Natural Selection | 自然选择考点精讲

📚 Natural Selection | 自然选择考点精讲

Natural selection is a fundamental concept in biology and the central mechanism of evolution. For CIE A-Level Biology, understanding natural selection is essential for explaining how species adapt, how antibiotic resistance develops, and how biodiversity arises. This article provides a detailed review of the key points, common exam questions, and how to avoid typical pitfalls.

自然选择是生物学的核心概念，也是进化的主要机制。在 CIE A-Level 生物学中，理解自然选择对于解释物种如何适应环境、抗生素耐药性如何产生以及生物多样性如何形成至关重要。本文将详细梳理考点、常见题型以及常见错误的避免方法。

1. Introduction to Natural Selection | 自然选择简介

Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring. This non-random process leads to changes in the allele frequencies of a population over generations.

自然选择是指更能适应环境的生物更有可能生存并繁殖更多后代的过程。这一非随机过程导致种群中等位基因频率在代际间发生变化。

The theory was independently proposed by Charles Darwin and Alfred Russel Wallace in the 19th century, and it remains the best explanation for the diversity of life on Earth.

该理论由查尔斯·达尔文和阿尔弗雷德·拉塞尔·华莱士在 19 世纪独立提出，至今仍是解释地球生命多样性的最佳理论。

Natural selection acts on individuals, but populations evolve. This distinction is important in exam answers.

自然选择作用于个体，但进化发生在种群层面。这一区别在考试答案中很重要。

2. Darwin’s Observations and Inferences | 达尔文的观察与推论

Darwin’s theory of natural selection was based on several key observations. First, all species have the potential for overproduction of offspring, which would lead to exponential population growth if unchecked.

达尔文的自然选择理论基于几个关键观察。首先，所有物种都有过度繁殖的潜力，如果不加限制，将导致种群呈指数增长。

Second, natural populations tend to remain stable in size over long periods. Third, resources such as food and space are limited.

其次，自然界中种群规模在长时间内趋于稳定。第三，食物和空间等资源是有限的。

From these observations, Darwin inferred that there is a struggle for existence, and that individuals with advantageous traits are more likely to survive and reproduce (differential survival and reproduction).

从这些观察中，达尔文推论存在生存斗争，拥有有利性状的个体更有可能生存和繁殖（差异生存与繁殖）。

He also noted that there is variation within populations, and much of this variation is heritable. Together, these ideas form the core of natural selection.

他还注意到种群内存在变异，且这种变异大多是可遗传的。这些观点共同构成了自然选择的核心。

3. Overproduction and Struggle for Existence | 过度繁殖与生存斗争

Most organisms produce more offspring than can possibly survive. For example, a single salmon can lay thousands of eggs, but only a few will reach adulthood.

大多数生物产生的后代数量超过了能够生存的数量。例如，一条鲑鱼可以产下数千颗卵，但只有少数能长成成鱼。

This leads to competition for limited resources (food, mates, territory) and a struggle for existence. Not all individuals will survive and reproduce; those with characteristics best suited to the environment have a selective advantage.

这导致了对有限资源（食物、配偶、领地）的竞争，即生存斗争。并非所有个体都能生存和繁殖；那些性状最适应环境的个体具有选择优势。

The result is that only a fraction of the offspring contribute genes to the next generation, and the population is shaped by this differential survival.

结果是只有一小部分后代将基因传递给下一代，种群在此差异生存作用下被塑造。

4. Variation and Its Sources | 变异及其来源

Phenotypic variation is essential for natural selection; without it, all individuals would be equally adapted and no differential survival could occur.

表型变异是自然选择的必要条件；没有变异，所有个体适应度相同，就不会发生差异生存。

Genetic variation arises from mutation, the ultimate source of new alleles. Additionally, sexual reproduction shuffles existing alleles through several mechanisms:

遗传变异来源于突变，它是新等位基因的最终来源。此外，有性生殖通过以下几种机制重组现有等位基因：

Mutations – random changes in DNA base sequence | 突变 – DNA碱基序列的随机变化
Crossing over during prophase I of meiosis | 减数分裂前期 I 的交叉互换
Independent assortment of homologous chromosomes in meiosis I | 减数分裂 I 中同源染色体的独立分配
Random fertilisation of gametes | 配子的随机受精

Environmental factors can also influence phenotype, but only heritable variation is subject to natural selection.

环境因素也能影响表型，但只有可遗传的变异才受自然选择作用。

5. Survival of the Fittest and Differential Reproduction | 适者生存与差异繁殖

‘Survival of the fittest’ is often misunderstood. Fitness in biology refers to an organism’s ability to survive and produce fertile offspring that in turn contribute to the gene pool of the next generation.

‘适者生存’常被误解。在生物学中，适合度（fitness）是指生物生存并产生可育后代的的能力，这些后代进而为下一代的基因库做贡献。

Individuals with higher fitness pass on their advantageous alleles more frequently. Over many generations, these alleles increase in frequency, leading to adaptation.

适合度更高的个体更频繁地传递有利等位基因。经过多代，这些等位基因频率上升，导致适应。

A classic example is the giraffe’s long neck: giraffes with slightly longer necks could reach higher leaves, had a better food supply during droughts, and thus survived and reproduced more than shorter-necked individuals.

一个经典例子是长颈鹿的长颈：脖子稍长的长颈鹿能吃到更高的树叶，在干旱时食物供应更好，因此比短颈个体生存和繁殖得更多。

6. Natural Selection in Action: Industrial Melanism | 自然选择的实例：工业黑化

The peppered moth (Biston betularia) is a classic case of directional selection driven by environmental change. Before the Industrial Revolution, the light-coloured (typica) form was well camouflaged against lichen-covered tree trunks, while the dark (carbonaria) form was rare and easily spotted by birds.

桦尺蛾是环境变化驱动定向选择的经典案例。工业革命前，浅色型（typica）在地衣覆盖的树干上伪装良好，而深色型（carbonaria）罕见，容易被鸟类发现。

During the Industrial Revolution, air pollution killed lichens and blackened tree trunks with soot. The dark form became camouflaged, while the light form was now conspicuous. As a result, the frequency of the carbonaria allele increased dramatically.

在工业革命期间，空气污染杀死了地衣，煤烟使树干变黑。深色型获得了伪装，而浅色型变得显眼。因此，深色型等位基因频率急剧上升。

After the Clean Air Acts in the 1950s, lichens returned, and the light form again became more common. This shows how natural selection can shift allele frequencies in response to environmental changes.

在 20 世纪 50 年代《清洁空气法案》之后，地衣恢复，浅色型又变得更为常见。这表明自然选择如何随环境变化而改变等位基因频率。

This example clearly illustrates the steps: pre-existing variation, a change in the environment creating a new selection pressure, and differential reproductive success leading to a change in the population’s gene pool.

这个例子清楚地说明了步骤：预先存在的变异，环境变化产生新的选择压力，差异繁殖成功导致种群基因库变化。

7. Natural Selection in Action: Antibiotic Resistance | 自然选择的实例：抗生素耐药性

Antibiotic resistance in bacteria such as Staphylococcus aureus (MRSA) is a major public health concern and an excellent example of natural selection observable over a short time scale.

细菌（如金黄色葡萄球菌 MRSA）的抗生素耐药性是重大公共卫生问题，也是自然选择在较短时间内可观察的绝佳实例。

A random mutation can give a bacterium resistance to an antibiotic. In the presence of the antibiotic, susceptible bacteria are killed, but resistant ones survive and reproduce, passing the resistance allele to their offspring.

随机突变可能使某个细菌对抗生素产生耐药性。在抗生素存在时，敏感细菌被杀死，而耐药细菌存活并繁殖，将耐药等位基因传给后代。

Over time, the population becomes dominated by resistant strains. This process is accelerated by the misuse

Published by TutorHao | A-Level Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Natural Selection | 自然选择考点精讲