Artificial Selection and Breeding | 人工选择与育种

📚 Artificial Selection and Breeding | 人工选择与育种

Artificial selection, also known as selective breeding, is the process by which humans choose organisms with desirable traits to reproduce, thereby shaping the genetic makeup of future generations. This practice has been central to the development of agriculture and livestock, and remains a key topic in CIE A-Level Biology.

人工选择，又称选择育种，是指人类选择具有理想性状的生物进行繁殖，从而塑造后代遗传组成的过程。这一实践对农业和畜牧业的发展至关重要，也是 CIE A-Level 生物学的重点内容。

1. What is Artificial Selection? | 什么是人工选择？

Artificial selection occurs when humans, rather than the natural environment, act as the selecting agent. By allowing only certain individuals to breed, humans increase the frequency of specific alleles in a population. For example, a farmer may select the largest tomatoes from a crop and grow only their seeds, gradually producing a population with larger fruit.

人工选择发生在人类而非自然环境充当选择压力时。通过只允许特定个体繁殖，人类增加了特定等位基因在群体中的频率。例如，农民可以从一茬作物中挑选最大的番茄，只播种它们的种子，逐渐产生果实更大的种群。

This process depends on pre-existing genetic variation. Without variation, there would be no distinct phenotypes for humans to select from. Variation arises from mutations, gene flow, and sexual reproduction, including crossing over and independent assortment during meiosis.

这一过程依赖于预先存在的遗传变异。没有变异，人类就没有可供选择的不同表型。变异来源于突变、基因流动以及有性生殖，包括减数分裂中的交叉互换和自由组合。

2. Principles of Selective Breeding | 选择育种的原理

Selective breeding operates on a simple principle: individuals with desirable phenotypes are allowed to breed, while those with less desirable phenotypes are prevented from doing so. Over successive generations, the favourable alleles become more common in the population. The strength of selection may be measured by the selection coefficient, which indicates the relative reduction in reproductive success of a particular phenotype.

选择育种的原理很简单：具有理想表型的个体被允许繁殖，而不太理想的个体则被阻止繁殖。经过连续多代，有利等位基因在种群中变得更加常见。选择强度可以用选择系数来衡量，该系数表示某一表型繁殖成功率相对下降的程度。

The success of a breeding program depends on several factors: the amount of genetic variation available, the heritability of the trait, the generation time of the organism, and the intensity of selection. Traits with high heritability, such as milk yield in dairy cattle, respond more quickly to selection than those with low heritability.

育种计划能否成功取决于多种因素：可用的遗传变异量、性状的遗传力、生物体的世代间隔以及选择强度。遗传力高的性状，例如奶牛产奶量，比遗传力低的性状对选择的反应更快。

3. Artificial vs Natural Selection | 人工选择与自然选择的比较

Both artificial and natural selection involve differential reproductive success based on phenotypes, but the selecting agent differs. In natural selection, the environment selects adaptations that increase survival and reproduction. In artificial selection, humans select traits that are often based on economic or aesthetic value, which may not improve survival in the wild.

人工选择和自然选择都涉及基于表型的差异繁殖成功，但选择者不同。自然选择中，环境选择能提高生存和繁殖的适应性状；人工选择中，人类选择通常基于经济或审美价值的性状，这些性状不一定提高在野外的生存能力。

In artificial selection, the selected traits may be controlled by dominant or recessive alleles. Humans may choose individuals showing a desired dominant phenotype, but heterozygotes may still carry undesirable recessive alleles. This explains why continued selection is necessary to maintain desirable traits.

在人工选择中，所选性状可能由显性或隐性等位基因控制。人类可能选择表现出所需显性表型的个体，但杂合子可能仍携带不利的隐性等位基因。这就解释了为什么需要持续选择以保持理想性状。

4. Methods in Plant Breeding | 植物育种方法

Several methods are used in plant breeding to improve crop varieties. Mass selection involves choosing many individuals with desirable traits from a population and growing their seeds together. This method is simple but slow, and it may not remove undesirable recessive alleles.

植物育种中有多种改良作物品种的方法。混合选择是从一个群体中挑选许多具有理想性状的个体，并将它们的种子一起种植。这种方法简单但较慢，且可能无法清除不利的隐性等位基因。

Pure-line selection is more rigorous: a single self-fertilising plant is chosen, and its offspring are grown to produce a uniform true-breeding line. Hybridisation, or crossing two different pure lines, can combine desirable traits from both parents. The F₁ hybrids often show increased vigour.

纯系选育更加严格：选择一株自花受精的植株，种植其后代以产生一致的真实遗传品系。杂交，即两个不同纯系之间的交配，可以结合双亲的理想性状。F₁ 杂种通常表现出更强的活力。

Backcrossing is often used to transfer a specific trait into an otherwise elite variety. A hybrid carrying the desired allele is crossed repeatedly with the elite parent

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