📚 AS Biology 4.3 Classification & Evolution | AS 生物 4.3 分类与进化
Classification is the process of placing living organisms into orderly groups based on shared characteristics, allowing biologists to organise the immense diversity of life, identify species reliably, and reveal evolutionary relationships. Evolution is the cumulative change in the heritable characteristics of biological populations over successive generations, driven primarily by natural selection acting on genetic variation.
分类是将生物根据共同特征归入有序列群的过程，使生物学家能够组织纷繁复杂的生命多样性、可靠地鉴别物种，并揭示进化亲缘关系。进化是生物种群在连续世代中可遗传特征的累积变化，主要由自然选择作用于遗传变异而驱动。
1. The Need for Classification | 分类的必要性
It is estimated that around 8.7 million species exist on Earth, yet fewer than two million have been formally described. Classification provides a universal framework for naming, cataloguing, and studying this enormous variety of organisms without ambiguity.
据估计，地球上约有870万个物种，但正式被描述的还不到两百万种。分类为命名、编目和研究这数量庞大的生物提供了一个无歧义的通用框架。
By grouping organisms according to shared features, classification reflects evolutionary history: organisms in the same group are more closely related to one another than to organisms in other groups. Once an organism is classified, its position within a taxon allows scientists to predict characteristics shared with other members, such as metabolic pathways, behaviour, or ecological roles.
通过将生物按共同特征分组，分类反映了进化历史：同一类群中的生物彼此之间的亲缘关系比与其他类群中生物的亲缘关系更近。一旦生物被分类，它在分类单元中的位置使科学家能预测其与其他成员共有的特征，如代谢途径、行为或生态角色。
The hierarchical classification system also enables accurate communication between researchers across the world, regardless of language barriers, because each species has a universally accepted scientific name.
层级分类系统还使世界各地的研究人员能够跨越语言障碍进行准确交流，因为每个物种都有一个被普遍接受的学名。
2. The Binomial System | 二名法
The binomial system, introduced by Carl Linnaeus in the 18th century, gives every species a two-part Latinised scientific name. The first part is the genus name (capitalised) and the second is the species name (lowercase); the whole name is written in italics or underlined.
二名法由卡尔·林奈在18世纪提出，赋予每个物种一个由两部分组成的拉丁化科学名称。第一部分是属名（首字母大写），第二部分是种名（小写）；整个名称用斜体书写或加下划线。
For example, humans are named Homo sapiens; the genus is Homo and the species is sapiens. No other species may share this exact two-part name, which makes it globally unique and avoids confusion caused by different common names in different regions.
例如，人类的学名是 Homo sapiens；属名为 Homo（人属），种名为 sapiens（智人种）。任何其他物种都不能使用这一确切的双名，因此它是全球唯一的，避免了不同地区俗名不同造成的混淆。
Key rules of binomial nomenclature: the genus name is always written with a capital initial letter, the species epithet is always written in lowercase, and both parts must be italicised in print or underlined when handwritten. The binomial system is accepted worldwide, making scientific communication precise and consistent.
二名法的关键规则是：属名首字母始终大写，种名始终小写，印刷时用斜体、手写时加下划线。二名法被全世界接受，使科学交流精确而一致。
3. Taxonomy Hierarchy | 分类学层级
The taxonomic hierarchy arranges organisms into nested ranks of increasing specificity. The full sequence is: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. A useful mnemonic is “Dear King Philip Came Over For Good Soup”.
分类学层级将生物按嵌套级别排列，范围逐级缩小。完整序列是：域、界、门、纲、目、科、属、种。常用助记句是”亲爱的菲利普国王来喝好汤”。
As we move down the hierarchy from Domain to Species, the number of organisms in each group decreases, while the number of shared characteristics increases. All organisms in the same group at any level share the characteristics that define that group; members of the same species share the greatest number of characteristics and can interbreed to produce fertile offspring.
从域到种逐级向下，每个类群中的生物数量逐渐减少，而共享特征的数量逐渐增多。任何层级同一类群中的所有生物都共享定义该类群的特征；同一物种的成员共享最多的特征，并能交配产生可育后代。
Comparing organisms at different levels of the hierarchy reveals their degree of evolutionary relatedness. For instance, a tiger and a lion share the genus Panthera, making them more closely related to each other than either is to a domestic cat, which belongs to a different genus.
在层级的不同水平比较生物，可以揭示其进化亲缘关系的亲疏程度。例如，虎和狮同属豹属（Panthera），因此它们之间的亲缘关系比它们各自与家猫（属于不同属）的关系更近。
4. The Three Domains | 三域系统
The three-domain system, proposed by Carl Woese in 1990, is based on differences in ribosomal RNA (rRNA) base sequences and other molecular characteristics. It divides all life into three domains: Bacteria, Archaea, and Eukaryota.
三域系统由卡尔·沃斯于1990年提出，基于核糖体RNA（rRNA）碱基序列及其他分子特征的差异。它将所有生命分为三个域：细菌域、古菌域和真核生物域。
Archaea were formerly grouped with Bacteria as prokaryotes, but molecular analysis revealed that they are genetically as different from Bacteria as they are from Eukaryota. Archaea often inhabit extreme environments such as hot springs, salt lakes, and deep-sea vents. Their cell walls lack peptidoglycan, their membrane lipids are ether-linked, and their DNA is associated with histone-like proteins, all of which distinguish them from true Bacteria.
古菌过去与细菌同被归为原核生物，但分子分析表明，古菌与细菌之间的遗传差异和古菌与真核生物之间的差异一样大。古菌常栖息于极端环境，如温泉、盐湖和深海热泉。其细胞壁不含肽聚糖，膜脂为醚键连接，DNA与类组蛋白结合，这些都将它们与真正的细菌区分开来。
Domain Eukaryota includes all organisms with a true nucleus and membrane-bound organelles, namely protoctists, fungi, plants, and animals. Comparing the three domains using molecular phylogeny shows that Archaea and Eukaryota share a more recent common ancestor than either shares with Bacteria, a key insight of the three-domain classification.
真核生物域包括所有具有真正细胞核和膜结合细胞器的生物，即原生生物、真菌、植物和动物。通过分子系统发育比较三个域显示，古菌与真核生物拥有比它们各自与细菌更近的共同祖先，这是三域分类系统的重要洞见。
5. The Five Kingdoms | 五界系统
Within Domain Eukaryota and Domain Bacteria, organisms may be further classified into five kingdoms: Prokaryotae, Protoctista, Fungi, Plantae, and Animalia. The five-kingdom classification is based largely on cell structure, mode of nutrition, and body organisation.
在真核生物域和细菌域内部，生物可进一步分为五个界：原核生物界、原生生物界、真菌界、植物界和动物界。五界分类主要基于细胞结构、营养方式和机体组织水平。
| Kingdom | Key Features | Example |
| Prokaryotae | No nucleus, no membrane-bound organelles, small ribosomes, cell wall of peptidoglycan | Escherichia coli |
| Protoctista | Eukaryotic, mostly unicellular, diverse nutrition | Amoeba |
| Fungi | Eukaryotic, chitin cell wall, saprophytic heterotrophs, hyphae | Mucor |
| Plantae | Eukaryotic, cellulose cell wall, autotrophic by photosynthesis, multicellular | Quercus (oak) |
| Animalia | Eukaryotic, no cell wall, heterotrophic, multicellular, nervous coordination | Homo sapiens |
Prokaryotae are the simplest organisms, lacking a true nucleus and membrane-bound organelles. Protoctists are a diverse group of mostly unicellular eukaryotes that do not fit into the other eukaryotic kingdoms. Fungi are absorptive heterotrophs with cell walls made of chitin, while plants are photosynthetic autotrophs with cellulose cell walls. Animals are ingestive heterotrophs that lack cell walls and show the greatest degree of tissue differentiation.
原核生物界是最简单的生物，缺乏真正细胞核和膜结合细胞器。原生生物界是一个多样化的类群，多为单细胞真核生物，不属于其他真核生物界。真菌是吸收式异养生物，细胞壁由几丁质组成；植物是光合自养生物，细胞壁由纤维素组成；动物是摄食式异养生物，无细胞壁，组织分化程度最高。
6. Courtship Behaviour as a Classification Mechanism | 求偶行为作为分类依据
Courtship behaviour consists of species-specific signals and actions performed by animals to attract a mate. These behaviours include visual displays, sounds, chemical signals (pheromones), and specific movements or dances. Because courtship rituals are unique to each species, they can be used to distinguish species that appear morphologically very similar.
求偶行为是动物为吸引配偶而进行的物种特异性信号和动作，包括视觉展示、声音、化学信号（信息素）以及特定的动作或舞蹈。由于求偶仪式对每个物种都是独特的，因此可用于区分外貌上非常相似的物种。
Courtship behaviour serves several vital functions. First, it enables individuals to recognise members of their own species, preventing wasteful cross-species mating. Second, it allows individuals to identify a partner of the opposite sex and one that is sexually mature and receptive. Third, it helps synchronise mating behaviour and timing so that gametes are released at the optimal moment. Finally, courtship builds a pair bond, increasing the likelihood that both parents will cooperate in raising offspring.
求偶行为具有几个重要功能。第一，它使个体能够识别同种成员，避免浪费精力的异种交配。第二，它使个体识别异性和性成熟、可接受交配的伴侣。第三，它有助于同步交配行为和时机，使配子在最佳时刻释放。最后，求偶建立成对关系，增加双亲合作抚育后代的可能性。
From a classification perspective, an animal that never displays or responds correctly to a specific courtship signal is unlikely to belong to that species. Thus, reproductive isolation through behaviour acts as a biological boundary that reinforces classification based on anatomy and genetics.
从分类角度看，一种动物若从不表现或不能正确回应某种特定的求偶信号，则它很可能不属于该物种。因此，通过行为实现的生殖隔离作为一种生物学边界，强化了基于解剖学和遗传学的分类。
7. Variation | 变异
Variation refers to the differences that exist between individuals of the same species. It is the raw material upon which natural selection acts. Variation has two broad sources: genetic and environmental.
变异指同一物种个体之间存在的差异。它是自然选择作用的原材料。变异有两个主要来源：遗传变异和环境变异。
Genetic variation arises from mutations, which create new alleles, and from meiosis. During meiosis, crossing over between homologous chromosomes shuffles alleles, and independent assortment of homologous chromosomes produces new combinations of alleles in gametes. Random fusion of gametes during fertilisation further increases genetic variation, as does the random mating of individuals within a population.
遗传变异源于产生新等位基因的突变，以及减数分裂。在减数分裂过程中，同源染色体之间的交叉互换重新组合等位基因，同源染色体的自由组合在配子中产生新的等位基因组合。受精过程中配子的随机融合进一步增加了遗传变异，种群内个体的随机交配也是如此。
Environmental variation is caused by differences in external factors such as diet, light intensity, temperature, and soil conditions. For example, two genetically identical plants may grow to very different heights if one receives abundant sunlight and water and the other does not. Most observable traits result from an interaction between genotype and environment.
环境变异由饮食、光照强度、温度和土壤条件等外部因素差异引起。例如，两株基因相同的植物，如果一株获得充足光照和水分而另一株没有，则会长得非常不同。多数可观察性状是基因型与环境相互作用的结果。
Variation may be continuous or discontinuous. Continuous variation, such as human height or mass, shows a range of phenotypes with no clear categories and is influenced by many genes (polygenic). Discontinuous variation, such as human blood group or seed shape in peas, falls into distinct categories and is controlled by one or few genes with little environmental influence.
变异可分为连续变异和不连续变异。连续变异如人类身高或体重，表现为没有明确类别的连续表型范围，受许多基因（多基因）影响。不连续变异如人类血型或豌豆种子形状，分为截然不同的类别，由一个或少数几个基因控制，受环境影响很小。
8. Natural Selection | 自然选择
Natural selection is the mechanism by which evolution occurs, first proposed by Charles Darwin and Alfred Russel Wallace. It depends on several key observations: populations produce more offspring than can survive; individuals in a population show variation; and resources such as food, space, and mates are limited.
自然选择是进化发生的机制，由查尔斯·达尔文和阿尔弗雷德·拉塞尔·华莱士首先提出。它依赖于几个关键事实：种群产生的后代数量超过能够存活的数量；种群内个体存在变异；食物、空间和配偶等资源是有限的。
Because resources are limited, individuals compete for survival. Those individuals with advantageous alleles are better adapted to their environment, making them more likely to survive and reproduce. These surviving individuals pass on their beneficial alleles to the next generation. Over many generations, the frequency of advantageous alleles in the population increases, while disadvantageous alleles decrease.
由于资源有限，个体之间发生生存竞争。携带有利等位基因的个体更适应其环境，更有可能生存和繁殖。这些存活下来的个体将有利等位基因传给下一代。经过许多世代，种群中有利等位基因的频率增加，而不利等位基因的频率降低。
An adaptation is a characteristic that enhances an organism’s ability to survive and reproduce in its particular environment. Examples include the cryptic colouration of peppered moths, the long neck of a giraffe, and antibiotic resistance in bacteria. Natural selection does not produce perfection; it simply favours traits that are best suited to the current environment, and when the environment changes, the direction of selection may also change.
适应是增强生物在特定环境中生存和繁殖能力的特征。例如桦尺蛾的保护色、长颈鹿的长颈以及细菌的抗生素耐药性。自然选择并不产生完美，它只是有利于最适应当前环境的性状；当环境改变时，选择的方向也可能改变。
9. Types of Selection | 选择的类型
Natural selection can act on a characteristic in three distinct ways: stabilising, directional, and disruptive selection. Each produces a different pattern in the distribution of phenotypes in a population over time.
自然选择可以三种不同方式作用于某一特征：稳定选择、定向选择和分裂选择。每种方式都会在种群表型分布上产生不同的模式。
Stabilising selection favours individuals with phenotypes near the mean of the population range and acts against extreme phenotypes. It reduces variation and maintains the status quo, and it is common in stable environments. For example, human birth weight is under stabilising selection: very small and very large babies have higher mortality, so most babies are born in the middle range.
稳定选择有利于种群范围内接近平均表型的个体，淘汰极端表型。它减少变异并维持现状，在稳定环境中常见。例如，人类出生体重受稳定选择作用：极轻和极重的婴儿死亡率较高，因此大多数婴儿出生体重在中间范围。
Directional selection favours one extreme phenotype, causing the mean of the population to shift in that direction. This occurs when the environment changes in a particular way. A classic example is industrial melanism in the peppered moth Biston betularia: dark forms became more common as soot darkened tree trunks, because dark moths were less visible to predators.
定向选择有利于一种极端表型，使种群平均值向该方向移动。这种情况发生在环境以特定方式改变时。经典例子是桦尺蛾（Biston betularia）的工业黑化：随着煤烟使树干变暗，黑色型变得更常见，因为黑蛾不易被捕食者发现。
Disruptive selection favours both extreme phenotypes and acts against the mean phenotype. It can lead to polymorphism and may eventually contribute to speciation. For example, in a habitat with patches of light and dark surfaces, both light-coloured and dark-coloured organisms survive better than intermediate forms, which are conspicuous on both backgrounds.
分裂选择同时有利于两种极端表型，淘汰中间表型。它可导致多态性，并可能最终促进物种形成。例如，在具有明暗斑块的环境中生境中，体色浅和体色深的生物都比中间型存活得好，因为中间型在两种背景下都显得显眼。
10. Speciation | 物种形成
A species is commonly defined as a group of organisms with similar morphological, physiological, and biochemical features that can interbreed and produce fertile offspring, and are reproductively isolated from other such groups. Speciation is the process by which new species arise through the accumulation of genetic differences over time.
物种通常被定义为一群在形态、生理和生化特征上相似、能够交配并产生可育后代的生物，它们与其他类似类群存在生殖隔离。物种形成是新物种通过遗传差异随时间的积累而产生的过程。
Allopatric speciation occurs when a population is physically divided by a geographical barrier such as a mountain range, river, or sea. The separated populations experience different environmental selection pressures and accumulate different mutations. Over time, the allele frequencies diverge until the two populations are genetically distinct; even if the geographical barrier later disappears, they can no longer interbreed and are therefore separate species.
异域物种形成发生在种群被山脉、河流或海洋等地理屏障物理分隔时。被分隔的种群经历不同的环境选择压力，积累不同的突变。随着时间推移，等位基因频率发生分化，直到两个种群在遗传上截然不同；即使地理屏障后来消失，它们也已无法交配，因此成为不同物种。
Sympatric speciation occurs without geographical separation, when reproductive isolation arises within the same habitat, often owing to behavioural barriers or polyploidy. Different courtship displays, mating seasons, or chromosome numbers prevent gene flow between subgroups. This mechanism is less common in animals but is important in plants, where polyploid individuals are reproductively isolated from their diploid progenitors.
同域物种形成不涉及地理分隔，而是在同一栖息地内因行为障碍或多倍体而产生生殖隔离。不同的求偶展示、交配季节或染色体数目阻止亚群之间的基因流动。这一机制在动物中较少见，但在植物中很重要，因为多倍体个体与其二倍体祖先之间存在生殖隔离。
Isolation is essential for speciation because it prevents gene flow, allowing divergence to occur. Beyond physical barriers, isolation may be ecological (different habitats), temporal (different breeding seasons), or behavioural (different courtship signals). All forms of isolation limit interbreeding and permit the independent evolution of separated gene pools.
隔离对物种形成至关重要，因为它阻止基因流动，使分化得以发生。除物理屏障外，隔离还可以是生态隔离（不同栖息地）、时间隔离（不同繁殖季节）或行为隔离（不同求偶信号）。所有形式的隔离都会限制杂交，允许被隔离的基因库独立进化。
11. Evidence for Evolution | 进化的证据
Evolution is supported by multiple independent lines of evidence drawn from the fossil record, comparative anatomy, comparative biochemistry, and geographical distribution.
进化得到多条独立证据的支持，这些证据来自化石记录、比较解剖学、比较生物化学和地理分布。
The fossil record provides direct evidence of past life and shows a sequence of change over geological time. Fossils of transitional organisms, such as Archaeopteryx linking reptiles and birds, document the gradual modification of lineages. The age and order of fossils in rock strata establish the historical pattern of evolutionary change.
化石记录提供了过去生命的直接证据，展示了地质时间中的变化序列。过渡生物化石，如连接爬行动物和鸟类的始祖鸟（Archaeopteryx），记录了谱系的渐进改造。岩层中化石的年龄和顺序确立了进化变化的历史模式。
Comparative anatomy reveals homologous structures: organs that have the same evolutionary origin but different functions, such as the pentadactyl (five-fingered) limb in mammals, birds, and amphibians. The presence of the same underlying bone structure in different groups strongly suggests descent from a common ancestor. In contrast, analogous structures such as insect and bird wings have similar functions but entirely different evolutionary origins.
比较解剖学揭示了同源结构：具有相同进化起源但功能不同的器官，如哺乳动物、鸟类和两栖动物的五趾型附肢。不同类群中存在相同的基础骨骼结构，强烈表明它们来自共同祖先。相比之下，昆虫翅膀和鸟翅膀等间功结构功能相似，但进化起源完全不同。
Comparative biochemistry compares molecules such as DNA, proteins, and cytochrome c across species. The more similar the base sequences of DNA and amino acid sequences of proteins, the more closely related the species are. Molecular clocks based on mutation rates allow biologists to estimate the time since two lineages diverged, and biochemical data now play a central role in constructing accurate evolutionary trees.
比较生物化学比较不同物种的DNA、蛋白质和细胞色素c等分子。DNA碱基序列和蛋白质氨基酸序列越相似，物种亲缘关系越近。基于突变率的分子钟使生物学家能够估计两个谱系分化的时间，生化数据如今在构建准确的进化树中发挥着核心作用。
Geographical distribution also provides evidence: species evolve differently in different parts of the world because of isolation. Island species, such as Darwin’s finches in the Galápagos, show adaptive radiation, where a single ancestral species gives rise to many new species each adapted to a different niche. All these lines of evidence converge to support evolution as the unifying principle of biology.
地理分布也提供证据：由于隔离，不同地区的物种以不同方式进化。岛上的物种，如加拉帕戈斯群岛的达尔文雀，表现出适应性辐射——一个祖先物种产生许多新物种，各自适应不同的生态位。所有这些证据汇合在一起，支持进化论是生物学的统一原则。
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