📚 AS Biology 4.3 Classification and Evolution | AS 生物 4.3 分类与进化
Classification and evolution are central to understanding biodiversity. This article explains how organisms are named, grouped and related, and how natural selection drives change over time.
分类与进化是理解生物多样性的核心。本文解释生物如何被命名、归类并建立亲缘关系，以及自然选择如何推动随时间发生的变化。
You will learn the rules of binomial nomenclature, the taxonomic hierarchy, the biological species concept, the evidence for evolution, natural selection, speciation and artificial selection. These ideas are examined together because classification is not just a filing system: it should reflect evolutionary ancestry.
你将学习双名法规则、分类阶层、生物学物种概念、进化证据、自然选择、物种形成和人工选择。这些内容通常一起考查，因为分类不只是归档系统，它还应反映进化上的亲缘关系。
1. Why Classification Matters | 为什么分类很重要
Classification arranges the estimated 8.7 million eukaryotic species into a manageable, international system. Without it, biologists could not communicate reliably about organisms.
分类将估计约 870 万个真核物种纳入一个可管理的国际系统。没有分类，生物学家就无法可靠地交流有关生物的信息。
Grouping by shared features allows scientists to identify unknown specimens, infer characteristics and plan conservation work.
按共同特征分组使科学家能够鉴定未知标本、推断特征并规划保护工作。
Classifying organisms also helps us understand evolutionary relationships. Species placed in the same group are thought to share a more recent common ancestor than species in different groups.
对生物分类还有助于我们理解进化关系。被归入同一类群的物种被认为比不同类群中的物种拥有更近的共同祖先。
- Identification: using keys and shared features to name an unknown organism. / 鉴定：利用检索表和共同特征为未知生物命名。
- Communication: one universal name avoids confusion between languages. / 交流：一个通用名称避免不同语言间的混淆。
- Prediction: related organisms may share similar biochemistry or habitat needs. / 预测：亲缘关系近的生物可能具有相似的生化特征或栖息地需求。
- Conservation: groups at risk can be monitored and protected together. / 保护：濒危类群可以被一起监测和保护。
2. Binomial Nomenclature | 双名法
Binomial nomenclature assigns every species a two-part Latin name. The first part is the genus and begins with a capital letter; the second part is the species and begins with a lower-case letter.
双名法为每个物种赋予一个两词拉丁学名。第一部分是属名，首字母大写；第二部分是种加词，首字母小写。
In print, the whole name is italicised, for example Homo sapiens for humans and Escherichia coli for a common gut bacterium.
印刷时整个名称用斜体，例如人类为 Homo sapiens，一种常见肠道细菌为 Escherichia coli。
This convention is universal, avoiding confusion caused by local common names. When handwritten, the name is underlined instead of italicised, and the genus is still capitalised.
这一规则在国际上通用，避免了地方俗名造成的混淆。手写时名称用下划线代替斜体，属名仍然大写。
After the first full mention, the genus can be abbreviated, so Escherichia coli becomes E. coli. The species name is never used alone without the genus, because "coli" by itself does not identify a species.
首次完整提及后，属名可以缩写，因此 Escherichia coli 可写成 E. coli。种加词不能单独使用而不带属名，因为单独 "coli" 无法鉴定一个物种。
3. Taxonomic Hierarchy | 分类阶层
The taxonomic hierarchy is a series of increasingly specific groups. The eight main ranks used today are domain, kingdom, phylum, class, order, family, genus and species.
分类阶层是一系列越来越具体的类群。当今使用的八个主要等级是域、界、门、纲、目、科、属、种。
As you move down the hierarchy, the number of organisms in each group decreases, while the number of shared features increases. Members of the same genus are more closely related than members of the same family.
沿阶层向下移动时，每一类群中的生物数量减少，而共有特征数量增加。同一属的成员比同一科的成员亲缘关系更近。
A useful memory phrase is: Dear King Philip Came Over For Good Soup. This gives Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
一个有用的记忆句是：Dear King Philip Came Over For Good Soup。它对应 Domain、Kingdom、Phylum、Class、Order、Family、Genus、Species。
| Rank / 等级 | Example / 例子 |
| Domain 域 | Eukarya 真核域 |
| Kingdom 界 | Animalia 动物界 |
| Phylum 门 | Chordata 脊索动物门 |
| Class 纲 | Mammalia 哺乳纲 |
| Order 目 | Primates 灵长目 |
| Family 科 | Hominidae 人科 |
| Genus 属 | Homo 人属 |
| Species 种 | Homo sapiens 智人 |
4. The Species Concept | 物种概念
A species is often defined as a group of organisms with similar morphology and physiology that can interbreed to produce fertile offspring. Members of the same species share a gene pool and are reproductively isolated from other species.
物种通常被定义为形态和生理相似、能够交配并产生可育后代的一群生物。同一物种的成员共享一个基因库，并与其他物种存在生殖隔离。
Reproductive isolation can be behavioural, temporal, mechanical or gametic. For example, two bird species may sing different mating songs, or two plants may release pollen at different times.
生殖隔离可以是行为上的、时间上的、机械上的或配子上的。例如，两种鸟可能唱不同的求偶歌曲，或两种植物在不同时间释放花粉。
The biological species concept has limits. It does not apply easily to asexual organisms, fossils or hybrids. Some distinct species can interbreed in captivity or in narrow hybrid zones, such as lions and tigers producing ligers.
生物学物种概念有局限。它不太适用于无性生殖生物、化石或杂交种。一些不同物种在圈养或狭窄杂交带中可以交配，例如狮和虎产生狮虎兽。
In exams, always mention fertile offspring rather than just offspring. A mule is the sterile offspring of a horse and a donkey, so the horse and donkey remain separate species.
考试中一定要提到可育后代，而不仅仅是后代。骡子是马和驴产生的不可育后代，因此马和驴仍是不同物种。
5. Classification Reflects Evolutionary Relationships | 分类反映进化关系
Modern classification aims to be phylogenetic. A phylogenetic group, or clade, contains an ancestor and all of its descendants. This is called a monophyletic group.
现代分类力求系统发育化。一个系统发育类群或分支包含一个祖先及其所有后代。这称为单系群。
If a group contains an ancestor but excludes some descendants, it is paraphyletic. If it contains organisms from different ancestors that evolved similar features separately, it is polyphyletic.
如果一个类群包含一个祖先但排除了一些后代，它是并系群。如果它包含来自不同祖先、却分别进化出相似特征的生物，它是多系群。
Homologous structures provide evidence for common ancestry. The pentadactyl limb of humans, bats, whales and horses has the same basic bone pattern, even though it performs different functions.
同源结构为共同祖先提供证据。人类、蝙蝠、鲸和马的五指型肢具有相同的基本骨骼模式，尽管功能不同。
Analogous structures, such as the wings of insects and birds, evolved independently. They show convergent evolution and do not indicate close relatedness.
同功结构，如昆虫和鸟的翅膀，是独立进化而来的。它们体现趋同进化，并不表示亲缘关系近。
6. Evidence for Evolution | 进化的证据
The fossil record shows that organisms have changed over time. Fossils can be dated by radioactive isotopes, and their position in rock strata indicates relative age. Transitional fossils, such as Archaeopteryx, show features of two groups.
化石记录显示生物随时间发生了变化。化石可通过放射性同位素测年，其在岩层中的位置表明相对年龄。过渡化石，如始祖鸟，显示了两个类群的特征。
Comparative anatomy provides evidence from homologous structures, vestigial organs and embryonic development. For example, vertebrate embryos share gill pouches and a tail at early stages.
比较解剖学从同源结构、痕迹器官和胚胎发育提供证据。例如，脊椎动物胚胎在早期具有鳃裂和尾。
Molecular evidence compares DNA base sequences, amino acid sequences and cytochrome c. The more closely related two species are, the more similar their sequences and the fewer the differences.
分子证据比较 DNA 碱基序列、氨基酸序列和细胞色素 c。两个物种亲缘关系越近，其序列越相似，差异越少。
Biogeography is also important. Islands often contain endemic species related to nearby mainland species, which supports the idea of dispersal followed by divergent evolution.
生物地理学也很重要。岛屿常含有与附近大陆物种亲缘关系近的特有物种，这支持扩散后趋异进化的观点。
7. Natural Selection | 自然选择
Natural selection was proposed by Charles Darwin and Alfred Russel Wallace. It requires variation, inheritance and differential survival and reproduction. Individuals with advantageous alleles are more likely to survive and pass those alleles to offspring.
自然选择由查尔斯·达尔文和阿尔弗雷德·拉塞尔·华莱士提出。它需要变异、遗传以及差异生存和繁殖。具有有利等位基因的个体更可能存活并将这些等位基因传给后代。
Over many generations, the frequency of advantageous alleles increases in the gene pool. This is evolution: a change in allele frequencies in a population over time.
经过许多代，有利等位基因在基因库中的频率增加。这就是进化：种群等位基因频率随时间的变化。
Natural selection is not goal-directed. It does not create perfect organisms, and it acts on existing variation. If the environment changes, previously favourable alleles may become disadvantageous.
自然选择不是有目的的。它不会创造完美生物，而是作用于已有的变异。如果环境改变，先前有利的等位基因可能变得不利。
eproduction. An organism that lives a long time but produces no offspring has low fitness.
Fitness = relative survival × reproduction
8. Variation, Adaptation and Allele Frequency | 变异、适应与等位基因频率
Variation arises from mutation, meiosis and random fertilisation. Mutation is the original source of new alleles, while crossing over and independent assortment reshuffle existing alleles.
变异来自突变、减数分裂和随机受精。突变是新等位基因的根本来源，而交换和独立分配则重新组合已有等位基因。
An adaptation is a feature that increases an organism's chance of survival and reproduction in its environment. Adaptations can be behavioural, physiological or anatomical.
适应是提高生物在环境中生存和繁殖机会的特征。适应可以是行为上的、生理上的或解剖上的。
Examples include the thick fur of Arctic mammals, the waxy cuticle of desert plants, and the long, thin beaks of nectar-feeding birds.
例子包括北极哺乳动物的厚毛、沙漠植物的蜡质角质层，以及食蜜鸟细长的喙。
Directional selection shifts the mean phenotype in one direction. Stabilising selection favours the average phenotype, and disruptive selection favours both extremes.
定向选择使平均表型向一个方向移动。稳定选择有利于平均表型，而分裂选择有利于两个极端。
Use the term allele frequency rather than gene frequency. A gene may have many alleles, and population genetics tracks how those allele frequencies change.
使用等位基因频率这一术语，而不是基因频率。一个基因可能有多个等位基因，群体遗传学追踪这些等位基因频率如何变化。
9. Speciation and Isolation | 物种形成与隔离
Speciation is the formation of new species from existing species. It usually requires isolation, genetic variation, natural selection and reproductive isolation.
物种形成是从现有物种形成新物种的过程。它通常需要隔离、遗传变异、自然选择和生殖隔离。
Allopatric speciation occurs when a physical barrier, such as a river, mountain range or sea, separates populations. The two populations experience different selection pressures and accumulate different mutations.
异域物种形成发生在物理屏障，如河流、山脉或海洋，将种群分隔时。两个种群经历不同的选择压力，并积累不同的突变。
Over time, the populations may become so different that they can no longer interbreed. At this point, they are separate species.
随着时间推移，这些种群可能差异大到无法再交配。此时它们就是不同物种。
Sympatric speciation occurs without a physical barrier, often through polyploidy in plants or through behavioural isolation in animals. Polyploid plants can produce fertile offspring with each other but not with the original diploid species.
同域物种形成没有物理屏障，通常通过植物多倍化或动物行为隔离发生。多倍体植物之间可产生可育后代，但与原始二倍体物种不能。
Exam questions may ask you to sequence the steps. A common order is: isolation, mutation, different selection pressures, differential survival, reproductive isolation, new species.
考试题可能要求你排列步骤。常见顺序是：隔离、突变、不同选择压力、差异生存、生殖隔离、新物种。
10. Artificial Selection and Selective Breeding | 人工选择与选择育种
Artificial selection is selection by humans for desired traits. It has produced crop plants and domesticated animals that differ greatly from their wild ancestors.
人工选择是人类对所需性状的选择。它产生了与野生祖先差异很大的农作物和家养动物。
Farmers choose individuals with high yield, disease resistance, drought tolerance or improved flavour. These individuals are bred, so the desired alleles become more common in the population.
农民选择具有高产、抗病、耐旱或更好风味的个体。这些个体被繁殖，因此所需等位基因在种群中变得更常见。
Brassica oleracea is a classic example. Selective breeding has produced cabbage, broccoli, cauliflower, kale and Brussels sprouts from the same wild species.
甘蓝是经典例子。选择育种从同一野生种产生了卷心菜、西兰花、花椰菜、羽衣甘蓝和抱子甘蓝。
Artificial selection can reduce genetic diversity and make populations vulnerable to disease. It may also cause unintended health problems in pedigree dogs.
人工选择可能降低遗传多样性，使种群易受疾病影响。它也可能在纯种狗中造成意外的健康问题。
Compare it with natural selection: both require variation and inheritance, but artificial selection is driven by human choice rather than environmental survival.
与自然选择比较：两者都需要变异和遗传，但人工选择由人类选择驱动，而不是环境生存。
11. Domains, Kingdoms and Modern Phylogenetics | 域、界与现代系统发育学
The three-domain system divides life into Bacteria, Archaea and Eukarya. It is based mainly on ribosomal RNA sequences and differences in cell membrane lipids and cell wall chemistry.
三域系统将生命分为细菌域、古菌域和真核域。它主要基于核糖体 RNA 序列以及细胞膜脂质和细胞壁化学的差异。
Archaea were once grouped with bacteria, but they have different membrane lipids and RNA polymerase. Many archaea live in extreme environments, though others live in soil and oceans.
古菌曾被归入细菌，但它们具有不同的膜脂质和 RNA 聚合酶。许多古菌生活在极端环境，但其他古菌生活在土壤和海洋中。
The five-kingdom system includes Prokaryotae, Protoctista, Fungi, Plantae and Animalia. It is still useful for introductory study, but phylogenetic classification now prefers monophyletic groups based on molecular data.
五界系统包括原核生物界、原生生物界、真菌界、植物界和动物界。它对入门学习仍有用，但系统发育分类现在更倾向于基于分子数据的单系群。
Molecular clocks use the rate of mutation in DNA or proteins to estimate when lineages diverged. DNA-DNA hybridisation and gene sequencing can quantify genetic similarity.
分子钟利用 DNA 或蛋白质的突变速率来估计谱系分化的时间。DNA-DNA 杂交和基因测序可以量化遗传相似性。
Classification is therefore a hypothesis. As new molecular evidence appears, groups may be reclassified. This is a strength of science, not a weakness.
因此分类是一种假说。随着新的分子证据出现，类群可能被重新分类。这是科学的优势，而不是缺陷。
12. Exam Technique and Common Mistakes | 考试技巧与常见错误
When answering classification questions, always give the full binomial name and use the correct format. Do not write homo sapiens or Homo Sapiens.
回答分类题时，始终写出完整双名法名称并使用正确格式。不要写成 homo sapiens 或 Homo Sapiens。
For natural selection questions, use causal connectives: because, therefore, so. A strong answer explains variation, selection pressure, differential survival and reproduction, inheritance, and changed allele frequency.
回答自然选择题时，使用因果连接词：because、therefore、so。高分答案要解释变异、选择压力、差异生存与繁殖、遗传，以及等位基因频率改变。
Do not say an organism "adapts" during its lifetime in a Lamarckian sense. Individuals do not evolve; populations evolve. Mutations occur randomly and are not caused by need.
不要以拉马克的方式说生物在一生中"适应"。个体不会进化，种群才会进化。突变随机发生，并非由需求引起。
Learn examples accurately: Homo sapiens, Escherichia coli, Archaeopteryx, Brassica oleracea and the pentadactyl limb. Named examples earn marks in longer answers.
准确记忆例子：Homo sapiens、Escherichia coli、始祖鸟、甘蓝和五指型肢。命名例子在长答案中可获得分数。
Finally, distinguish homologous from analogous structures, and monophyletic from paraphyletic groups. These distinctions are common in multiple-choice and extended-response questions.
最后，区分同源结构与同功结构，以及单系群与并系群。这些区分在选择题和扩展回答题中很常见。
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