📚 Natural Selection | 自然选择
Natural selection is the process by which individuals with heritable traits that increase fitness tend to survive and reproduce more successfully than others in a given environment. This mechanism, first clearly described by Charles Darwin and Alfred Russel Wallace, explains how populations adapt over generations without any purposeful design.
自然选择是指具有提高适应度的可遗传性状的个体，在特定环境中往往比其他个体存活和繁殖得更成功。这一机制由查尔斯·达尔文和阿尔弗雷德·拉塞尔·华莱士首先清晰描述，解释了种群如何在世代间适应环境，而无需任何有目的的设计。
1. The Core Principle | 核心原理
Natural selection requires three conditions: variation in traits within a population, heritability of those traits, and differential reproductive success linked to the traits. When these conditions are met, the alleles that produce advantageous phenotypes become more common in subsequent generations.
自然选择需要三个条件：种群内性状存在变异、这些性状可遗传、以及与性状相关的繁殖成功率存在差异。当这些条件满足时，产生有利表型的等位基因在后续世代中会变得更加常见。
A key point is that natural selection acts on phenotypes but changes allele frequencies in the gene pool. The environment determines which phenotypes are advantageous, so selection is not random and does not create perfect organisms.
关键点在于自然选择作用于表型，但改变的是基因库中的等位基因频率。环境决定了哪些表型是有利的，因此选择不是随机的，也不会创造出完美的生物体。
2. Sources of Genetic Variation | 遗传变异来源
Genetic variation arises mainly through mutation, independent assortment during meiosis, crossing over, and random fertilisation. Mutation is the ultimate source of new alleles, although most mutations are neutral or harmful.
遗传变异主要来源于突变、减数分裂中的独立分配、交叉互换以及随机受精。突变是新等位基因的根本来源，尽管大多数突变是中性的或有害的。
In a diploid organism, sexual reproduction shuffles existing alleles into new combinations, producing offspring that differ from both parents. Without this variation, natural selection would have no raw material on which to act.
在二倍体生物中，有性生殖将现有等位基因重新组合成新的组合，产生与父母双方不同的后代。没有这种变异，自然选择就没有可供作用的原材料。
3. Overproduction and the Struggle for Existence | 过度繁殖与生存斗争
Most species produce far more offspring than can possibly survive to maturity. For example, a single bacterial cell can divide every 20 minutes under ideal conditions, and many plants release thousands of seeds each year.
大多数物种产生的后代数量远远超过能够存活到成熟的个体数。例如，一个细菌细胞在理想条件下每 20 分钟分裂一次，许多植物每年释放数千粒种子。
Limited resources such as food, water, space and mates lead to competition within and between species. This ecological struggle means that only a fraction of offspring survive and reproduce.
食物、水、空间和配偶等有限资源导致种内和种间竞争。这种生态斗争意味着只有一小部分后代能够存活并繁殖。
4. Differential Survival and Fitness | 差异存活与适应度
Individuals whose phenotypes are better suited to local environmental conditions tend to survive longer and leave more offspring. Fitness in biology is defined as reproductive success relative to other individuals in the population, not simply physical strength.
表型更适合当地环境条件的个体往往存活时间更长并留下更多后代。生物学中的适应度被定义为相对于种群中其他个体的繁殖成功率，而不仅仅是体力。
Differential survival means that advantageous alleles are passed on at a higher rate. Over many generations, this can shift the phenotypic distribution of the population towards better-adapted forms.
差异存活意味着有利等位基因以更高的速率传递给后代。经过许多代后，这可以使种群的表型分布转向更适应环境的形式。
5. Heritability and Allele Frequency Change | 遗传力与等位基因频率变化
For natural selection to cause evolutionary change, the advantageous trait must have a genetic basis and be heritable. Acquired characteristics, such as increased muscle mass from exercise, are not inherited and therefore cannot be directly selected.
要使自然选择引起进化变化，有利性状必须具有遗传基础并且是可遗传的。后天获得的特征，例如运动增加的肌肉量，不会被遗传，因此不能被直接选择。
As individuals with beneficial alleles reproduce more, the frequency of those alleles increases in the gene pool. This can be represented as a change from one generation to the next, for example p frequency of a favoured allele rising from 0.40 to 0.55.
随着携带有利等位基因的个体繁殖更多，这些等位基因在基因库中的频率增加。这可以表示为从一代到下一代的变化，例如受青睐的等位基因频率 p 从 0.40 上升到 0.55。
6. Selection Pressures | 选择压力
Selection pressures are environmental factors that influence survival and reproduction. They can be abiotic, such as temperature, drought or salinity, or biotic, such as predation, disease and competition for mates.
选择压力是影响生存和繁殖的环境因素。它们可以是非生物因素，如温度、干旱或盐度，也可以是生物因素，如捕食、疾病和配偶竞争。
A change in selection pressure often changes which phenotypes are favoured. For example, a colder climate may favour individuals with thicker fur, whereas a warmer climate may favour thinner fur.
选择压力的变化通常会改变哪些表型受到青睐。例如，较冷的气候可能有利于毛发较厚的个体，而较暖的气候可能有利于毛发较薄的个体。
7. Types of Selection | 选择类型
Stabilising selection favours intermediate phenotypes and removes extreme values. It reduces variation and is common in stable environments, such as human birth weight where very small or very large babies have lower survival.
稳定选择有利于中间表型并淘汰极端值。它减少变异，在稳定环境中很常见，例如人类出生体重，非常小或非常大的婴儿存活率较低。
Directional selection favours one extreme phenotype, shifting the population mean. This occurs when the environment changes, for example a darker background favouring darker-coloured insects.
方向选择有利于某一极端表型，使种群平均值发生偏移。这发生在环境变化时，例如较暗的背景有利于颜色较深的昆虫。
Disruptive selection favours two or more extreme phenotypes and selects against intermediate forms. It can increase variation and may lead to sympatric speciation if reproductive isolation develops.
分裂选择有利于两个或多个极端表型，并淘汰中间类型。它可以增加变异，如果发展出生殖隔离，可能导致同域物种形成。
8. Case Study: Antibiotic Resistance | 案例研究：抗生素耐药性
Antibiotic resistance in bacteria is a clear example of directional selection. When a patient takes an antibiotic, susceptible bacteria are killed, but rare mutants with resistance alleles survive and reproduce without competition.
细菌的抗生素耐药性是方向选择的一个清晰例子。当患者服用抗生素时，敏感细菌被杀死，但罕见的携带耐药等位基因的突变体在没有竞争的情况下存活并繁殖。
Over time, the resistant strain becomes dominant in the bacterial population. This is why incomplete antibiotic courses and overuse of antibiotics accelerate the evolution of multidrug-resistant pathogens.
随着时间推移，耐药菌株在细菌种群中占据优势。这就是为什么未完成抗生素疗程和过度使用抗生素会加速多重耐药病原体的进化。
The selection pressure is the antibiotic; the advantageous trait is the resistance mechanism, which may involve enzymes that break down the drug, efflux pumps, or altered target sites.
选择压力是抗生素；有利性状是耐药机制，可能涉及分解药物的酶、外排泵或改变的目标位点。
9. Case Study: Sickle Cell Anaemia and Malaria | 案例研究：镰状细胞性贫血与疟疾
Sickle cell anaemia is caused by a recessive allele that alters haemoglobin. Individuals with two copies of the sickle cell allele have severe anaemia, but heterozygotes have a survival advantage in malaria-endemic regions.
镰状细胞性贫血由改变血红蛋白的隐性等位基因引起。携带两个镰状细胞等位基因的个体患有严重贫血，但杂合子在疟疾流行地区具有生存优势。
The heterozygote advantage maintains both the normal and sickle cell alleles in the population. This balanced polymorphism is an example of stabilising selection favouring the heterozygous genotype in certain environments.
杂合子优势使正常等位基因和镰状细胞等位基因在种群中得以维持。这种平衡多态性是在特定环境中有利于杂合基因型的稳定选择的一个例子。
Where malaria is absent, the sickle cell allele is selected against because homozygotes have low fitness, so its frequency is much lower.
在没有疟疾的地区，镰状细胞等位基因会被选择淘汰，因为纯合子适应度低，因此其频率要低得多。
10. Industrial Melanism in Peppered Moths | 桦尺蠖的工业黑化
The peppered moth Biston betularia exists in light and dark forms. Before the industrial revolution, light moths were camouflaged on lichen-covered trees and dark moths were easily eaten by birds.
桦尺蠖存在浅色和深色两种类型。在工业革命之前，浅色蛾在覆盖地衣的树干上具有伪装，深色蛾容易被鸟类捕食。
During industrialisation, soot killed lichens and darkened tree bark. The dark form then had higher survival, and its allele frequency increased dramatically in polluted areas.
工业化期间，煤烟杀死地衣并使树皮变黑。深色类型因此具有更高的存活率，其等位基因频率在污染地区急剧上升。
As air pollution declined in the late twentieth century, light forms became more common again. This reversible change provides strong evidence that natural selection responds to changing environments.
随着二十世纪后期空气污染减少，浅色类型再次变得更为常见。这种可逆变化提供了有力证据，表明自然选择会对环境变化作出响应。
11. Sexual Selection | 性选择
Sexual selection is a special form of natural selection based on the ability to obtain mates. It often produces traits that appear harmful to survival, such as bright plumage or elaborate antlers, because the reproductive benefit outweighs survival costs.
性选择是基于获得配偶能力的一种特殊形式的自然选择。它通常会产生似乎不利于生存的性状，例如鲜艳的羽毛或精致的鹿角，因为繁殖收益超过了生存成本。
Intrasexual selection involves competition between members of the same sex, while intersexual selection involves mate choice, often by females. Both mechanisms can drive rapid evolution of display traits.
性内选择涉及同一性别个体之间的竞争，而性间选择涉及配偶选择，通常由雌性进行。这两种机制都能推动展示性状的快速进化。
12. Natural Selection, Genetic Drift and Speciation | 自然选择、遗传漂变与物种形成
Natural selection is not the only mechanism that changes allele frequencies. Genetic drift is the random change in allele frequencies due to chance events, especially in small populations, and it is not adaptive. Unlike genetic drift, natural selection is non-random with respect to fitness: it consistently increases alleles that improve survival and reproduction in a given environment.
自然选择并不是改变等位基因频率的唯一机制。遗传漂变是由于偶然事件导致的等位基因频率随机变化，尤其是在小种群中，它不具有适应性。与遗传漂变不同，自然选择在适应度方面是非随机的：它持续增加在特定环境中提高生存和繁殖的等位基因。
When natural selection operates on geographically isolated populations under different environmental conditions, populations may diverge and eventually become reproductively isolated. This is allopatric speciation.
当自然选择在不同环境条件下作用于地理隔离的种群时，种群可能发生分化并最终形成生殖隔离。这就是异域物种形成。
Reproductive isolation can be prezygotic, such as differences in mating behaviour or breeding time, or postzygotic, such as hybrid infertility. Once gene flow stops, the two lineages evolve independently.
生殖隔离可以是
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