Alevel生物遗传多样性自然选择 AQA Edexcel

Introduction to Genetic Diversity

Genetic diversity is the total number of different alleles present in a population. It is the raw material upon which natural selection acts, and it determines a species’ ability to adapt to changing environments. Without genetic diversity, populations become vulnerable to disease, climate shifts, and other environmental pressures. The greater the genetic variation within a population, the higher the probability that some individuals will possess traits suited to survival under new conditions.

遗传多样性是指一个种群中存在的不同等位基因的总数。它是自然选择作用的原材料，决定了一个物种适应环境变化的能力。没有遗传多样性，种群就会容易受到疾病、气候变化和其他环境压力的影响。种群内的遗传变异越大，某些个体在新条件下拥有适合生存的特征的可能性就越高。

Sources of Genetic Variation

Genetic variation arises from three primary sources: mutation, meiosis, and random fertilisation. Mutations introduce new alleles into a gene pool by altering the DNA sequence. Meiosis creates variation through independent assortment of chromosomes and crossing over between homologous chromosomes. Random fertilisation further amplifies diversity because any sperm can fuse with any egg, producing a unique combination of parental alleles in each zygote.

遗传变异来自三个主要来源：突变、减数分裂和随机受精。突变通过改变DNA序列将新的等位基因引入基因库。减数分裂通过染色体的独立分配和同源染色体之间的交叉互换产生变异。随机受精进一步扩大了多样性，因为任何精子都可以与任何卵子融合，在每个受精卵中产生独特的亲本等位基因组合。

Mutations as a Source of Genetic Diversity

Mutations are permanent changes to the DNA sequence that can occur spontaneously during DNA replication or be induced by mutagens such as UV radiation, ionising radiation, and certain chemicals. Point mutations affect a single nucleotide and include substitutions, insertions, and deletions. Substitution mutations may be silent (no change to the amino acid), missense (different amino acid), or nonsense (premature stop codon). Frameshift mutations, caused by insertions or deletions that are not multiples of three nucleotides, alter every amino acid downstream of the mutation site and typically produce non-functional proteins.

突变是DNA序列的永久性改变，可能在DNA复制过程中自发发生，或由诱变剂（如紫外线、电离辐射和某些化学物质）诱导。点突变影响单个核苷酸，包括替换、插入和缺失。替换突变可能是沉默的（不改变氨基酸）、错义的（改变氨基酸）或无义的（提前出现终止密码子）。移码突变由非三的倍数的插入或缺失引起，会改变突变位点下游的所有氨基酸，通常产生无功能的蛋白质。

Meiosis and Genetic Recombination

Meiosis is the specialised cell division that produces haploid gametes from diploid germline cells. During prophase I of meiosis, homologous chromosomes pair up and form bivalents, at which point crossing over occurs. Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. This process creates recombinant chromosomes that carry new combinations of alleles, dramatically increasing genetic diversity. The chiasmata, or crossover points, are visible under a microscope as X-shaped structures where chromatids are physically linked.

减数分裂是一种特殊的细胞分裂，从二倍体的生殖细胞产生单倍体的配子。在减数第一次分裂前期I，同源染色体配对并形成二价体，此时发生交叉互换。交叉互换是同源染色体非姐妹染色单体之间遗传物质的交换。这一过程产生携带新等位基因组合的重组染色体，显著增加了遗传多样性。交叉点（交叉结）在显微镜下可见，是染色单体物理连接的X形结构。

Independent Assortment and Random Fertilisation

During metaphase I of meiosis, homologous chromosomes line up on the metaphase plate independently of each other. The orientation of each homologous pair is random, meaning that the maternal and paternal chromosomes are distributed into daughter cells in a vast number of possible combinations. For a human with 23 pairs of chromosomes, independent assortment alone can produce 2^23, or approximately 8.4 million, different gamete types. When combined with random fertilisation, where any one of millions of sperm can fertilise any one egg, the total number of possible genetic combinations in offspring is astronomically large.

在减数第一次分裂中期I，同源染色体彼此独立地排列在赤道板上。每个同源对的朝向是随机的，这意味着母本和父本染色体以大量可能的组合方式分配到子细胞中。对于具有23对染色体的人类而言，仅独立分配就可以产生2的23次方（约840万）种不同的配子类型。当与随机受精（数百万精子中的任何一个都可以与任何一个卵子结合）结合时，后代中可能的遗传组合总数极其庞大。

Natural Selection: The Mechanism of Evolution

Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It requires three conditions: variation in traits within a population, heritability of those traits, and differential reproductive success based on those traits. Organisms with advantageous alleles are more likely to survive to reproductive age and pass those alleles to the next generation. Over many generations, the frequency of beneficial alleles increases in the population while harmful alleles become rarer or are eliminated entirely. This process is the primary driver of adaptive evolution.

自然选择是指由于表型差异导致的个体生存和繁殖差异。它要求三个条件：种群内性状的变异、这些性状的可遗传性以及基于这些性状的差异化繁殖成功率。具有有利等位基因的生物体更有可能存活到繁殖年龄并将这些等位基因传递给下一代。经过许多代后，有利等位基因的频率在种群中增加，而有害等位基因变得更稀少或被完全消除。这一过程是适应性进化的主要驱动力。

Types of Selection

Stabilising selection favours the intermediate phenotype and acts against extremes. A classic example is human birth weight: babies with very low or very high birth weights have lower survival rates, so the intermediate range is favoured. Directional selection favours one extreme phenotype, shifting the population mean over time. Antibiotic resistance in bacteria is a well-documented case of directional selection, where bacteria possessing resistance alleles survive antibiotic treatment and reproduce. Disruptive selection favours both extreme phenotypes at the expense of the intermediate, and can lead to sympatric speciation if reproductive isolation develops between the two extremes.

稳定化选择有利于中间表型，并对极端表型不利。一个经典的例子是人类出生体重：出生体重极低或极高的婴儿存活率较低，因此中等范围受到青睐。方向性选择有利于其中一个极端表型，随时间推移改变种群平均值。细菌中的抗生素耐药性是方向性选择的一个充分记录的例子，拥有耐药性等位基因的细菌在抗生素治疗中存活并繁殖。分裂选择有利于两个极端表型而不利于中间表型，如果两个极端之间发展出生殖隔离，可能导致同域物种形成。

Genetic Drift and the Founder Effect

Genetic drift is a random change in allele frequencies that occurs in all populations but has its most pronounced effects in small populations. Unlike natural selection, genetic drift is not adaptive: it does not favour alleles that increase fitness. The founder effect is a special case of genetic drift that occurs when a small group of individuals colonises a new area. The new population’s gene pool is a small, non-representative sample of the original population. Over time, the allele frequencies in the founder population can diverge significantly from the source population, even in the absence of natural selection. The bottleneck effect is another form of genetic drift where a catastrophic event drastically reduces population size, randomly eliminating many alleles.

遗传漂变是等位基因频率的随机变化，发生在所有种群中，但在小种群中影响最为显著。与自然选择不同，遗传漂变不是适应性的：它不有利于增加适应性的等位基因。奠基者效应是遗传漂变的一个特殊情况，发生在一小群个体在新区域定居时。新种群的基因库是原始种群的一个小的、非代表性的样本。随着时间的推移，即使没有自然选择，奠基者种群中的等位基因频率也可能与来源种群显著不同。瓶颈效应是遗传漂变的另一种形式，灾难性事件急剧减少种群规模，随机消除许多等位基因。

Speciation and Reproductive Isolation

Speciation is the evolutionary process by which new biological species arise. Allopatric speciation occurs when a population is geographically divided by a physical barrier such as a mountain range, river, or ocean. Over time, the separated populations experience different selection pressures and accumulate distinct mutations, eventually becoming reproductively isolated. Sympatric speciation occurs without geographic separation, typically through polyploidy in plants or disruptive selection creating reproductive barriers within the same habitat. Reproductive isolation can be prezygotic (preventing fertilisation) or postzygotic (producing non-viable or infertile hybrid offspring).

物种形成是新生物物种产生的进化过程。异域物种形成发生在一个种群被物理屏障（如山脉、河流或海洋）在地理上分隔时。随着时间的推移，分隔的种群经历不同的选择压力并积累不同的突变，最终变得生殖隔离。同域物种形成发生在没有地理分隔的情况下，通常通过植物的多倍体或在同一栖息地内产生生殖屏障的分裂选择。生殖隔离可以是合子前的（阻止受精）或合子后的（产生无法存活或不育的杂交后代）。

Exam Techniques and Common Pitfalls

When answering exam questions on genetic diversity, always distinguish clearly between mutation (the origin of new alleles), meiosis (the reshuffling of existing alleles), and natural selection (the change in allele frequency over time). A common mistake is conflating natural selection with evolution: natural selection acts on individuals, but populations evolve. Another frequent error is stating that organisms “develop” adaptations to survive. The correct view is that variation already exists within the population, and those with advantageous traits are selected for. Use precise terminology: “allele frequency increases” rather than “the allele becomes stronger.”

在回答关于遗传多样性的考试题目时，始终清楚地区分突变（新等位基因的来源）、减数分裂（现有等位基因的重新组合）和自然选择（等位基因频率随时间的变化）。一个常见错误是将自然选择与进化混淆：自然选择作用于个体，但进化发生在种群层面。另一个常见错误是说生物体”发展”适应来生存。正确的观点是种群内已经存在变异，那些具有有利性状的个体被选择。使用精确的术语：”等位基因频率增加”而不是”等位基因变得更强”。

Artificial Selection and Its Relationship to Natural Selection

Artificial selection is the process by which humans deliberately breed organisms for desired traits. Unlike natural selection, where the environment determines which individuals survive and reproduce, artificial selection imposes human choice as the selective force. Domesticated crops such as maize (derived from teosinte) and domesticated animals such as dogs (derived from wolves) are striking examples. Over thousands of generations, artificial selection has produced extraordinary phenotypic divergence from wild ancestors. Artificial selection demonstrates the same underlying principle as natural selection: heritable variation exposed to differential reproductive success leads to evolutionary change. The key difference is the agent of selection: humans versus the environment.

人工选择是人类有意识地为了所需性状而繁育生物的过程。与自然选择不同（环境决定哪些个体存活和繁殖），人工选择将人类的选择作为选择力。驯化作物如玉米（源自类蜀黍）和驯化动物如狗（源自狼）是显著的例子。经过数千代的繁育，人工选择产生了与野生祖先相比非凡的表型差异。人工选择展示了与自然选择相同的基本原则：可遗传变异受到差异化繁殖成功率的作用导致进化变化。关键区别在于选择的主体：人类与环境。

Key Bilingual Terms

等位基因 · Allele | 遗传多样性 · Genetic Diversity | 突变 · Mutation | 减数分裂 · Meiosis | 交叉互换 · Crossing Over | 独立分配 · Independent Assortment | 自然选择 · Natural Selection | 选择压力 · Selection Pressure | 稳定化选择 · Stabilising Selection | 方向性选择 · Directional Selection | 分裂选择 · Disruptive Selection | 遗传漂变 · Genetic Drift | 奠基者效应 · Founder Effect | 瓶颈效应 · Bottleneck Effect | 物种形成 · Speciation | 生殖隔离 · Reproductive Isolation | 异域物种形成 · Allopatric Speciation | 同域物种形成 · Sympatric Speciation | 合子前隔离 · Prezygotic Isolation | 合子后隔离 · Postzygotic Isolation