📚 Artificial Selection: Selective Breeding for Desired Traits | 人工选择：为理想性状而进行的选择育种
Artificial selection is the process by which humans breed plants and animals for particular genetic characteristics. Unlike natural selection, where the environment determines which individuals survive and reproduce, artificial selection has humans acting as the selective agent, consistently choosing organisms with traits that are useful or attractive. This deliberate shaping of species has been practised for thousands of years and is responsible for the vast diversity of crops, livestock and companion animals we see today.
人工选择是人类为了特定的遗传特征而对植物和动物进行育种的过程。与自然选择不同，在自然选择中环境决定哪些个体存活并繁殖，人工选择则将人类作为选择媒介，持续挑选具有有用或吸引人特征的生物。这种有意识地塑造物种的做法已经实践了数千年，并造就了我们今天看到的极其多样的作物、家畜和伴侣动物。
1. Definition of Artificial Selection | 人工选择的定义
Artificial selection, also known as selective breeding, involves humans identifying desirable traits in organisms and then using those individuals as parents for the next generation. Over many generations, the frequency of alleles that confer the preferred traits increases in the population. This is essentially a form of evolution driven by human preference rather than survival advantage in the wild.
人工选择，也称为选择育种，是指人类识别出生物中理想的特征，然后利用这些个体作为下一代的亲本。经过许多世代，赋予优先性状的等位基因频率在种群中上升。这本质上是一种由人类偏好而非野外生存优势驱动的进化形式。
2. Artificial Selection vs Natural Selection | 人工选择与自然选择的对比
While both natural and artificial selection change allele frequencies in populations, the driving force behind each is fundamentally different. In natural selection, environmental pressures such as predation, climate and disease select for characteristics that improve fitness – the ability to survive and reproduce. In artificial selection, humans intentionally choose which individuals reproduce based on traits that meet human needs, regardless of whether those traits would be advantageous in nature.
尽管自然选择和人工选择都改变种群中的等位基因频率，但各自的驱动力有着根本的不同。在自然选择中，捕食、气候和疾病等环境压力选出能提高适合度——即生存和繁殖能力的性状。而在人工选择中，人类根据满足自身需求的性状有意识地选择哪些个体繁殖，而不管这些性状在自然界中是否有利。
The table below summarises key differences:
下表总结了主要区别：
| Feature 特征 | Natural Selection 自然选择 | Artificial Selection 人工选择 |
|---|---|---|
| Selective agent 选择媒介 | Environment and ecological factors 环境和生态因素 | Humans 人类 |
| Traits favoured 受青睐的性状 | Traits that increase fitness and survival 提高适合度与生存的性状 | Traits useful or pleasing to humans 对人类有用或悦人的性状 |
| Speed of change 变化速度 | Usually slow, over many generations 通常缓慢，历经多代 | Can be rapid due to strong selective pressure 由于强选择压力而可快速进行 |
| Outcome 结果 | Adaptation to the environment 对环境的适应 | Organisms meeting human requirements 满足人类需求的生物 |
3. Mechanisms of Selective Breeding | 选择育种的机制
The mechanism underlying artificial selection involves choosing a small number of individuals with the most desirable expression of a trait from a variable population and allowing only those to breed. Their offspring are then screened again for the trait, and the process is repeated. Each generation narrows the genetic variation for that particular trait, concentrating the alleles responsible for the preferred phenotype.
人工选择的机制包括从一个具有变异的种群中挑选出极少数具有最理想性状表现的个体，并只允许这些个体繁殖。随后对它们的后代再次筛选该性状，并重复此过程。每一代都缩小了针对该特定性状的遗传变异，集中了导致优先表现型的等位基因。
For many quantitative traits such as milk production or grain size, selective breeding progressively shifts the mean value of the population. Breeders use techniques such as line breeding and outcrossing to balance trait improvement with genetic diversity.
对于许多数量性状，如产奶量或谷粒大小，选择育种逐步使种群的平均值发生偏移。育种者采用品系繁育和异型杂交等方法，在性状改良与遗传多样性之间取得平衡。
4. Examples in Agriculture: Crops | 农业实例：作物
Nearly all staple crops are a product of intensive artificial selection. Modern bread wheat (Triticum aestivum), for instance, originated from wild grasses through repeated selection for larger seeds, non-shattering seed heads and higher yield. Similarly, maize (corn) was domesticated from teosinte, a wild grass with tiny, hard seeds. Over centuries, farmers selected for larger cobs and softer kernels, dramatically transforming the plant.
几乎所有的粮食作物都是密集人工选择的产物。例如，现代面包小麦(Triticum aestivum)源自野草，通过反复选择更大的谷粒、不脱落穗头和更高的产量而形成。同样，玉米从大刍草驯化而来，大刍草是一种籽粒细小坚硬的野草。经过数个世纪，农民选出果穗更大、籽粒更软的植株，使该植物发生了巨大的改变。
Another remarkable example is Brassica oleracea, a single wild mustard species from which cabbage, broccoli, cauliflower, kale and Brussels sprouts have all been produced by selecting different parts of the plant – leaves, stems, flower clusters and lateral buds respectively.
另一个显著的例子是甘蓝(Brassica oleracea)，一种野生的芥菜物种，通过分别选择植株的不同部位——叶片、茎、花簇和侧芽——培育出了卷心菜、西蓝花、花椰菜、羽衣甘蓝和抱子甘蓝等。
5. Examples in Animal Husbandry: Livestock | 畜牧业实例：家畜
Livestock such as cattle, pigs and sheep have been selectively bred for economically important traits. Dairy cows like the Holstein-Friesian have been selected for extremely high milk yield, while beef cattle breeds such as Angus are selected for muscle mass and marbling. Sheep are bred for both fine wool (Merino) and rapid weight gain for meat (Suffolk).
牛、猪和羊等家畜已针对经济上重要的性状进行了选择育种。像荷斯坦-弗里斯兰这样的奶牛已被选出极高的产奶量，而如安格斯牛之类的肉牛品种则根据肌肉量和大理石纹进行选择。羊既为生产细羊毛（美利奴）而育种，也为快速增重产肉（萨福克）而育种。
The selection intensity can be extreme: modern broiler chickens grow twice as fast as breeds from 50 years ago, reaching market weight in just a few weeks. This is achieved by selecting birds with the fastest growth rate and greatest breast muscle development over many generations.
选择强度可以是极端的：现代肉鸡的生长速度是50年前品种的两倍，短短几周便达到上市体重。这是通过许多世代中选择生长最快、胸肌最发达的鸡来实现的。
6. The Domestication of Dogs | 狗的驯化
The domestic dog (Canis lupus familiaris) is one of the most diverse species on Earth due to artificial selection. All dog breeds are descended from wolves, with domestication beginning at least 15,000 years ago. Humans selected for tameness, hunting ability, herding behaviour and guarding instincts, leading to the creation of hundreds of distinct breeds, each with characteristic size, coat colour, skull shape and temperament.
家犬(Canis lupus familiaris)是地球上由于人工选择而产生的多样性最高的物种之一。所有犬种都源自狼，驯化至少在15,000年前就开始了。人类针对温顺、狩猎能力、放牧行为和守卫本能进行选择，从而创造出数百个各具特色的犬种，每个犬种都有独特的体型、毛色、头骨形状和性情。
The extreme morphological differences between a Chihuahua and a Great Dane, for example, result from selecting for or against genes influencing growth factors and bone development. This provides a powerful demonstration of how quickly artificial selection can produce remarkable phenotypic divergence.
例如，吉娃娃和大丹犬之间极端的外形差异，就是由选择取舍影响生长因子和骨骼发育的基因造成的。这有力地证明了人工选择能以多快的速度产生显著的表型分化。
7. Consequences: Inbreeding Depression | 后果：近交衰退
When selective breeding focuses heavily on a small number of elite individuals, it inevitably increases the chance of mating between closely related organisms. This inbreeding boosts homozygosity, which raises the likelihood that harmful recessive alleles – normally masked in heterozygous condition – are expressed. The resulting decline in fitness, known as inbreeding depression, often appears as reduced fertility, lower growth rates and increased susceptibility to disease.
当选择育种严重集中在少数优秀个体上时，便不可避免地增加了近亲生物之间交配的机会。这种近交通提高了纯合性，从而增加了有害隐性等位基因（通常在杂合状态下被掩盖）表现出来的可能性。由此造成的适合度下降被称为近交衰退，常表现为生育力降低、生长速度减慢和疾病易感性增加。
Many purebred dog lines, for example, suffer from breed-specific disorders such as hip dysplasia in German Shepherds or respiratory problems in flat-faced breeds like Pugs. In agriculture, inbred crop lines can lose vigour and yield, although hybrid vigour (heterosis) is often deliberately exploited to counteract this in production systems.
例如，许多纯种犬品系都患有品种特异的疾病，如德国牧羊犬的髋关节发育不良或哈巴狗等扁平脸犬种的呼吸系统问题。在农业上，近交作物品系会丧失活力和产量，尽管在生产系统中常有意地利用杂种优势（杂种活力）来抵消这一问题。
8. Impact on Gene Pool and Genetic Diversity | 对基因库和遗传多样性的影响
Intense artificial selection narrows the gene pool of a population by concentrating only those alleles that contribute to the desired trait. Alleles that do not directly affect the selected trait – and even linked alleles nearby on chromosomes – may be lost through genetic hitchhiking or simply through the elimination of individuals lacking the preferred phenotype. Over time, this erosion of genetic diversity reduces the ability of a population to adapt to new challenges such as emerging diseases or climate change.
高强度的人工选择通过仅聚焦那些贡献于理想性状的等位基因，缩小了种群的基因库。与所选性状不直接相关的等位基因——甚至染色体上邻近的连锁等位基因——都可能通过遗传搭车效应或因缺乏优先表现型的个体被淘汰而丢失。随着时间推移，遗传多样性的侵蚀降低了种群适应新挑战（如新出现疾病或气候变化）的能力。
The Irish potato famine is a classic example; the dependence on a few genetically uniform potato varieties made the crop catastrophically susceptible to late blight (Phytophthora infestans). Maintaining seed banks and wild relatives is therefore critical for long-term food security.
爱尔兰土豆饥荒就是一个经典案例；依赖少数几个遗传上均一的马铃薯品种，使作物对晚疫病(Phytophthora infestans)具有灾难性的易感性。因此，维持种子库和野生近缘种对于长期粮食安全至关重要。
9. Artificial Selection and Modern Genetics | 人工选择与现代遗传学
Traditional selective breeding is based on observable phenotype, but modern genetics now allows breeders to understand the underlying alleles. Quantitative trait loci (QTL) mapping identifies regions of the genome associated with complex traits such as drought tolerance or milk composition. Marker-assisted selection (MAS) uses molecular markers linked to these QTLs to select parents at the DNA level, even before the trait is expressed, dramatically speeding up the breeding cycle.
传统选择育种基于可观察的表型，但现代遗传学如今使育种者能够了解背后的等位基因。数量性状位点(QTL)作图确定了与复杂性状（如耐旱性或乳成分）相关的基因组区域。标记辅助选择(MAS)利用与这些QTL连锁的分子标记，在性状表达之前就在DNA水平上选择亲本，从而显著加快了育种周期。
These techniques do not involve recombinant DNA technology, so they remain distinct from genetic engineering. Instead, they refine the precision of selective breeding by integrating Mendelian genetics and genomics with traditional practices.
这些技术不涉及重组DNA技术，因此它们仍然有别于基因工程。相反，它们通过将孟德尔遗传学与基因组学融入传统实践，提高了选择育种的精确性。
10. Selective Breeding vs Genetic Engineering | 选择育种与基因工程的比较
It is important to distinguish artificial selection from genetic engineering (genetic modification). Selective breeding works within the existing gene pool of a species and relies on sexual reproduction to recombine alleles. Genetic engineering, by contrast, directly alters an organism's DNA, often introducing genes from unrelated species that would never cross naturally. This can produce novel traits instantly, such as bacterial pesticide production in Bt corn.
将人工选择与基因工程（遗传改造）加以区分是很重要的。选择育种是在一个物种现有的基因库内进行，并依赖有性繁殖重组等位基因。相反，基因工程直接改变生物的DNA，往往引入来自不相关物种的基因，这在自然情况下永远不会发生。这可以即刻产生新的性状，例如Bt玉米中的细菌杀虫剂生产。
While both approaches are human-driven, selective breeding reshapes the genetic makeup of populations gradually over generations, whereas genetic engineering can combine genetic material from any source in a single generation. Selective breeding's products are generally considered 'natural' variations, while genetically modified organisms (GMOs) are subject to strict regulation and labelling in many countries.
虽然这两种方法都是人类驱动的，但选择育种历经多代逐渐重塑种群的基因构成，而基因工程则可在一代之内组合来自任何来源的遗传物质。选择育种的产物通常被视为“自然”变异，而转基因生物(GMOs)在许多国家受到严格的监管和标签制度。
11. Ethical and Environmental Considerations | 伦理与环境考量
The application of artificial selection raises ethical questions, particularly regarding animal welfare. Breeding for extreme traits – like large breast muscles in turkeys that prevent natural mating, or short noses in dogs that cause breathing distress – can cause suffering. Critics argue that prioritising human needs or aesthetics over animal health is ethically problematic.
人工选择的应用引发了伦理问题，尤其是在动物福利方面。为极端性状育种——如火鸡过大的胸肌导致无法自然交配，或狗的短鼻引发呼吸窘迫——可能造成痛苦。批评者认为，将人类需求或审美置于动物健康之上在伦理上是有问题的。
Environmentally, the narrow genetic base of artificially selected monocultures makes entire ecosystems vulnerable. Pest or pathogen outbreaks can sweep through genetically identical crops, leading to heavy pesticide use. Preserving landraces and wild relatives is essential to maintain the genetic reservoir that both traditional breeding and future biotechnologies depend on.
在环境方面，人工选择产生的单一栽培其狭窄的遗传基础使整个生态系统变得脆弱。病虫害爆发可能会席卷遗传上相同的作物，导致大量使用农药。保存地方品种和野生近缘种对于维持传统育种和未来生物技术所依赖的遗传储备库至关重要。
12. Summary and Key Points | 总结与要点
Artificial selection is a powerful evolutionary force directed by human choice. It operates by consistently breeding from individuals with favoured traits, leading to a rapid shift in allele frequencies. This process has produced an extraordinary range of domesticated organisms but also risks inbreeding depression and loss of genetic diversity. Understanding the similarities and differences between artificial selection, natural selection and genetic engineering is a core learning outcome in A-Level Biology. Students should be able to explain selective breeding examples, describe its genetic consequences, and discuss associated ethical issues.
人工选择是一种由人类选择导引的强大演化力量。它通过持续从具有偏好性状的个体进行繁殖来运作，导致等位基因频率迅速偏移。该过程已产生出极为多样化的驯化生物，但同时也带来近交衰退和遗传多样性丧失的风险。理解人工选择、自然选择和基因工程之间的异同是A-Level生物的核心学习成果。学生应当能够解释选择性育种实例，描述其遗传后果，并讨论相关的伦理问题。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图