WJEC生物 多样性 分类 三域 辛普森指数
WJEC 的 AS Unit 2 第一个 topic 叫 All organisms are related through their evolutionary history，考的是分类与生物多样性。这一块在卷面上常被学生当成背诵题处理，结果是定义背得很熟，一到比较两个生境就写不出句子。考纲在这一节里把 (a) 到 (n) 十四处考点列得相当细，从三域系统一直延伸到取样实操。下面按考纲的编号顺序把这些考点拆开，重点放在能拿分的那几句话上。
Topic 1 of AS Unit 2 in the WJEC specification covers classification and biodiversity. Most candidates can recite the kingdoms, yet fewer can compare two habitats in a sentence. The assessment points run from the three domain system down to a sampling investigation, so the notes below follow that order and stay close to what the mark scheme asks for.
分层分类与分类的暂定性
考纲点 (a) 要求把生物按进化关系分进离散而分层的组里，点 (b) 补了一句分类具有暂定性质。域、界、门、纲、目、科、属、种这一串名字是嵌套关系，不是并列的标签，一个属内的物种共享的共同祖先比同科不同属的更晚。很多学生以为分类就是把生物摆进几个盒子，忽略了盒子本身可以重排。三域系统出现之后，原来同属原核生物的细菌与古菌被拆开，五界系统里的原核生物界随之失去原有位置。分类随证据改变，这就是考纲用 tentative 这个词的原因。分类单元之间的距离靠共同祖先的远近衡量，而不是靠外形差别的大小。
Classification is hierarchical and discrete, so each group sits inside a larger one instead of beside it. The sequence runs domain, kingdom, phylum, class, order, family, genus, species, and members of a genus share a more recent common ancestor than members of the same family do. When the three domain system split the old prokaryotes into bacteria and archaea, one whole kingdom lost its place, which is why the specification calls classification tentative.
三域系统与五界系统的比较
三域系统由 Woese 等人在 1990 年提出，依据是核糖体 RNA 序列的差异，把生物分成细菌域、古菌域与真核域。五界系统保留原核生物界、原生生物界、植物界、真菌界与动物界。两套系统的分歧点集中在原核生物要不要拆开。古菌在光学显微镜下与细菌难分，细胞壁成分、膜脂与基因序列都不一样，还能在高温高盐环境里正常代谢。细菌域的成员多出现在中温环境，古菌则常见于热泉、盐湖与海底沉积物。考纲要求的是比较，不是把名字列一遍。常见的错误是只写三域更准确，阅卷时看不到比较维度就不给分。
The three domain system separates life on ribosomal RNA sequence into bacteria, archaea and eukaryotes, while the five kingdom system keeps prokaryotes together. Archaea look like bacteria down a light microscope but differ in cell wall chemistry, membrane lipids and gene sequence. A comparison answer has to name the criterion behind the split rather than announce that one system is newer.
五个界的识别特征
原核生物界没有真正的细胞核与膜结合细胞器，核糖体是 70S 型。原生生物界是单细胞或简单多细胞的真核生物，营养方式兼有类似植物与类似动物两类，分类上组成了一个并系群。这一类群里既有能游动的鞭毛类，也有固着生活的多细胞藻类。植物界为多细胞体，细胞壁含纤维素，含叶绿体，光合作用放出氧气。真菌界细胞壁含几丁质，以腐生方式吸收营养，没有叶绿体。动物界多细胞、无细胞壁、摄食式异养，多数能运动。考纲用的是 Prokaryotae 这类拉丁界名，答题写原名最稳。
The kingdoms are told apart by nucleus, cell wall chemistry and nutrition. Prokaryotae lacks a true nucleus and carries 70S ribosomes, Plantae builds cellulose walls and photosynthesises, and Fungi uses chitin walls with saprotrophic absorption. Animalia is multicellular, wall-less and heterotrophic by ingestion, so no single feature settles the whole classification.
物种概念与双名法
考纲用相邻的两点处理命名问题：物种是能够交配并产生可育后代的群体，双名法用属名加种名来表示一个物种。属名首字母大写，种名全部小写，印刷时整串斜体，手写时下面画线。Homo sapiens 与 Quercus robur 是两个标准写法。很多学生以为第二个词描述外形特征，其实它只是一个标识，同一个种名可以出现在完全不同的属里。物种概念本身有边界问题，无性繁殖的生物与化石材料都套不进去。同一物种的不同种群在地理隔离之后，各自的等位基因频率会慢慢分开。
A species is a group whose members can interbreed to produce fertile offspring. The binomial system pairs a genus with a specific epithet, capitalises the genus, lowercases the epithet and italicises the pair in print or underlines it by hand. Asexual organisms and fossil material do not fit the definition neatly, so the concept works as a tool rather than a fixed rule.
趋同进化与分子证据
外形相似不等于亲缘相近。仙人掌科植物与大戟科的某些成员都在干旱环境里长出肉质茎与刺，这是趋同进化的结果，两者的共同祖先并没有这些结构。考纲点 (e) 要求用 DNA 指纹与酶的研究判断亲缘关系，因为分子层面的相似性不受形态趋同干扰。比较细胞色素 c 的氨基酸序列，或做 DNA 杂交测相似度，都能给出可量化的证据。分子证据背后的假设是序列差异随分开时间累积，差异量因此可以换算成大致的分开时长。常见的错误是只凭外形下结论，答案里缺一条生化证据。
Similar shape is not evidence of shared ancestry, because cacti and several Euphorbia species both evolved fleshy stems and spines in dry habitats. DNA fingerprinting and enzyme studies give measures of relatedness that morphological convergence cannot fake. An answer built on appearance alone leaves the biochemical evidence out of the mark scheme.
生物多样性的定义与时空变化
考纲点 (h) 把生物多样性定义为特定地理区域内生物的数量与种类，这里有两个限定语容易被读漏：一是区域，二是数量与种类两个成分。点 (i) 补充说多样性在空间与时间上都在变化。一处林地里的物种数量会随海拔、土壤酸碱度与光照改变，也会随季节与演替阶段改变。热带雨林的物种数目远高于同面积的温带针叶林。同一条河流的上游与下游，物种组成往往完全不同。评估一个生境时，一次取样只能代表当时当地那一小块。
Biodiversity counts both the number of organisms and the variety of kinds within a stated region, so the region matters as much as the count. It shifts with altitude, soil pH and light, and it shifts again with season and successional stage. One visit therefore describes one moment in one place, not the habitat as a whole.
辛普森指数的公式与算例
辛普森多样性指数同时考虑物种丰富度与均匀度，公式写成 D 等于 1 减去各物种个体数占比的平方和，其中 n 表示某物种的个体数，N 表示全部个体数。举一个样方：五个物种的个体数分别为 12、8、5、4、1，N 等于 30。各占比依次是 0.4、0.267、0.167、0.133、0.033，平方后求和约 0.278，D 约等于 0.72。算到小数点后两位足够，题干若给出别的修正形式，就按题干给的式子代入。代入时分子与分母都用个体数，不要换成物种数。
Simpson's index combines species richness with evenness, subtracting the squared proportions from one. With counts of 12, 8, 5, 4 and 1 the total is 30, the proportions fall from 0.4 to 0.033 and the squared sum comes to about 0.278. That leaves D at roughly 0.72, and two decimal places are enough to earn the mark.
辛普森指数的判读与常见错误
D 的取值范围是 0 到 1，数值越大表示多样性越高。0 意味着全部个体属于同一个物种，接近 1 意味着个体比较均匀地分布在很多物种上。比较两个生境要看 D 的大小，不能只看物种数目。另一个样方同样是五个物种，个体数为 20、6、2、1、1，N 也是 30，D 约等于 0.51。物种数目相同而 D 更低，差在均匀度上。很多学生以为物种数一样就代表多样性一样，这一步最容易丢分。
D runs from zero to one, and a higher value means greater diversity. A second quadrat holding counts of 20, 6, 2, 1 and 1 also has five species and 30 individuals, yet its D falls to about 0.51. Equal richness with unequal evenness gives a different answer, and that comparison is where marks usually disappear.
遗传多样性：多态位点与基因库
考纲点 (k) 要求在物种内部评估多样性，做法是看种群基因库里等位基因的丰富程度，而不是看个体。判定标准是一个位点存在两个或更多等位基因，并且频率高于突变单独能维持的水平，这样的位点叫多态位点。基因组里多态位点所占的比例越高，这个种群的遗传多样性越大。小种群经历瓶颈之后，多态位点的比例会明显下降。保护遗传学里常用这个指标判断种群的适应潜力。比例高说明这个种群在过去长期维持了较大的有效群体规模。
Genetic diversity is read from the gene pool rather than from individual organisms. A locus counts as polymorphic when two or more alleles persist at frequencies that mutation alone could not maintain, and the proportion of such loci across the genome supplies the measure. Populations that pass through a bottleneck lose polymorphic loci, which limits how they can respond to change.
DNA 指纹与测序
考纲点 (l) 把评估推到分子层面，工具是 DNA 指纹与测序。DNA 指纹利用非编码区重复序列的长度差异，酶切后电泳，条带的位置与数目构成图谱。测序直接读出碱基顺序，单个碱基的差异称为 SNP。亲缘关系越近，序列差异越少。两种方法都不依赖形态，因此在隐存种与幼体鉴定上特别有用。取样只需少量组织，但要避免交叉污染，否则条带会混在一起。
Molecular assessment replaces morphology with DNA fingerprinting or sequencing. Fingerprinting compares the lengths of repeated non-coding regions after digestion and electrophoresis, while sequencing reads bases directly and treats single base differences as SNPs. Closely related organisms share more sequence, and a small tissue sample is enough provided contamination is avoided.
自然选择与等位基因频率
考纲点 (m) 说多样性由自然选择产生。种群内部本来就有变异，突变与减数分裂时的重组是变异的来源。环境压力让某些表型留下更多可育后代，相应等位基因的频率逐代上升。方向性选择让分布整体向一侧移动，稳定化选择压缩两端，分裂性选择把种群推向两个峰。抗生素耐药与工业黑化都是可以直接观察的例子。这里的关键是频率变化，不是个体在活着的时候改变自己。
Natural selection generates diversity by shifting allele frequencies rather than by improving individuals on demand. Mutation and recombination supply the variation, and a selective pressure lets some phenotypes leave more fertile offspring behind. Directional, stabilising and disruptive selection describe three ways a distribution can move, with antibiotic resistance as the fastest example to observe.
三类适应：结构、生理与行为
考纲点 (n) 要求区分结构适应、生理适应与行为适应。结构适应是身体形态上的改变，例如旱生植物下陷的气孔与卷曲的叶片。生理适应是代谢或功能上的调整，例如骆驼在缺水时浓缩尿液、减少排汗。行为适应是动作与节律上的调整，例如沙漠动物夜行、蜗牛在干季封壳休眠。一个物种往往同时具备几类，答题时要按题目给的类别归位。把行为适应写成生理适应是很常见的失分点。
Adaptations come in three kinds, and the mark depends on putting each example in the right box. Anatomy covers form, such as sunken stomata and rolled leaves in xerophytes; physiology covers function, such as the camel's concentrated urine; behaviour covers timing and movement, such as nocturnal desert activity. Most species combine all three, so one example rarely earns full credit.
生境取样实操与实验评估
考纲指定的实验是生境生物多样性调查。样方调查必须随机取样，坐标由随机数生成，不能凭眼睛挑一块看起来有代表性的地方。动物取样可用扫网、踢样或诱捕，不同方法对体型与活动速度都有偏向，扫网漏掉贴底的种类，踢样则把底栖的冲进网里。取样量要足够，单个样方只能代表一小块地面。评估方法时要写清随机化、样本量与重复次数，再指出一种具体偏差来源。很多学生以为算出 D 就完成了评估，其实评分点在后半段。
The specified practical is an investigation into biodiversity in a habitat, and random sampling is the part that earns method marks. Quadrat positions should come from random numbers, and animal techniques such as kick sampling or sweep netting each bias the catch towards particular sizes and speeds. An evaluation scores by naming the randomisation, the sample size and one concrete source of error.
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