部位，它就会变得更加发达，而这种发育会传递给它的后代。例如，长颈鹿为够到树叶而伸长脖子，会遗传更长的脖子。
Darwin’s theory, supported by genetics, shows that only traits controlled by genes can be inherited. Acquired changes (such as increased muscle size from exercise) are not passed on. The critical difference is the source of variation: random genetic mutations and sexual reproduction, not an organism’s needs or efforts.
达尔文的理论得到了遗传学的支持，表明只有由基因控制的性状才能被遗传。获得性变化（如因锻炼而增大的肌肉）不会被传递。关键区别在于变异的来源：随机的基因突变和有性生殖，而不是生物的需求或努力。
12. Summary and Exam Tips | 总结与考试技巧
To describe natural selection in an exam answer, always follow the sequence: 1. variation exists in the population; 2. overproduction leads to competition; 3. individuals with advantageous characteristics are more likely to survive; 4. they reproduce and pass on the beneficial alleles; 5. over many generations, these alleles become more common in the population.
在考试答案中描述自然选择时，始终遵循以下顺序：1. 种群中存在变异；2. 过度繁殖导致竞争；3. 具有有利特征的个体更有可能生存；4. 它们繁殖并将有利等位基因传递给后代；5. 经过许多代，这些等位基因在种群中变得更加常见。
Be ready to apply these steps to specific examples such as antibiotic resistance, peppered moths, or Darwin’s finches. Use precise terminology: allele frequency, selective advantage, phenotype, genotype. Avoid Lamarckian phrasing such as ‘they needed to change’ or ‘they adapted themselves’.
准备好将这些步骤应用于具体例子，如抗生素耐药性、桦尺蛾或达尔文雀。使用精确术语：等位基因频率、选择优势、表现型、基因型。避免拉马克式的措辞，如“它们需要改变”或“它们自身适应了”。
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