Selection and Evolution | 选择与进化

📚 Selection and Evolution | 选择与进化

Selection and evolution are central ideas in biology that explain how populations change over time. In Cambridge A-Level Biology, you need to link genetic variation, allele frequencies, natural selection, and the formation of new species. This article reviews the key definitions, the Hardy–Weinberg principle, types of selection, speciation, and the evidence for evolution.

选择与进化是生物学中解释种群如何随时间发生改变的核心概念。在剑桥 A-Level 生物课程中，你需要把遗传变异、等位基因频率、自然选择和新物种形成联系起来。本文梳理关键定义、哈迪–温伯格原理、选择类型、物种形成以及进化证据。

1. Genetic Variation and the Gene Pool | 遗传变异与基因库

Genetic variation is the raw material for evolution. It arises through mutation, crossing over and independent assortment during meiosis, and random fertilisation. These processes produce new allele combinations without changing the overall gene pool size.

遗传变异是进化的原材料。它来源于突变、减数分裂中的交叉互换和自由组合，以及随机的受精过程。这些过程产生新的等位基因组合，但不会改变整个基因库的大小。

The gene pool is the total collection of all alleles of all genes in a population at a given time. An allele frequency is the proportion of a particular allele among all copies of that gene in the gene pool. Evolution can be defined as a change in these allele frequencies from one generation to the next.

基因库是指某一时刻一个种群中所有基因的全部等位基因的总和。等位基因频率是指某个特定等位基因在该基因所有拷贝中所占的比例。进化可以定义为这些等位基因频率从一个世代到下一个世代发生的变化。

2. Allele Frequencies and the Hardy–Weinberg Principle | 等位基因频率与哈迪–温伯格原理

The Hardy–Weinberg principle states that allele and genotype frequencies remain constant from generation to generation if a population is large, mates randomly, and experiences no mutation, no selection, no gene flow, and no genetic drift. Such a population is in Hardy–Weinberg equilibrium.

哈迪–温伯格原理指出，如果一个种群足够大、随机交配，并且没有突变、没有选择、没有基因流动和遗传漂变，那么等位基因频率和基因型频率会逐代保持不变。这样的种群处于哈迪–温伯格平衡。

p + q = 1 and p² + 2pq + q² = 1

In these equations, p and q represent the frequencies of two alleles. The terms p² and q² represent the frequencies of the two homozygous genotypes, while 2pq represents the heterozygous genotype frequency. If observed genotype frequencies differ significantly from these expected values, evolutionary forces are likely acting on the population.

在这些方程中，p 和 q 代表两个等位基因的频率。p² 和 q² 表示两种纯合基因型的频率，2pq 表示杂合基因型的频率。如果观察到的基因型频率与这些预期值有显著差异，说明进化力量很可能正在作用于该种群。

3. Natural Selection as a Mechanism of Evolution | 作为进化机制的自然选择

Natural selection occurs when individuals with phenotypes better suited to their environment survive and reproduce more successfully. This leads to a greater contribution of their alleles to the next generation, so advantageous alleles increase in frequency over time.

当具有更适应环境表型的个体能够更好地生存和繁殖时，自然选择就发生了。这使它们的等位基因对下一代有更大的贡献，因此有利的等位基因频率会随时间增加。

The conditions for natural selection include variation within the population, overproduction of offspring, competition for limited resources, and differential survival and reproduction. Selection acts on phenotypes, but it changes allele frequencies in the genotype.

自然选择发生的条件包括种群内存在变异、后代数量过剩、对有限资源的竞争，以及生存和繁殖的差异。选择作用于表型，但它改变的是基因型中的等位基因频率。

4. Types of Selection: Stabilising, Directional and Disruptive | 选择类型：稳定选择、定向选择与分裂选择

Stabilising selection favours intermediate phenotypes and removes extreme phenotypes. Human birth weight is a classic example: very small or very large babies have higher mortality, so intermediate weights are favoured. This reduces phenotypic variation and keeps allele frequencies around an optimum.

稳定选择有利于中间表型，淘汰极端表型。人类出生体重是一个典型例子：过小或过大的婴儿死亡率较高，因此中间体重受到青睐。这种选择降低表型变异，使等位基因频率保持在最适值附近。

Directional selection favours one extreme phenotype. Antibiotic resistance in bacteria and the evolution of increased body size in some lineages are examples. Directional selection shifts the mean phenotype toward that extreme and changes allele frequencies in one direction.

定向选择有利于某一极端表型。细菌的抗生素耐药性和某些谱系中体型增大的进化都是例子。定向选择使平均表型向该极端方向移动，并使等位基因频率向一个方向改变。

Disruptive selection favours both extreme phenotypes over the intermediate phenotype. For example, birds with very small or very large beaks may be favoured when only small and large seeds are available. This can increase variation and may contribute to speciation if the extremes become reproductively isolated.

分裂选择有利于两个极端表型，而不利于中间表型。例如，当只有小种子和大种子可用时，喙非常小或非常大的鸟类可能更受青睐。这种选择会增加变异，如果极端表型之间形成生殖隔离，就可能促进物种形成。

5. Evolution and Adaptation | 进化与适应

Evolution is the cumulative change in the heritable characteristics of a population over generations. It does not act on individuals, and it has no predetermined goal. Adaptation is a feature that increases an organism’s fitness in a particular environment.

进化是种群可遗传特征在世代间的累积变化。它不作用于个体，也没有预先设定的目标。适应是指能提高生物体在特定环境中适合度的特征。

Fitness is the ability of an organism to survive and reproduce, passing its alleles to the next generation. Adaptations can be structural, behavioural, or physiological, and they arise through the gradual action of natural selection on existing variation.

适合度是指生物体生存和繁殖并将自身等位基因传递给下一代的能力。适应可以是结构性的、行为性的或生理性的，它们是通过自然选择对已有变异逐步作用而产生的。

6. Isolation and Speciation | 隔离与物种形成

Speciation is the formation of new species from an ancestral population. A species is generally defined as a group of organisms that can interbreed to produce fertile offspring under natural conditions.

物种形成是指从祖先种群形成新物种的过程。物种通常被定义为在自然条件下能够相互交配并产生可育后代的一群生物。

For speciation to occur, gene flow between populations must be reduced or prevented. Isolation can be geographical, leading to allopatric speciation, or reproductive within the same area, leading to sympatric speciation. When populations cannot exchange alleles, natural selection and genetic drift act separately, causing divergence.

要发生物种形成，种群之间的基因流动必须被减少或阻断。隔离可以是地理上的，导致异域物种形成；也可以是同一地区内的生殖隔离，导致同域物种形成。当种群无法交换等位基因时，自然选择和遗传漂变分别发挥作用，导致种群分化。

7. Allopatric and Sympatric Speciation | 异域物种形成与同域物种形成

Allopatric speciation occurs when a geographical barrier such as a mountain range, river, or ocean separates populations. Different selection pressures favour different alleles in each population. Over many generations, the populations diverge and may become reproductively isolated even if the barrier is removed.

异域物种形成发生在地理屏障（如山脉、河流或海洋）将种群分隔开时。不同的选择压力使每个种群中不同的等位基因受到青睐。经过许多世代，种群发生分化，即使屏障消失，它们也可能已经形成生殖隔离。

Sympatric speciation occurs within the same geographical area without physical separation. It often involves polyploidy in plants, or ecological and behavioural isolation. Reproductive isolation develops because different groups exploit different niches or mating signals.

同域物种形成发生在同一地理区域内，没有物理分隔。它通常涉及植物的多倍体化，或生态隔离和行为隔离。由于不同群体利用不同的生态位或交配信号，生殖隔离得以形成。

8. Reproductive Isolating Mechanisms | 生殖隔离机制

Reproductive isolating mechanisms prevent gene flow between species. They can be prezygotic, occurring before fertilisation, or postzygotic, occurring after fertilisation.

生殖隔离机制阻止物种之间的基因流动。它们可以是合子前隔离，发生在受精之前；也可以是合子后隔离，发生在受精之后。

Prezygotic barriers: habitat isolation, temporal isolation, behavioural isolation, mechanical isolation, and gametic isolation.
合子前屏障：栖息地隔离、时间隔离、行为隔离、机械隔离和配子隔离。
Postzygotic barriers: hybrid inviability, hybrid sterility, and hybrid breakdown.
合子后屏障：杂种不活、杂种不育和杂种衰败。

9. Evidence for Evolution | 进化证据

Evidence for evolution comes from several independent sources. Fossils show changes in organisms over geological time and include transitional forms. Comparative anatomy reveals homologous structures that indicate common ancestry, such as the pentadactyl limb in vertebrates.

进化证据来自多个相互独立的来源。化石显示了生物在地质时间尺度上的变化，并包含过渡类型。比较解剖学揭示了表明共同祖先的同源结构，例如脊椎动物的五趾型肢。

Molecular biology provides strong evidence: closely related species have more similar DNA and protein sequences. Biogeography shows that species in different regions have evolved from local ancestors, and embryology reveals shared developmental stages among related organisms.

分子生物学提供了强有力的证据：亲缘关系较近的物种具有更相似的 DNA 和蛋白质序列。生物地理学表明，不同地区的物种是从当地祖先进化而来的；胚胎学则揭示了相关生物之间共享的发育阶段。

10. Antibiotic Resistance and Industrial Melanism | 抗生素耐药性与工业黑化

Antibiotic resistance is a clear example of directional natural selection. In a bacterial population, a few cells may carry alleles for resistance. When antibiotics are used, sensitive cells die, while resistant cells survive and reproduce, increasing the frequency of resistance alleles.

抗生素耐药性是定向自然选择的一个明显例子。在细菌种群中，少数细胞可能携带耐药性等位基因。使用抗生素时，敏感细胞死亡，而耐药细胞存活并繁殖，从而增加耐药性等位基因的频率。

Industrial melanism in the peppered moth Biston betularia shows how environmental change can shift phenotype frequencies. Dark moths became more common in polluted areas because they were better camouflaged on soot-covered trees, while light moths were favoured in clean areas.

桦尺蛾的工业黑化现象表明环境变化如何改变表型频率。在污染地区，深色蛾变得更常见，因为它们在布满煤烟的树上伪装得更好；而在清洁地区，浅色蛾更受青睐。

11. Genetic Drift, Bottleneck and Founder Effect | 遗传漂变、瓶颈效应与奠基者效应

Genetic drift is the random change in allele frequencies from one generation to the next. Its effect is much stronger in small populations and can lead to the loss of alleles independently of selection.

遗传漂变是指等位基因频率从一个世代到下一个世代的随机变化。它在小种群中的作用要强得多，并且可以在没有选择的情况下导致等位基因丢失。

A genetic bottleneck occurs when a population is drastically reduced in size, so the surviving gene pool may not represent the original population. The founder effect occurs when a small group colonises a new area, carrying only a fraction of the original genetic variation.

遗传瓶颈发生在种群数量急剧减少时，因此幸存下来的基因库可能无法代表原有种群。奠基者效应发生在小群体迁入新地区时，它们只携带原有遗传变异的一小部分。

12. Artificial Selection and Evolution | 人工选择与进化

Artificial selection is the process by which humans breed organisms with desirable traits. Unlike natural selection, it is directed by human choice rather than environmental fitness, but it relies on the same genetic variation and differential reproduction.

人工选择是人类培育具有优良性状的生物体的过程。与自然选择不同，它由人类的选择而非环境适合度来主导，但它同样依赖于遗传变异和繁殖差异。

Examples include selective breeding of crops for higher yield, dogs from wolves, and dairy cattle for increased milk production. Artificial selection demonstrates that selection can change allele frequencies rapidly, supporting evolutionary theory.

例子包括为提高产量而对作物进行选择育种、由狼培育出狗，以及为提高产奶量而对奶牛进行选育。人工选择表明选择可以迅速改变等位基因频率，为进化理论提供支持。

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Selection and Evolution | 选择与进化