📚 Artificial Selection: Breeding Desired Traits | 人工选择：定向培育优良性状
Artificial selection is one of the central mechanisms of evolutionary change that Cambridge A-Level Biology expects you to apply and evaluate. This article explains how humans have shaped gene pools in crops, livestock and companion animals, and why the process creates both remarkable phenotypes and serious genetic risks.
人工选择是剑桥 A-Level 生物要求掌握并评价的核心进化机制之一。本文解释人类如何塑造农作物、家畜和伴侣动物的基因库，以及为什么该过程既能产生显著表型，也会带来严重的遗传风险。
1. Defining Artificial Selection | 人工选择的定义
Artificial selection is the process by which humans deliberately choose individual organisms with desirable characteristics to reproduce. Over many generations, this non-random breeding changes the allele frequencies in a population, producing phenotypes that suit human needs.
人工选择是人类有目的地挑选具有理想特征的个体进行繁殖的过程。经过多个世代，这种非随机育种会改变群体中的等位基因频率，从而产生符合人类需求的表型。
It is also called selective breeding. Unlike natural selection, where environmental pressures determine survival and reproduction, artificial selection has a human-defined selection pressure, such as higher milk yield, larger seeds or a calmer temperament.
人工选择又称选择性育种。与自然选择由环境压力决定生存和繁殖不同，人工选择的选择压力由人类设定，例如更高的产奶量、更大的种子或更温顺的性情。
The starting point is always existing genetic variation. Without differences in alleles among individuals, selection cannot cause evolutionary change because there is nothing to select for or against.
人工选择的起点始终是已有的遗传变异。如果个体之间不存在等位基因差异，选择就无法引起进化改变，因为根本没有可供选择或淘汰的变异。
2. Artificial Selection vs Natural Selection | 人工选择与自然选择的比较
| Feature | Artificial selection | Natural selection |
|---|---|---|
| Selective agent | Humans | Environment |
| Aim | Human benefit | Survival and reproduction |
| Speed | Can be rapid | Usually slow |
| Outcome | Traits may reduce fitness in wild | Traits usually increase fitness |
The table summarises the key differences. In artificial selection, traits that are useful to humans may be harmful in the wild, such as the large flight muscles of broiler chickens, which reduce their ability to escape predators.
上表总结了关键区别。在人工选择中，对人类有用的性状在野外可能是有害的，例如肉鸡发达的胸肌反而降低了它们逃避捕食者的能力。
Exam questions often ask you to compare the two processes. Always link both to allele frequency change, but stress that the selective pressure comes from different sources.
考题常要求比较这两种过程。回答时一定要把两者都与等位基因频率变化联系起来，但强调选择压力来自不同来源。
3. Selective Breeding in Crop Plants | 农作物中的选择性育种
A classic example is wild mustard, Brassica oleracea. From one ancestral species, humans selected different parts of the plant: terminal buds produced cabbage, lateral buds produced Brussels sprouts, stems produced kohlrabi, and flower clusters produced broccoli and cauliflower.
经典例子是野生甘蓝（Brassica oleracea）。从同一个祖先物种出发，人类选择植物的不同部位：顶芽培育出卷心菜，侧芽培育出抱子甘蓝，茎培育出球茎甘蓝，花簇培育出西兰花和花椰菜。
This shows how strong directional selection can split one gene pool into many cultivated forms. Each variety has a different set of alleles enriched by choosing only plants with the desired enlarged organ.
这表明强烈的定向选择可以把一个基因库分成许多栽培类型。每个品种都通过只选择具有所需膨大器官的植株，富集了不同的等位基因组合。
Wheat domestication also involved artificial selection. Early farmers selected mutant plants whose grains did not shatter from the ear, allowing easier harvesting. This non-shattering trait is controlled by major genes and became fixed in cultivated wheat.
小麦驯化也涉及人工选择。早期农民挑选籽粒不易从穗上脱落的突变植株，便于收获。这种不易脱粒性状由主效基因控制，并在栽培小麦中被固定下来。
4. Animal Domestication and Breed Development | 动物驯化与品种培育
Domestic dogs descend from wolves. For thousands of years, humans selected wolves that were less fearful and more social. Later, different working and aesthetic traits gave rise to breeds as different as greyhounds, huskies and dachshunds.
家犬起源于狼。数千年来，人类选择那些恐惧感较低、更亲近人类的狼。后来，不同的工作需求和审美标准产生了差异巨大的品种，如灵缇、哈士奇和腊肠犬。
The Russian farm-fox experiment provides experimental evidence for artificial selection on behaviour. By selecting only the tamest foxes for breeding, researchers produced fox populations that behaved like domestic dogs within a few decades, with changes in coat colour and tail carriage also appearing.
俄罗斯农场狐狸实验为行为上的人工选择提供了实验证据。研究人员只选择最温顺的狐狸进行繁殖，几十年内就培育出行为类似家犬的狐狸种群，同时还出现了毛色和尾巴姿态的变化。
In cattle, breeders have developed specialised dairy and beef breeds. Holstein cattle have been selected for high milk yield, while Aberdeen Angus have been selected for muscle growth and meat quality. These breeds differ greatly in body shape and metabolism.
在牛中，育种者已培育出专门的乳用和肉用品种。荷斯坦牛被选择用于高产奶量，而安格斯牛被选择用于肌肉生长和肉质。这些品种在体型和代谢方面差异巨大。
5. Genetic Consequences: Reduced Variation | 遗传后果：变异减少
Because artificial selection allows only a small number of individuals with favoured traits to breed, it reduces genetic diversity in the population. Alleles that do not contribute to the selected trait may be lost by chance or by active culling.
由于人工选择只允许少数具有优良性状的个体繁殖，它会降低群体的遗传多样性。与所选性状无关的等位基因可能因随机漂变或被主动淘汰而丢失。
This is a form of genetic bottleneck. A reduced gene pool means the population has fewer alleles to cope with new diseases, climate change or other environmental challenges.
这是一种遗传瓶颈。基因库缩小意味着群体应对新疾病、气候变化或其他环境挑战的等位基因更少。
Modern crop monocultures are especially vulnerable. If all plants of a variety share the same disease-susceptibility allele, a single pathogen strain can wipe out the entire crop, as occurred in the Irish potato famine.
现代作物单一栽培尤其脆弱。如果一个品种的所有植株都带有相同的感病等位基因，单一病原体株系就可能摧毁整片作物，爱尔兰马铃薯饥荒就是这样的例子。
6. Inbreeding Depression and Hybrid Vigour | 近交衰退与杂种优势
Selective breeding often involves mating closely related individuals to fix desirable traits. This increases homozygosity, including homozygosity for harmful recessive alleles, leading to inbreeding depression: reduced fertility, lower survival and poor growth.
选择性育种常涉及近亲交配以固定理想性状。这增加了纯合性，包括有害隐性等位基因的纯合，从而导致近交衰退：生育力下降、存活率降低和生长不良。
Inbreeding depression is observed in pedigree dogs, where many breeds suffer from inherited disorders such as hip dysplasia, heart defects and immune problems because harmful alleles have become common in closed gene pools.
近交衰退在纯种犬中很常见，许多品种因封闭基因库中有害等位基因变得常见，而患上髋关节发育不良、心脏缺陷和免疫问题等遗传病。
Hybrid vigour, or heterosis, is the opposite effect. Crossing two different inbred lines produces heterozygous offspring that often outperform both parents in size, yield and disease resistance. This is widely used in maize production.
杂种优势（heterosis）是相反的效果。将两个不同的近交系杂交，产生的杂合后代在体型、产量和抗病性上往往优于双亲。这在玉米生产中应用广泛。
7. Directional Selection and Polygenic Traits | 定向选择与多基因性状
Most traits targeted by artificial selection, such as milk yield, body mass and grain size, are polygenic. They show continuous variation controlled by many genes and environmental factors, so breeders see a gradual shift in the mean phenotype rather than sudden jumps.
人工选择所针对的大多数性状，如产奶量、体重和籽粒大小，都是多基因性状。它们表现出由许多基因和环境因素控制的连续变异，因此育种者观察到的是平均表型的逐渐移动，而不是突然跳跃。
Artificial selection is a strong form of directional selection. Each generation, only individuals at one extreme of the phenotype distribution are chosen as parents, shifting the normal distribution curve towards that extreme.
人工选择是一种强烈的定向选择。每一代只选择表型分布一端极端的个体作为亲本，使正态分布曲线向该极端移动。
The response to selection depends on heritability. If a trait has high heritability, offspring resemble their parents closely and selection is effective. If environmental influence is large, the response is weaker.
选择响应取决于遗传力。如果性状遗传力高，后代表型与亲本相似，选择效果就好。如果环境影响因素大，选择响应就较弱。
8. Modern Tools: Marker-Assisted Selection | 现代工具：标记辅助选择
Traditional breeding relies on observing phenotypes, which can be slow and imprecise. Marker-assisted selection uses DNA markers linked to desirable alleles to identify superior individuals at the seedling or embryo stage, before the trait is visible.
传统育种依赖表型观察，既缓慢又不精确。标记辅助选择利用与优良等位基因连锁的 DNA 标记，在苗期或胚胎阶段、性状尚未显现时就能识别优良个体。
A marker is a short DNA sequence located close to a gene or QTL that affects the trait. If the marker is present, the linked favourable allele is likely to be present too, allowing breeders to screen large numbers quickly.
标记是位于影响性状的基因或数量性状位点附近的短 DNA 序列。如果标记存在，与之连锁的有利等位基因也很可能存在，这样育种者就能快速筛选大量个体。
Marker-assisted selection is especially useful for disease resistance and for traits expressed late in life. It can also be used in backcrossing programmes to introduce a single useful allele from a wild relative into an elite cultivar while recovering most of the elite genome.
标记辅助选择对抗病性状和晚期才表达的性状特别有用。它还可用于回交计划，把野生近缘种中的单个有利等位基因引入优良栽培品种，同时恢复大部分优良基因组。
9. Artificial Selection vs Genetic Engineering | 人工选择与基因工程比较
Artificial selection works only with alleles already present in the gene pool or arising by mutation. It cannot transfer genes between species. Genetic engineering, by contrast, can isolate a gene from any organism and insert it into a crop or animal.
人工选择只能利用基因库中已有的等位基因或突变产生的新等位基因，不能跨物种转移基因。相比之下，基因工程可以从任何生物中分离基因，并将其插入作物或动物体内。
Selective breeding changes many genes at once, often with unknown side effects because linked alleles are inherited together. Genetic engineering usually changes one or a few known genes, giving more precise control over the phenotype.
选择性育种一次改变许多基因，常因连锁等位基因共同遗传而产生未知副作用。基因工程通常只改变一个或少数已知基因，对表型的控制更加精确。
However, both methods ultimately alter allele frequencies or genotypes in a population. Exam answers should recognise that conventional selection is not ‘unnatural’ in the sense of violating genetics; it accelerates and directs processes that also occur in nature.
然而，两种方法最终都会改变群体中的等位基因频率或基因型。答题时应认识到，传统选择并非违背遗传规律的“非自然”过程，而是加速并引导自然界也会发生的过程。
10. Ethical and Conservation Issues | 伦理与保护问题
Artificial selection raises welfare concerns when it produces extreme phenotypes that harm animal health. Brachycephalic dog breeds such as pugs often have breathing difficulties, and some dairy cows suffer lameness and mastitis under high milk-production demands.
当人工选择产生危害动物健康的极端表型时，会引发动物福利问题。巴哥等短头犬常出现呼吸困难，一些奶牛在高产奶要求下会患跛行和乳腺炎。
Conservation of rare traditional breeds is important because they retain alleles that may be needed for future breeding. Gene banks, seed banks and rare breed trusts preserve genetic resources that high-yielding commercial breeds may have lost.
保护稀有传统品种很重要，因为它们保留了未来育种可能需要的等位基因。基因库、种子库和稀有品种信托机构保存了高产商业品种可能已丢失的遗传资源。
There is also a social dimension: farmers in developing countries may depend on locally adapted landraces that are resilient to drought and poor soils, even if their yield is lower than modern uniform varieties.
还有社会层面的问题：发展中国家的农民可能依赖适应当地条件的农家品种，这些品种虽然产量低于现代单一品种，但对干旱和贫瘠土壤具有抗性。
11. Exam Focus: Analysing Selection Experiments | 考点聚焦：分析选择实验
Cambridge questions may give data on changes in phenotype distribution over several generations. You should describe the shift in the mean, explain that humans selected extreme individuals as parents, and link the change to altered allele frequencies in the gene pool.
剑桥考题可能给出若干世代中表型分布变化的数据。你应描述平均值的移动，解释人类选择极端个体作为亲本，并将变化与基因库中等位基因频率的改变联系起来。
A common task is to outline the steps of a selective breeding programme: choose parents with desired trait, allow them to reproduce, select the best offspring, repeat over many generations, and sometimes cross with a different line to restore vigour.
常见任务是概述选择性育种计划的步骤：选择具有理想性状的亲本，让其繁殖，挑选最佳后代，重复多代，有时与不同品系杂交以恢复活力。
Evaluation questions often ask why artificial selection can lead to loss of disease resistance or increased inherited disorders. Use terms such as reduced gene pool, inbreeding depression, homozygosity and genetic bottleneck to gain marks.
评价题常问为什么人工选择会导致抗病性丧失或遗传病增加。使用“基因库缩小”“近交衰退”“纯合性”和“遗传瓶颈”等术语可以得分。
When interpreting graphs, mention that selection has a limit: once all individuals carry the favoured alleles, variation is exhausted and the response plateaus unless new mutations or crosses introduce fresh variation.
解释图表时，要提到选择有极限：一旦所有个体都携带有利等位基因，变异耗尽，选择响应就会进入平台期，除非有新的突变或杂交引入新变异。
12. Summary and Key Terms | 总结与关键术语
Artificial selection is a powerful demonstration of evolution by changing allele frequencies. It has produced the crops, livestock and pets we depend on, but it also reduces genetic diversity and can create health and welfare problems when taken to extremes.
人工选择是通过改变等位基因频率来体现进化的有力例证。它创造了我们赖以生存的作物、家畜和宠物，但当走向极端时，也会降低遗传多样性并带来健康和福利问题。
Key terms to remember include: selective breeding, gene pool, allele frequency, directional selection, continuous variation, polygenic trait, genetic bottleneck, inbreeding depression, heterosis, heritability, marker-assisted selection and genetic engineering.
需要记住的关键术语包括：选择性育种、基因库、等位基因频率、定向选择、连续变异、多基因性状、遗传瓶颈、近交衰退、杂种优势、遗传力、标记辅助选择和基因工程。
In an exam, always frame artificial selection as a human-directed change in allele frequency, supported by concrete examples such as Brassica oleracea, wheat, dogs and dairy cattle, and evaluate its genetic and ethical consequences.
在考试中，要始终把人工选择表述为人类引导的等位基因频率变化，并用甘蓝、小麦、犬和奶牛等具体例子支撑，同时评价其遗传和伦理后果。
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