High School Biology — Evolution and Natural Selection — Solutions

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Section A · Worked SolutionsA 部分 · 详细解答

Q1EASY易 🇺🇸 US美 AP-style MCQAP 风格选择题 §1 Evidence for Evolution进化的证据 · HS-LS4-1 [3 marks][3 分]

The forelimbs of humans, whales, bats, and horses all share the same arrangement of bones (humerus, radius, ulna, carpals) even though the limbs perform very different functions. This evidence is best described as:人类、鲸鱼、蝙蝠和马的前肢骨骼排列相同（肱骨、桡骨、尺骨、腕骨），尽管功能各异。这一证据最恰当的描述是：

Answer:答案： (B) Homologous structures showing common ancestry同源结构，表明共同祖先

(a) Identify the correct term and justify确定正确术语并说明理由 A1·A1·A1

Homologous structures are anatomical features inherited from a common ancestor that share the same underlying bone arrangement (same evolutionary origin) despite serving different functions in different species. The shared humerus-radius-ulna-carpals pattern across mammals is the defining example: it reflects descent from a common tetrapod ancestor, not convergent adaptation to similar environments.同源结构是从共同祖先遗传而来的解剖特征，尽管在不同物种中功能各异，但具有相同的基本骨骼排列（相同的进化起源）。哺乳动物共享的肱骨-桡骨-尺骨-腕骨模式正是典型案例：它反映了源自共同四肢动物祖先的遗传关系，而非对相似环境的趋同适应。

Why the distractors fail.干扰项分析。

(A) Analogous structures (e.g., bird wing and insect wing) share function but NOT common ancestry or bone arrangement. Here the bones ARE homologous.同功结构（如鸟翼与昆虫翅膀）功能相似，但不具有共同祖先或相同骨骼排列，此题骨骼排列相同，故排除。

(C) Vestigial structures are reduced or non-functional remnants (e.g., human coccyx); adaptive radiation is a pattern of diversification, not a type of structural evidence.痕迹结构是退化或无功能的残余（如人体尾椎）；适应辐射是多样化模式，并非结构证据的类型。

(D) DNA sequencing is biochemical evidence, not structural/anatomical evidence. The question describes bone arrangement, making (B) the only match.DNA 测序属于生化证据，而非结构/解剖证据。题目描述的是骨骼排列，因此 (B) 是唯一正确答案。

Homologous vs. analogous: origin vs. function.同源结构与同功结构：起源与功能之别。 The key distinction is evolutionary origin, not current function. Homologous structures (same origin, different function) reveal common descent. Analogous structures (different origin, similar function) reveal convergent evolution under similar selective pressures. Remembering the mnemonic: Homologous = same History; Analogous = same Action. The forelimb example is the most tested homology in high school biology because it spans swimming (whale flipper), flying (bat wing), running (horse leg), and grasping (human arm) -- four radically different functions from one ancestral limb plan.关键区别在于进化起源，而非当前功能。同源结构（起源相同，功能各异）揭示共同祖先。同功结构（起源不同，功能相似）揭示在相似选择压力下的趋同进化。记忆口诀：同源 = 同历史（Homologous = same History）；同功 = 同动作（Analogous = same Action）。前肢案例是高中生物中考查最多的同源性示例，因为它涵盖游泳（鲸鳍）、飞翔（蝙蝠翼）、奔跑（马腿）和抓握（人臂）四种截然不同的功能，均源自同一祖先肢体方案。

Q2EASY易 🇺🇸 US美 AP-style MCQAP 风格选择题 §2 Darwin and Natural Selection达尔文与自然选择 · HS-LS4-2 [3 marks][3 分]

Which of the following is NOT one of Darwin's four postulates for natural selection to occur?以下哪项不是达尔文自然选择发生的四个公设之一？

Answer:答案： (C) Individuals with favorable traits always survive to reproduce具有有利性状的个体总能存活并繁殖

(a) State Darwin's four postulates and identify the false one陈述达尔文四个公设并找出错误项 A1·A1·A1

Darwin's four postulates are: (1) Variation: individuals differ in their traits. (2) Heritability: some variation is passed from parent to offspring. (3) Overproduction (struggle for existence): more offspring are produced than can survive. (4) Differential survival/reproduction: individuals with favorable traits are more likely (not certain) to survive and reproduce. Option (C) replaces "more likely" with "always," which is false. Natural selection is probabilistic, not deterministic: even the best-adapted individual can die by chance (predation, disease, accident).达尔文四个公设为：(1) 变异：个体在性状上存在差异。(2) 遗传性：部分变异可从亲代传给后代。(3) 过度繁殖（生存竞争）：产生的后代多于能存活的数量。(4) 差异存活/繁殖：具有有利性状的个体更有可能（并非必然）存活并繁殖。选项 (C) 将"更有可能"改为"总能"，这是错误的。自然选择是概率性的，而非决定性的：即使是适应性最强的个体也可能因偶然因素（捕食、疾病、意外）而死亡。

Natural selection is differential reproductive success, not guaranteed success.自然选择是差异性繁殖成功，而非有保障的成功。 The word "always" in option (C) is the trap. Natural selection shifts the statistical distribution of traits across generations; it does not guarantee any individual's survival. This probabilistic nature is why small populations can lose beneficial alleles by genetic drift even while natural selection favors them. Examiners frequently test whether students confuse "increased probability of survival" (correct) with "guaranteed survival" (incorrect). The four postulates form a logical syllogism: given (1)-(3), (4) must follow as a consequence, producing evolution.选项 (C) 中的"总能"是陷阱所在。自然选择改变的是世代间性状的统计分布，并不保证任何个体的存活。正是这种概率性，使得小种群即使在自然选择偏向某有益等位基因时，也可能因基因漂变而失去该等位基因。考题经常考查学生是否将"存活概率更高"（正确）与"必然存活"（错误）混淆。四个公设构成一个逻辑三段论：给定 (1)-(3)，(4) 必然随之而来，从而产生进化。

Q3MEDIUM中 🇨🇦 ON安 ON Provincial-style安大略省考风格 §4 Mechanisms of Evolution进化机制 · SBI3U [5 marks][5 分]

A small group of 15 birds blown to a remote island; island population later shows very different allele frequencies from the mainland.15 只鸟被暴风雨吹到偏远岛屿，该岛屿种群后来显示出与大陆截然不同的等位基因频率。

Answer:答案： (a) Genetic drift基因漂变 · (b) Sampling error is proportionally larger in small populations小种群中抽样误差比例更大 · (c) Founder effect奠基者效应

(a) Identify the mechanism指出进化机制 A1·A1

The mechanism is genetic drift: random changes in allele frequency due to chance events rather than natural selection. The 15 birds carry only a random subset of the mainland gene pool, so allele frequencies differ by chance from the start.该机制为基因漂变：等位基因频率因偶然事件而发生随机变化，而非由自然选择驱动。这 15 只鸟只携带了大陆基因库的随机子集，因此等位基因频率从一开始就因偶然性而与大陆种群不同。

(b) Explain why small populations are more strongly affected解释小种群受影响更强的原因 A1·A1

In a large population, random deviations in allele frequency average out across many individuals, so drift has minimal effect. In a small population of 15 birds, the loss or gain of even a single individual represents a large fraction of the gene pool. Random events (death before reproduction, failure to find a mate) can drastically change allele frequencies in one generation. The statistical principle is that variance in allele frequency change is inversely proportional to population size: $\text{Var}(\Delta p) \approx p(1-p)/(2N)$. Smaller $N$ means greater variance and more powerful drift.在大种群中，等位基因频率的随机偏差在众多个体间相互抵消，基因漂变影响微弱。在仅有 15 只鸟的小种群中，哪怕只损失或增加一个个体，也代表基因库的较大比例。随机事件（繁殖前死亡、找不到配偶）可在一个世代内大幅改变等位基因频率。统计原理表明，等位基因频率变化的方差与种群大小成反比：$\text{Var}(\Delta p) \approx p(1-p)/(2N)$。$N$ 越小，方差越大，漂变效应越强。

(c) Name this specific type of event写出该事件的具体名称 A1

This is the founder effect: a specific type of genetic drift that occurs when a small group (the "founders") establishes a new population, carrying only a non-representative sample of the original gene pool.这是奠基者效应：一种特定类型的基因漂变，发生在一小群"奠基者"建立新种群时，它们只携带了原始基因库的非代表性样本。

Founder effect vs. bottleneck: both are genetic drift, different triggers.奠基者效应与瓶颈效应：同属基因漂变，触发因素不同。 The founder effect occurs when a small group colonizes a new area (geographical dispersal). A population bottleneck occurs when an existing population is dramatically reduced in size by a catastrophe (drought, disease, hunting), then recovers. Both reduce genetic diversity and fix allele frequencies by chance. The Kaibab squirrel and Amish populations are classic founder effect examples; the cheetah's near-extinction in the ice age is a classic bottleneck. Distinguishing these two is a common exam question: ask "did they move to a new place?" (founder) or "did the existing population crash?" (bottleneck).奠基者效应发生在一小群个体迁入新地区（地理扩散）时。种群瓶颈效应发生在现有种群因灾难（干旱、疾病、猎杀）骤然减少后又恢复时。两者都因偶然性减少遗传多样性并固定等位基因频率。凯巴布松鼠和阿米什人群体是奠基者效应的经典案例；猎豹在冰河时代几乎灭绝是瓶颈效应的经典案例。区分两者是常见考题：问"他们是否迁往新地点？"（奠基者效应）还是"现有种群是否骤然崩溃？"（瓶颈效应）。

Q4MEDIUM中 🇨🇦 BC卑 BC Provincial-style卑诗省考风格 §3 Variation, Adaptation and Fitness变异、适应与适合度 · Biology 12 [6 marks][6 分]

Beetle population: body color from light tan to dark brown; birds preferentially eat light beetles against dark soil.甲虫种群体色从浅棕黄色到深棕色不等；鸟类优先捕食在深色土壤上更显眼的浅色甲虫。

Answer:答案： (a) Directional selection; distribution shifts toward darker color定向选择；分布向深色偏移 · (b) Dark beetles have highest relative fitness深色甲虫相对适合度最高 · (c) Mutation (accept: recombination)突变（可接受：重组）

(a) Type of selection and change in distribution选择类型及分布变化 A1·A1·A1

This is directional selection: selection consistently favors one extreme phenotype (dark color) over the other extreme and the middle. Over time the frequency of dark-body alleles increases each generation as lighter beetles are disproportionately removed by predation. The distribution of body color shifts from a bell curve centered on intermediate color toward a distribution with a peak at the dark end. Eventually most beetles in the population will be dark.这是定向选择：选择持续偏向一种极端表现型（深色），而非另一极端或中间型。随着时间推移，深色体色等位基因的频率每代递增，因为浅色甲虫被捕食的比例更高。体色分布从以中间色为中心的钟形曲线向以深色为峰值的分布偏移。最终种群中大多数甲虫将变为深色。

(b) Define relative fitness and identify highest-fitness beetles定义相对适合度并指出适合度最高的甲虫 A1·A1

Relative fitness is the reproductive success of one genotype/phenotype compared to the most successful genotype in the same environment (scaled 0 to 1, where 1 = the most successful). In this population, dark-colored beetles have the highest relative fitness (fitness = 1) because they are least visible to predators on dark soil, survive longer, and therefore leave more offspring than light-colored beetles.相对适合度是某一基因型/表现型与同一环境中最成功基因型相比的繁殖成功率（取值 0 至 1，1 = 最成功）。在该种群中，深色甲虫相对适合度最高（适合度 = 1），因为它们在深色土壤上对捕食者的可见度最低，存活时间更长，因此比浅色甲虫留下更多后代。

(c) Source of heritable variation可遗传变异的来源 A1

Mutation is the original source of new alleles for body-color genes (e.g., a point mutation in a pigmentation gene could create a new, darker variant). Genetic recombination during meiosis can shuffle existing alleles into new combinations that produce different color phenotypes.突变是体色基因新等位基因的原始来源（例如，色素基因的点突变可以产生新的更深色变体）。减数分裂过程中的基因重组可将现有等位基因重新排列，产生不同的颜色表现型组合。

Three types of natural selection: directional, stabilizing, and disruptive.自然选择的三种类型：定向选择、稳定化选择和分裂选择。 Directional selection (this question) favors one extreme: the mean shifts, and variance may decrease. Stabilizing selection favors the intermediate phenotype: the mean stays the same but variance decreases (e.g., human birth weight). Disruptive (diversifying) selection favors both extremes over the middle: variance increases and the population may split (e.g., beak size in finches with bimodal seed distributions). The peppered moth (Q7) is another directional selection example. Distinguishing these three by which phenotypes are favored is a recurring AP and provincial exam item.定向选择（本题）偏向某一极端：均值偏移，方差可能减小。稳定化选择偏向中间表现型：均值不变但方差减小（如人类出生体重）。分裂（多样化）选择偏向两个极端而非中间型：方差增大，种群可能分裂（如双峰种子分布中的雀喙大小）。椒花蛾（Q7）是另一个定向选择案例。区分这三种类型（依据哪种表现型受到偏向）是 AP 和省级考试的常见考点。

Q5MEDIUM中 🇨🇦 AB阿 AB Diploma-style阿尔伯塔毕业考风格 §5 Speciation物种形成 · Biology 30 D1.4k [8 marks][8 分]

Grand Canyon formed ~5-6 million years ago, splitting squirrel population. Kaibab squirrel (North Rim) and Abert squirrel (South Rim) differ in coat color and cannot interbreed successfully.大峡谷约 500-600 万年前形成，将松鼠种群一分为二。凯巴布松鼠（北缘）与阿伯特松鼠（南缘）毛色不同，无法成功交配繁殖。

Answer:答案： (a) Allopatric speciation; geographic isolation异域物种形成；地理隔离 · (b) See sequence below见以下顺序 · (c) Unlikely to merge; reproductive isolation already established不太可能重新融合；生殖隔离已建立

(a) Type of speciation and isolation物种形成类型及隔离类型 A1·A1

This is allopatric speciation ("allo" = other, "patric" = homeland): speciation driven by geographic isolation. The Grand Canyon is a physical barrier (geographic isolation) that prevented gene flow between the two populations for millions of years.这是异域物种形成（"allo" = 另外，"patric" = 家园）：由地理隔离驱动的物种形成。大峡谷是一道物理屏障（地理隔离），数百万年来阻止了两个种群之间的基因流。

(b) Sequence of events from one to two species从一个到两个物种的事件顺序 A1·A1·A1

One ancestral squirrel population lived on both sides before the canyon formed.大峡谷形成前，一个祖先松鼠种群生活在两侧。
The canyon formed, creating a geographic barrier that prevented migration and gene flow between the two groups.大峡谷形成，产生地理屏障，阻止了两群个体之间的迁徙和基因流。
Each isolated population accumulated different random mutations and experienced different selective pressures (different vegetation, climate, predators on each rim), causing divergence in allele frequencies.每个隔离种群积累了不同的随机突变，并经历了不同的选择压力（两侧植被、气候、捕食者不同），导致等位基因频率分化。
Over millions of generations, genetic divergence accumulated to the point where the two populations can no longer interbreed successfully: reproductive isolation evolved, completing speciation.经过数百万代，遗传分化积累到两个种群无法成功交配的程度：生殖隔离演化形成，物种形成完成。

(c) Evaluate the "fill the canyon" suggestion评价"填平大峡谷"的建议 A1·A1·A1

The suggestion is unlikely to be correct. Reproductive isolation has already been established: the two populations cannot interbreed successfully, meaning they have evolved different mating signals, different gamete compatibility, or post-zygotic incompatibilities. Removing the geographic barrier would allow the two species to come into contact, but reproductive isolation would prevent them from exchanging genes and merging into one gene pool. Geographic isolation was only the initiating mechanism; the reproductive isolation that now exists is biological and cannot be reversed simply by reconnecting the habitats. The populations would coexist as two species, not merge back into one.该建议不太可能正确。生殖隔离已经建立：两个种群无法成功交配，意味着它们已进化出不同的交配信号、不同的配子相容性，或合子后不兼容性。移除地理屏障会让两个物种接触，但生殖隔离会阻止它们交换基因并融合为同一基因库。地理隔离只是启动机制；现在存在的生殖隔离是生物学上的，无法仅仅通过重新连接栖息地来逆转。两个种群将作为两个物种共存，而非重新融合为一个。

Geographic isolation initiates allopatric speciation; reproductive isolation completes it.地理隔离启动异域物种形成；生殖隔离完成物种形成。 A critical conceptual point: geographic isolation is not sufficient by itself to define two species. Two populations that are geographically separated but could still interbreed if brought together are subspecies or varieties, not separate species (under the biological species concept). Speciation is complete only when reproductive isolation evolves. This is why the test of whether the canyon squirrels are truly different species is whether they can interbreed -- and the answer is no. The biological species concept (Mayr 1942) defines a species as a group of populations that actually or potentially interbreed and are reproductively isolated from other groups.一个关键概念：地理隔离本身并不足以定义两个物种。两个地理上隔离但若聚到一起仍可交配的种群，是亚种或变种，而非独立物种（根据生物物种概念）。只有当生殖隔离进化形成时，物种形成才算完成。这就是为什么检验大峡谷松鼠是否真正属于不同物种的标准是它们是否能够交配，答案是不能。生物物种概念（迈尔，1942）将物种定义为实际上或潜在地相互交配、并与其他群体生殖隔离的种群集合。

Section B · Worked SolutionsB 部分 · 详细解答

Q6EASY易 🇺🇸 US美 AP-feeder FRQAP 衔接简答题 §1 Evidence for Evolution进化的证据 · HS-LS4-1 [7 marks][7 分]

Multiple lines of evidence for evolution by common descent: fossil record, comparative embryology, molecular evidence; plus one fossil record limitation.支持共同祖先进化论的多条证据：化石记录、比较胚胎学、分子证据；及化石记录的一个局限性。

Answer: See detailed sub-part solutions below.答案：详见以下各子问题解析。

(a) Fossil record化石记录 A1·A1

The fossil record is the preserved remains (bones, shells, imprints) of organisms from the past, organized in sedimentary rock layers (strata) from oldest (deepest) to youngest (shallowest). It supports evolution by common descent because: (1) fossils show a progression of forms over time -- simpler organisms appear in older strata, more complex and diverse forms in younger strata; (2) transitional fossils (e.g., Tiktaalik, Archaeopteryx) show intermediate features between ancestral and descendant groups, exactly as predicted by gradual descent with modification.化石记录是保存在沉积岩层（地层）中的古代生物遗骸（骨骼、贝壳、印迹），按从最古老（最深）到最年轻（最浅）排列。它支持共同祖先进化论，原因是：(1) 化石显示出随时间推移的生命形式演进，较简单的生物出现在较老地层，更复杂多样的形式出现在较新地层；(2) 过渡化石（如提塔利克鱼、始祖鸟）显示出祖先类群与后代类群之间的中间特征，正如渐进式"有修改的后代"理论所预测的。

(b) Comparative embryology比较胚胎学 A1·A1

Comparative embryology is the study and comparison of embryonic development across species. It supports common descent because: distantly related vertebrates (fish, frogs, reptiles, birds, mammals -- including humans) share strikingly similar embryonic stages, including pharyngeal (gill) slits and a post-anal tail, even in species that lack these structures as adults. These shared developmental stages reveal conserved genetic programs inherited from a common ancestor. The embryos are more similar to each other than the adults are, because developmental pathways branch only later in embryogenesis.比较胚胎学是对不同物种胚胎发育过程的研究与比较。它支持共同祖先进化论，原因是：亲缘关系较远的脊椎动物（鱼类、蛙类、爬行类、鸟类、哺乳类，包括人类）共享惊人相似的胚胎发育阶段，包括咽弓（鳃）裂和肛后尾，即使在成体中不具备这些结构的物种中也存在。这些共同的发育阶段揭示了遗传自共同祖先的保守遗传程序。胚胎彼此之间的相似程度高于成体之间，因为发育路径只在胚胎发生较晚期才发生分歧。

(c) Molecular/biochemical evidence分子/生化证据 A1·A1

DNA and protein sequence comparisons provide quantitative evidence for common descent. Species that share a more recent common ancestor have more similar DNA and protein sequences. For example, human and chimpanzee DNA is approximately 98-99% identical; human and yeast share many of the same core metabolic genes. Cytochrome c (a protein involved in cellular respiration) is nearly identical across all eukaryotes, reflecting its ancient origin. Shared non-functional DNA sequences (e.g., shared endogenous retroviruses, shared pseudogenes) provide especially compelling evidence because non-functional sequences could not have been selected for independently.DNA 和蛋白质序列比较为共同祖先进化论提供了定量证据。共享更近期共同祖先的物种具有更相似的 DNA 和蛋白质序列。例如，人类与黑猩猩的 DNA 约有 98-99% 相同；人类与酵母共享许多相同的核心代谢基因。细胞色素 c（参与细胞呼吸的蛋白质）在所有真核生物中几乎相同，反映了其古老起源。共享的非功能性 DNA 序列（如共享的内源性逆转录病毒、共享的假基因）提供了尤为有力的证据，因为非功能性序列不可能被独立地选择出来。

(d) One limitation of the fossil record化石记录的一个局限性 A1

Accept any one of: (1) Soft-bodied organisms rarely fossilize, so the record is biased toward hard-bodied species and underrepresents the true diversity of past life. (2) The fossil record is incomplete because most organisms decompose before fossilization can occur; preservation requires specific conditions (burial in sediment, absence of oxygen, etc.). (3) Many fossils have not yet been discovered; the record is a work in progress.接受以下任一：(1) 软体生物很少留下化石，因此记录偏向硬体物种，低估了古代生命的真实多样性。(2) 化石记录不完整，因为大多数生物在化石化之前就已分解；保存需要特定条件（被沉积物掩埋、缺氧环境等）。(3) 许多化石尚未被发现；化石记录是一项持续进行中的工作。

Consilience of evidence: multiple independent lines converge on the same conclusion.证据的协调一致性：多条独立证据线索指向同一结论。 What makes the case for evolution so strong is not any single line of evidence but the convergence of multiple independent methods. Fossils, embryology, anatomy (homologous structures, vestigial organs), biogeography (island species patterns), and molecular biology all independently support the same pattern: life has diverged from common ancestors over time. No single limitation of any one method undermines the overall picture because the other methods compensate. This is called consilience of inductions. Examiners often ask students to name the limitation of a particular evidence type to test whether they understand that science builds its case through redundant, cross-checking evidence streams.进化论证据如此有力，不在于任何单一证据，而在于多种独立方法的汇聚。化石、胚胎学、解剖学（同源结构、痕迹器官）、生物地理学（岛屿物种模式）和分子生物学都独立支持同一模式：生命随时间从共同祖先分化而来。任何单一方法的局限性都不会动摇整体图景，因为其他方法可以弥补。这被称为归纳法的协调一致性。考题经常要求学生指出某类特定证据的局限性，以检验他们是否理解科学依靠冗余的、相互印证的证据流来构建其论据。

Q7MEDIUM中 🇨🇦 ON安 ON Provincial-style安大略省考风格 §2 + §3 Darwin + Variation and Adaptation达尔文 + 变异与适应 · SBI3U [8 marks][8 分]

Peppered moth (Biston betularia): light moths common before industrialization; dark moths became dominant after soot blackened tree bark; light moths rebounded after clean-air legislation.椒花蛾：工业革命前浅色蛾较多；工业化使树皮变黑后深色蛾占主导；清洁空气立法后浅色蛾再次增多。

Answer:答案： (a) See four postulates below见以下四个公设解析 · (b) Selection pressure reversed; adaptation tracks the new environment选择压力反转；适应追踪新环境 · (c) Microevolution小进化

(a) Darwin's four postulates applied to peppered moths after industrialization达尔文四个公设在工业化后椒花蛾案例中的应用 A1·A1·A1·A1

Variation:变异： The moth population contains individuals with both light (speckled) and dark (melanic) color phenotypes. This heritable variation already exists before industrialization.蛾种群包含浅色（斑点型）和深色（黑化型）两种表现型的个体。这种可遗传变异在工业化之前已存在。
Heritability:遗传性： Body color is genetically controlled and passed from parents to offspring. The melanic form is caused by dominant alleles at a pigmentation gene.体色受遗传控制，由亲代传给后代。黑化型由色素基因的显性等位基因控制。
Overproduction:过度繁殖： Moths produce many more offspring than can survive to reproduce, creating competition for survival.蛾产生的后代远多于能存活并繁殖的数量，形成生存竞争。
Differential survival:差异存活： After industrialization blackened tree bark with soot, dark moths are better camouflaged against the dark bark and are less frequently eaten by predatory birds. Light moths are more visible and are eaten at a higher rate. Dark moths therefore survive longer and produce more offspring, increasing the frequency of dark alleles each generation until dark moths dominate.工业化使树皮被烟尘染黑后，深色蛾在深色树皮上的伪装效果更好，被捕食鸟类吃掉的频率更低。浅色蛾更显眼，被捕食率更高。深色蛾因此存活更久，产生更多后代，使深色等位基因频率逐代增加，直至深色蛾占主导。

(b) Recovery of light moths after clean-air legislation清洁空气立法后浅色蛾的恢复 A1·A1

When pollution decreased and tree bark returned to its natural light, lichen-covered color, the selection pressure reversed. Light moths became better camouflaged on the now-lighter bark, while dark moths became more conspicuous to predators. Light moths now had higher survival and reproductive success. Because the variation (both light and dark alleles) was still present in the population (the dark alleles had not been completely eliminated), natural selection could quickly shift allele frequencies back toward the light phenotype. This demonstrates that adaptation is a response to the current environment, not a permanent progressive trend.当污染减少、树皮恢复为天然的浅色地衣覆盖状态时，选择压力反转。浅色蛾在现在较浅的树皮上伪装效果更好，而深色蛾对捕食者变得更为显眼。浅色蛾现在具有更高的存活率和繁殖成功率。由于变异（浅色和深色等位基因）仍然存在于种群中（深色等位基因尚未被完全消除），自然选择能够迅速将等位基因频率重新偏向浅色表现型。这表明适应是对当前环境的响应，而非永久的进步趋势。

(c) Microevolution or macroevolution?小进化还是大进化？ A1·A1

This is microevolution: a change in allele frequency within a single population of one species over a relatively short time period. The peppered moths remained the same species (Biston betularia) throughout; no new species formed and no major body plan changes occurred. Macroevolution refers to evolution at or above the species level (speciation, adaptive radiation, extinction events, the origin of major body plans). The peppered moth example is the most celebrated demonstration of microevolution because the allele frequency change was documented in real time by Kettlewell's studies in the 1950s.这是小进化：同一物种的单个种群内等位基因频率在相对较短时间内的变化。椒花蛾自始至终仍是同一物种（Biston betularia）；没有新物种形成，也没有发生主要的体制变化。大进化是指物种水平及以上的进化（物种形成、适应辐射、灭绝事件、主要体制的起源）。椒花蛾案例是最著名的小进化演示，因为 Kettlewell 在 1950 年代的研究实时记录了等位基因频率的变化。

Natural selection only acts on existing variation; it cannot create new mutations on demand.自然选择只作用于现有变异；它不能按需产生新突变。 A common misconception is that industrialization "caused" moths to become darker. In fact, the dark (melanic) form existed as a rare variant before industrialization, controlled by a pre-existing dominant allele. Industrialization changed the environment (selection pressure), not the mutation rate. Natural selection merely shifted the relative proportions of existing variants by differentially favoring dark moths. The same logic applies to antibiotic resistance (Q9): the resistant bacteria existed before the antibiotic was introduced. Selection acts on standing variation; mutation provides the raw material, but on its own timescale. This is why populations with greater genetic diversity respond faster to environmental change.一个常见误解是工业化"导致"蛾变得更深色。实际上，深色（黑化型）形态在工业化之前就以罕见变体形式存在，由一个预先存在的显性等位基因控制。工业化改变的是环境（选择压力），而非突变率。自然选择仅仅通过差异性偏向深色蛾改变了现有变体的相对比例。同样的逻辑适用于抗生素耐药性（Q9）：耐药细菌在抗生素引入之前就已存在。选择作用于现有变异；突变提供原材料，但按照自身的时间尺度进行。这就是为什么遗传多样性更高的种群对环境变化的响应更快。

Q8MEDIUM中 🇨🇦 BC卑 BC Provincial-style卑诗省考风格 §4 + §7 Mechanisms of Evolution + Patterns进化机制 + 进化模式 · Biology 12 [8 marks][8 分]

Compare natural selection, mutation, gene flow, and genetic drift: random vs. non-random; consistently adaptive. Gene flow vs. drift effects. Divergent vs. convergent evolution.比较自然选择、突变、基因流和基因漂变：随机与非随机；是否持续导致适应。基因流与漂变的效应。趋异进化与趋同进化。

Answer: See sub-part solutions below.答案：详见以下各子问题解析。

(a) Table: four mechanisms表格：四种机制 A1·A1·A1·A1

Mechanism机制	Random or Non-random?随机还是非随机？	Consistently leads to adaptation?持续导致适应？
Natural selection自然选择	Non-random (favors traits that increase fitness)非随机（偏向提高适合度的性状）	Yes是
Mutation突变	Random (occurs at random loci, regardless of need)随机（发生在随机位点，与需求无关）	No (most are neutral or harmful)否（多数为中性或有害）
Gene flow基因流	Neither (movement of individuals between populations)两者均非（个体在种群间移动）	Not consistently (can introduce adaptive or non-adaptive alleles)不持续（可引入适应性或非适应性等位基因）
Genetic drift基因漂变	Random (chance events determine which alleles are passed on)随机（偶然事件决定哪些等位基因被传递）	No (random; can fix harmful alleles)否（随机；可固定有害等位基因）

(b) Gene flow reduces differences; genetic drift increases them基因流减小差异；基因漂变增大差异 A1·A1

Gene flow occurs when individuals (or their gametes) move between populations and reproduce. When a migrant from population A enters population B and breeds, it introduces alleles from A into B's gene pool. If this happens regularly between two populations, their allele frequencies converge toward each other, reducing genetic differences. Conversely, genetic drift causes random fluctuations in allele frequencies that are independent in each isolated population. Because chance events differ between populations, drift causes the populations to diverge from each other in random directions, increasing differences over time -- eventually to the point of genetic incompatibility (speciation).基因流发生在个体（或其配子）在种群间移动并繁殖时。当来自种群 A 的移入者进入种群 B 并繁殖时，它将 A 的等位基因引入 B 的基因库。若两个种群间定期发生这种情况，它们的等位基因频率会相互趋近，从而减小遗传差异。相反，基因漂变导致每个隔离种群中等位基因频率发生独立的随机波动。由于各种群中的偶然事件不同，漂变使种群朝随机方向相互分化，随时间推移增大差异，最终可能导致遗传不相容（物种形成）。

(c) Divergent vs. convergent evolution趋异进化与趋同进化 A1·A1

Divergent evolution: related species evolve increasingly different traits as they adapt to different environments. Example: Darwin's finches on the Galapagos Islands all descended from one ancestor but diverged into ~18 species with different beak shapes adapted to different food sources. The forelimbs of mammals (Q1) are another example. Convergent evolution: unrelated species independently evolve similar traits because they face similar selection pressures. Example: dolphins (mammals) and sharks (fish) both evolved streamlined body shapes and dorsal fins for efficient swimming in water, despite having very different ancestors. Convergent traits are analogous (similar function, different origin), not homologous.趋异进化：相关物种随着适应不同环境而进化出越来越不同的性状。示例：加拉帕戈斯群岛的达尔文雀均源自同一祖先，但分化为约 18 个物种，各具适应不同食物来源的不同喙形。哺乳动物前肢（Q1）是另一例。趋同进化：不相关物种因面临相似选择压力而独立进化出相似性状。示例：海豚（哺乳类）和鲨鱼（鱼类）都进化出流线型体形和背鳍以在水中高效游泳，尽管它们的祖先截然不同。趋同性状是同功的（功能相似，起源不同），而非同源的。

Only natural selection is reliably adaptive; the other three mechanisms are evolutionary noise.只有自然选择是可靠的适应性机制；其他三种机制是进化中的"噪音"。 Students often assume all evolutionary mechanisms produce adaptation. Only natural selection is systematically adaptive because it is the only mechanism that consistently filters for fitness-enhancing traits. Mutation provides the raw genetic variation that selection acts on, but mutations themselves are random and mostly neutral or deleterious. Gene flow and genetic drift change allele frequencies without regard to fitness. In the real world all four operate simultaneously; the relative importance of each depends on population size (drift is stronger in small populations), geographic connectivity (gene flow requires migration routes), and the strength of selection.学生常常认为所有进化机制都会产生适应性。只有自然选择是系统性适应性的，因为它是唯一持续筛选提高适合度性状的机制。突变提供选择作用的原始遗传变异，但突变本身是随机的，多数为中性或有害。基因流和基因漂变改变等位基因频率时不考虑适合度。现实世界中四种机制同时运作；各自的相对重要性取决于种群大小（漂变在小种群中更强）、地理连通性（基因流需要迁移路径）和选择强度。

Q9HARD难Honors荣誉级 🇺🇸 US美 AP-feeder FRQAP 衔接简答题 §2 + §3 + §4 Synthesis: Selection, Variation, Mechanisms综合：选择、变异、机制 · HS-LS4-4 [7 marks][7 分]

Antibiotic resistance: bacterial population exposed to antibiotic; small fraction initially resistant due to pre-existing mutation.抗生素耐药性：细菌种群暴露于抗生素；由于预先存在的突变，最初只有极少数个体具有耐药性。

Answer: See sub-part solutions below.答案：详见以下各子问题解析。

(a) Why natural selection, not the antibiotic, produces resistant bacteria为何是自然选择而非抗生素本身产生了耐药细菌 A1·A1·A1

The antibiotic does not cause mutations or direct the bacterium to change. The resistant mutation pre-existed in the population as a rare variant, arising by random mutation independently of the antibiotic. When the antibiotic is introduced, it acts as a selection agent: susceptible bacteria (the vast majority) are killed, while the rare pre-existing resistant variants survive and reproduce. Their offspring inherit the resistance mutation. Over successive generations, the frequency of the resistance allele increases because resistant bacteria have higher fitness (survival and reproduction) in the antibiotic environment. This is natural selection acting on pre-existing variation -- the antibiotic selects which variants survive, but does not create the resistance. This distinction is critical: organisms do not "try" to adapt; beneficial variants already present are simply more likely to survive.抗生素不会导致突变，也不会直接引导细菌改变。耐药突变作为罕见变体预先存在于种群中，由随机突变产生，与抗生素无关。抗生素被引入时，它作为选择因素：易感细菌（绝大多数）被杀死，而罕见的预先存在的耐药变体存活并繁殖。它们的后代遗传了耐药突变。随着世代更替，耐药等位基因的频率增加，因为耐药细菌在抗生素环境中适合度更高（存活率和繁殖率更高）。这是自然选择作用于预先存在的变异，抗生素只选择哪些变体存活，但并不创造耐药性。这一区别至关重要：生物体不会"尝试"适应；已经存在的有益变体只是更有可能存活。

(b) How incomplete antibiotic courses accelerate resistance evolution不完整的抗生素疗程如何加速耐药性进化 A1·A1

When a patient stops taking antibiotics early, the antibiotic concentration drops below the minimum inhibitory concentration. At this point, some partially resistant (intermediate-resistance) bacteria that would have been killed by the full course can now survive. These bacteria, which may carry partial resistance mutations, survive and reproduce, increasing the frequency of resistance alleles in the surviving population. The most resistant variants -- the ones that can survive even at full antibiotic concentration -- are then more likely to be selected from this enriched resistant pool. Stopping early thus leaves a partially selected population with elevated resistance allele frequencies, from which full resistance can evolve more quickly than if the full course had eliminated all but the most resistant individuals.患者提前停止服用抗生素时，抗生素浓度降至最低抑菌浓度以下。此时，一些本来会被完整疗程杀死的部分耐药（中等耐药）细菌现在可以存活。这些携带部分耐药突变的细菌存活并繁殖，增加了残存种群中耐药等位基因的频率。最耐药的变体（即使在全剂量抗生素下也能存活的那些）更有可能从这个耐药性增强的种群中被选择出来。因此，提前停药留下了一个耐药等位基因频率升高的部分选择种群，从中比完整疗程消灭所有但最耐药个体之外全部细菌时，能更快速地进化出完全耐药性。

(c) Why this example shows evolution is not goal-directed为何该案例说明进化不是目标导向的 A1·A1

The bacteria did not "want" to become resistant or "try" to outsmart the antibiotic. The resistant mutation arose long before the antibiotic was ever used, for reasons entirely unrelated to antibiotic exposure (e.g., a random error in DNA replication). Evolution has no foresight and no goal. Natural selection simply filters existing variation based on current conditions. There is nothing inherently "progressive" about antibiotic resistance: it is only advantageous in the specific context of antibiotic exposure. In an environment without antibiotics, resistance alleles may carry a fitness cost (e.g., the resistance mechanism may divert cellular energy) and non-resistant bacteria may outcompete resistant ones. The appearance of "directed" evolution is an illusion created by selection consistently favoring one phenotype under a fixed environmental pressure.细菌并不"想要"变得耐药，也没有"尝试"智胜抗生素。耐药突变早在抗生素被使用之前就已出现，其原因与抗生素暴露完全无关（例如，DNA 复制过程中的随机错误）。进化没有远见，也没有目标。自然选择只是根据当前条件筛选现有变异。抗生素耐药性本身并不存在"进步性"：它只在抗生素暴露的特定背景下才具有优势。在没有抗生素的环境中，耐药等位基因可能带来适合度代价（例如，耐药机制可能消耗细胞能量），非耐药细菌可能会胜过耐药细菌。"定向"进化的表象是一种幻觉，由固定环境压力下选择持续偏向某一表现型所造成的。

Antibiotic resistance is the single most important public-health application of evolutionary biology.抗生素耐药性是进化生物学最重要的公共卫生应用案例。 Understanding that resistance evolves by selection on pre-existing variation has direct clinical implications. It explains why: (1) completing the full antibiotic course matters (eliminate all but the most resistant); (2) rotating antibiotics slows resistance evolution (different selection pressures); (3) using antibiotics in agriculture creates human health risks (resistance alleles spread via gene flow through bacterial populations); (4) combination therapy (multiple antibiotics at once) is more effective because the probability of a bacterium having pre-existing resistance to all antibiotics simultaneously is vanishingly small. The same logic applies to evolution of pesticide resistance, cancer cell resistance to chemotherapy, and antiviral resistance -- all are natural selection on pre-existing genetic variation.理解耐药性通过对预先存在的变异进行选择而进化，具有直接的临床意义。这解释了为何：(1) 完成完整的抗生素疗程很重要（消灭除最耐药个体外的所有细菌）；(2) 轮换使用抗生素会减缓耐药性进化（不同的选择压力）；(3) 在农业中使用抗生素会带来人类健康风险（耐药等位基因通过基因流在细菌种群中传播）；(4) 联合疗法（同时使用多种抗生素）更有效，因为细菌同时对所有抗生素预先存在耐药性的概率极低。同样的逻辑适用于农药耐药性进化、癌细胞对化疗的耐药性，以及抗病毒药物耐药性，这些都是对预先存在的遗传变异进行自然选择的结果。

Section C · Worked SolutionsC 部分 · 详细解答

Q10MEDIUM中 🇨🇦 AB阿 AB Diploma-style阿尔伯塔毕业考风格 §5 Speciation (applied)物种形成（应用） · Biology 30 D1.4k [8 marks][8 分]

Darwin's finches: single ancestral species colonized Galapagos; today ~18 species adapted to different food sources. Similar DNA, different beak morphologies.达尔文雀：单一祖先物种迁入加拉帕戈斯群岛；如今约 18 个物种各自适应不同食物来源。DNA 相似，喙部形态各异。

Answer:答案： (a) See definition + island rationale below见以下定义及岛屿理由 · (b) Similar DNA indicates recent common ancestorDNA 相似表明近期共同祖先 · (c) Yes, same species under the biological species concept if they can interbreed and produce fertile offspring若能交配产生可育后代，则在生物物种概念下属同一物种

(a) Define adaptive radiation; explain why islands favor it定义适应辐射；解释岛屿环境有利于适应辐射的原因 A1·A1·A1

Adaptive radiation is the rapid evolutionary diversification of a single ancestral lineage into many new species, each adapted to a different ecological niche or resource. It occurs when a group enters a new environment and encounters many unoccupied niches with little competition. Islands are particularly favorable because: (1) Geographic isolation prevents gene flow from the mainland, allowing populations to diverge independently. (2) Vacant ecological niches: oceanic islands typically have fewer species than comparable mainland areas, so newly arrived species face reduced competition for resources. This provides strong directional selection pressure to exploit each available niche. (3) Multiple islands create a mosaic of distinct habitats with different food availability, further driving divergence. Together these factors allow rapid speciation from a single colonizing ancestor.适应辐射是单一祖先谱系迅速多样化为许多新物种的过程，各物种分别适应不同的生态位或资源。当一个类群进入新环境并遇到许多未占据的生态位而竞争极少时，适应辐射就会发生。岛屿环境特别有利，原因是：(1) 地理隔离阻止了来自大陆的基因流，允许种群独立分化。(2) 空置生态位：海洋岛屿通常比同等大陆地区的物种少，因此新到达的物种在资源竞争中面临较少压力。这为开发每个可用生态位提供了强烈的定向选择压力。(3) 多座岛屿形成具有不同食物供应的独特栖息地镶嵌体，进一步推动分化。这些因素共同作用，使单一殖民祖先能够迅速发生物种形成。

(b) Similar DNA supports common descent despite different beak morphologiesDNA 相似支持共同祖先假说，尽管喙部形态各异 A1·A1·A1

If the 18 finch species had independent origins (arose separately from different ancestors), we would expect their DNA sequences to be very different from each other, reflecting millions of years of independent evolution. Instead, all 18 species have highly similar DNA, indicating they share a very recent common ancestor. The small number of DNA differences among the species reflects the relatively short time (a few million years) since their divergence from that ancestor. Meanwhile, the dramatic differences in beak morphology are explained by strong directional selection on just a few genes controlling beak development (e.g., the BMP4 and calmodulin genes have been shown to control beak size and shape in finches). This is a powerful demonstration that large morphological change can occur rapidly through selection on a small number of genes, without requiring wholesale DNA divergence.如果这 18 种雀鸟有独立起源（分别从不同祖先独立产生），我们预期它们的 DNA 序列彼此之间会有很大差异，反映数百万年的独立进化。然而，所有 18 个物种的 DNA 高度相似，表明它们共享一个非常近期的共同祖先。物种间 DNA 差异数量较少，反映了自它们从该祖先分化以来相对较短的时间（几百万年）。与此同时，喙部形态的显著差异由对控制喙部发育的少数基因（如已被证实控制雀喙大小和形状的 BMP4 和钙调蛋白基因）的强定向选择来解释。这有力地证明了大规模形态变化可以通过对少数基因的选择迅速发生，而不需要整体 DNA 分化。

(c) Biological species concept applied to hypothetically interbreeding finch species将生物物种概念应用于假设中能够交配的雀种 A1·A1

Under the biological species concept (BSC), a species is defined as a group of organisms that actually or potentially interbreed with each other in nature and are reproductively isolated from other such groups. If two finch populations that evolved on different islands can be brought together and can successfully interbreed and produce fertile offspring, then by the BSC they should be considered the same species (or, more precisely, they have not yet achieved full reproductive isolation). Their classification as two different species would need to be revised. However, if they fail to interbreed or produce infertile hybrids, they remain separate species regardless of geographic proximity. The test is reproductive compatibility, not geography.根据生物物种概念（BSC），物种被定义为在自然界中实际上或潜在地相互交配、并与其他此类群体生殖隔离的一组生物。如果在不同岛屿上进化的两个雀种被置于一起，能够成功交配并产生可育后代，那么根据 BSC，它们应被视为同一物种（或更准确地说，它们尚未实现完全生殖隔离）。它们被划分为两个不同物种的分类需要修订。然而，如果它们不能交配或产生不育杂交后代，则无论地理上如何接近，它们仍然是不同物种。检验标准是生殖相容性，而非地理位置。

Adaptive radiation requires both ecological opportunity and geographic structure.适应辐射需要生态机遇与地理结构的共同作用。 The Galapagos finches are the textbook example, but adaptive radiation has occurred many times in Earth's history: cichlid fish in the African Rift Valley lakes, Hawaiian honeycreepers (birds), marsupials in Australia, and placental mammals after the dinosaur extinction. Each case shares the same conditions: a colonizing ancestor, ecological opportunity (empty niches), geographic structure (isolation), and sufficient time. The biological species concept, while powerful, has limitations: it applies only to sexually reproducing organisms, cannot apply to extinct organisms (no way to test interbreeding), and breaks down for asexually reproducing organisms and bacteria. Alternative species concepts (morphological, phylogenetic, ecological) address these gaps.加拉帕戈斯雀鸟是教科书上的案例，但适应辐射在地球历史上发生过很多次：非洲裂谷湖中的慈鲷鱼、夏威夷管舌鸟（鸟类）、澳大利亚的有袋类动物，以及恐龙灭绝后的有胎盘哺乳动物。每种情况都具备相同条件：殖民祖先、生态机遇（空置生态位）、地理结构（隔离）和充足时间。生物物种概念虽然有力，但也有局限性：它只适用于有性生殖生物，不能应用于已灭绝的生物（无法检验交配），对无性生殖生物和细菌则失效。形态学、系统发育学、生态学等替代性物种概念弥补了这些不足。

Q11HARD难Honors荣誉级 🇨🇦 AB阿 AB Diploma-style阿尔伯塔毕业考风格 §6 Hardy-Weinberg Equilibrium哈迪-温伯格平衡 · Biology 30 D1.3k [9 marks][9 分]

Population of 1000 individuals: PTC tasting ability; T (tasting) dominant over t (non-tasting). Observed genotypes: 490 TT, 420 Tt, 90 tt.1000 人的种群：PTC 感知能力；T（感知）对 t（不感知）为显性。观测基因型：490 TT，420 Tt，90 tt。

Answer:答案： $p = 0.70$, $q = 0.30$ · Expected: TT = 0.49, Tt = 0.42, tt = 0.09期望频率：TT = 0.49，Tt = 0.42，tt = 0.09 · Population IS in HW equilibrium种群处于哈迪-温伯格平衡

(a) Calculate allele frequencies计算等位基因频率 A1·A1·A1

Total number of alleles $= 2 \times 1000 = 2000$.等位基因总数 $= 2 \times 1000 = 2000$。

Count T alleles: each TT individual contributes 2 T alleles; each Tt individual contributes 1 T allele:计算 T 等位基因数：每个 TT 个体贡献 2 个 T 等位基因；每个 Tt 个体贡献 1 个 T 等位基因：

$$ \text{T alleles} = 2(490) + 1(420) = 980 + 420 = 1400 $$ $$ p = \frac{1400}{2000} = 0.70 $$

Count t alleles: each tt individual contributes 2 t alleles; each Tt individual contributes 1 t allele:计算 t 等位基因数：每个 tt 个体贡献 2 个 t 等位基因；每个 Tt 个体贡献 1 个 t 等位基因：

$$ \text{t alleles} = 2(90) + 1(420) = 180 + 420 = 600 $$ $$ q = \frac{600}{2000} = 0.30 $$

Verification: $p + q = 0.70 + 0.30 = 1.00$ ✓验证：$p + q = 0.70 + 0.30 = 1.00$ ✓

(b) Calculate expected Hardy-Weinberg genotype frequencies计算期望的哈迪-温伯格基因型频率 A1·A1·A1

Using $p^2 + 2pq + q^2 = 1$ with $p = 0.70$ and $q = 0.30$:使用 $p^2 + 2pq + q^2 = 1$，其中 $p = 0.70$，$q = 0.30$： $$ p^2 = (0.70)^2 = 0.49 \quad \Rightarrow \quad \text{Expected freq}(TT) = 0.49 $$ $$ 2pq = 2(0.70)(0.30) = 0.42 \quad \Rightarrow \quad \text{Expected freq}(Tt) = 0.42 $$ $$ q^2 = (0.30)^2 = 0.09 \quad \Rightarrow \quad \text{Expected freq}(tt) = 0.09 $$

Verification: $0.49 + 0.42 + 0.09 = 1.00$ ✓验证：$0.49 + 0.42 + 0.09 = 1.00$ ✓

Expected numbers (out of 1000): TT = 490, Tt = 420, tt = 90.期望数量（共 1000 人）：TT = 490，Tt = 420，tt = 90。

(c) Compare observed vs. expected; assess HW equilibrium比较观测值与期望值；评估哈迪-温伯格平衡 A1·A1·A1

Genotype基因型	Observed frequency观测频率	Expected frequency (HW)期望频率（HW）	Match?是否吻合？
TT	490/1000 = 0.49	0.49	Yes ✓是 ✓
Tt	420/1000 = 0.42	0.42	Yes ✓是 ✓
tt	90/1000 = 0.09	0.09	Yes ✓是 ✓

The observed genotype frequencies match the Hardy-Weinberg expected frequencies exactly. This population appears to be in Hardy-Weinberg equilibrium: there is no statistical evidence of any evolutionary force acting on this locus at this time. (Accept any one of the following mechanisms that could explain a deviation if one were observed): natural selection on the tasting phenotype, non-random mating (e.g., tasters preferentially mating with tasters), genetic drift in a small population, gene flow from a population with different allele frequencies, or a high mutation rate at this locus.观测基因型频率与哈迪-温伯格期望频率完全吻合。该种群目前似乎处于哈迪-温伯格平衡状态：没有统计证据表明此时有任何进化力量作用于该位点。（若存在偏差，以下任一机制均可接受作为解释）：对感知表现型的自然选择、非随机交配（如感知者倾向于与感知者交配）、小种群中的基因漂变、来自具有不同等位基因频率种群的基因流，或该位点的高突变率。

Hardy-Weinberg equilibrium is a null hypothesis: it tells us what to expect when evolution is NOT occurring.哈迪-温伯格平衡是一个零假设：它告诉我们当进化不发生时应该期望什么。 The HW principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences -- specifically: no natural selection, no mutation, random mating, no genetic drift (infinitely large population), and no gene flow. In reality, no population perfectly satisfies all five conditions; real populations are always evolving to some degree. The value of HW is as a baseline: when observed frequencies deviate significantly from HW expectations, it is evidence that one or more evolutionary forces are acting. In this question, the perfect match between observed and expected frequencies is a designed feature of the numbers (490 TT, 420 Tt, 90 tt with $p = 0.70, q = 0.30$ was constructed to be in equilibrium), which is a common AB diploma exam convention.哈迪-温伯格原理指出，在没有其他进化影响的情况下，种群中的等位基因和基因型频率将世代保持不变，具体条件为：无自然选择、无突变、随机交配、无基因漂变（无限大种群）和无基因流。现实中，没有种群能完全满足所有五个条件；真实种群总是在某种程度上进化。哈迪-温伯格平衡的价值在于提供基线：当观测频率显著偏离哈迪-温伯格期望时，这是一种或多种进化力量正在发挥作用的证据。本题中观测值与期望值的完全吻合是数字设计的特点（490 TT，420 Tt，90 tt，$p = 0.70, q = 0.30$ 被构造为处于平衡状态），这是阿尔伯塔省毕业考试的常见惯例。

Q12HARD难 🇺🇸 US美 AP-feeder FRQAP 衔接简答题 §2 + §4 + §7 Synthesis: Selection types + Patterns综合：选择类型 + 进化模式 · HS-LS4-2 · HS-LS4-4 [10 marks][10 分]

Wild horses: leg length variation; moderate leg length gives best survival balance. Tracked 100 years. Drought reduces population from 2000 to 40 individuals; slowly recovers.野马：腿长从短到长各有变化；中等腿长提供最佳存活平衡。追踪 100 年。干旱使种群从 2000 匹减至 40 匹；缓慢恢复。

Answer:答案： (a) Stabilizing selection; distribution narrows around moderate length稳定化选择；分布在中等腿长附近收窄 · (b) Bottleneck effect (genetic drift)瓶颈效应（基因漂变） · (c) Reduced genetic variation limits evolutionary potential遗传变异减少限制了进化潜力 · (d) Genetic diversity provides raw material for future adaptation遗传多样性为未来适应提供原材料

(a) Type of selection on leg length; change in distribution over 100 years作用于腿长的选择类型；100 年内分布的变化 A1·A1·A1

This is stabilizing selection: the environment favors the intermediate phenotype (moderate leg length) and selects against both extremes (very short and very long legs). Over 100 years, the distribution of leg length in the population would narrow: horses with moderate leg length survive and reproduce at higher rates, while those at both extremes are removed by selection (very short legs are too slow to escape predators; very long legs are energetically inefficient). The mean of the distribution remains roughly constant (around moderate length), but the variance decreases as extreme phenotypes become rarer. The bell curve becomes taller and narrower over time.这是稳定化选择：环境偏向中间表现型（中等腿长），并对两个极端（极短腿和极长腿）进行选择淘汰。在 100 年内，种群腿长分布会收窄：中等腿长的马以更高的速率存活并繁殖，而两个极端的马被选择淘汰（极短腿跑得太慢，无法逃脱捕食者；极长腿的能量效率太低）。分布均值大致保持不变（在中等腿长附近），但方差减小，因为极端表现型变得越来越罕见。钟形曲线随时间推移变得更高更窄。

(b) How the drought changes allele frequencies unrelated to adaptation干旱如何以与适应无关的方式改变等位基因频率 A1·A1·A1

This event is the bottleneck effect, a specific form of genetic drift. When the population crashes from 2000 to 40 individuals, the surviving 40 carry only a random sample of the alleles present in the original 2000-horse population. By chance alone (not by fitness), certain alleles that were common in the original population may be absent or rare among the 40 survivors, while alleles that were rare may be over-represented. This random change in allele frequency is unrelated to the fitness of those alleles in the current environment -- it is purely due to the random sampling of which 40 individuals happened to survive the drought (e.g., those that found a water source by chance). After the bottleneck, genetic drift will continue to have a strong effect as the population slowly recovers from a small founding group.这一事件是瓶颈效应，是基因漂变的一种具体形式。当种群从 2000 匹骤降至 40 匹时，幸存的 40 匹只携带了原始 2000 匹种群中等位基因的随机样本。仅仅因为偶然（而非适合度），原始种群中常见的某些等位基因在 40 名幸存者中可能缺失或罕见，而罕见的等位基因可能被过度代表。这种等位基因频率的随机变化与这些等位基因在当前环境中的适合度无关，纯粹是因为随机采样决定了哪 40 匹马碰巧在干旱中存活下来（例如，那些碰巧找到水源的个体）。经历瓶颈后，随着种群从小的奠基群体缓慢恢复，基因漂变将继续产生强烈影响。

(c) Recovered population is less fit: genetic variation and evolutionary potential恢复后的种群适应性更低：遗传变异与进化潜力 A1·A1

The scientist's argument is that the recovered population, descended from only 40 individuals, has much lower genetic diversity than the original 2000-horse population. Many alleles present in the original population were lost during the bottleneck. A population with low genetic variation has limited evolutionary potential: it has fewer allele variants available for natural selection to act upon if the environment changes in the future. For example, if a new disease emerges, a diverse population is more likely to contain some individuals with pre-existing resistance alleles; a genetically uniform population is more vulnerable because all individuals may lack resistance. Lower genetic diversity therefore represents lower long-term fitness of the population (not necessarily lower individual fitness in the current stable environment).科学家的论点是，仅由 40 匹马繁殖而来的恢复种群遗传多样性远低于原始 2000 匹种群。瓶颈期间，原始种群中存在的许多等位基因丢失了。遗传变异低的种群进化潜力有限：如果未来环境发生变化，可供自然选择作用的等位基因变体更少。例如，若出现新疾病，多样化的种群更有可能包含一些预先存在耐药等位基因的个体；遗传上均一的种群更脆弱，因为所有个体都可能缺乏耐药性。因此，遗传多样性降低代表种群长期适应性降低（不一定代表当前稳定环境中个体适合度降低）。

(d) Why maintaining genetic diversity is important for long-term survival为何维持遗传多样性对长期存续重要 A1·A1

Genetic diversity is the raw material for evolution. A species with high genetic diversity has a larger reservoir of alleles, some of which may confer advantages under future environmental conditions that differ from the present (e.g., climate change, new pathogens, altered food availability). Without this variation, a species cannot adapt and may go extinct when conditions change. This is why conservation biologists prioritize genetic diversity in endangered species management: inbreeding in small, isolated populations (low diversity) increases the risk of extinction. The cheetah is a real-world example: a bottleneck ~10,000 years ago left cheetahs nearly genetically identical, making them extremely vulnerable to disease and environmental change. Maintaining genetic diversity is thus an investment in a species' evolutionary future.遗传多样性是进化的原材料。遗传多样性高的物种拥有更大的等位基因储备，其中一些可能在不同于现在的未来环境条件下（如气候变化、新病原体、食物供应改变）赋予优势。没有这种变异，一个物种就无法适应，当条件改变时可能灭绝。这就是为什么保护生物学家在濒危物种管理中优先考虑遗传多样性：小型隔离种群中的近亲繁殖（低多样性）会增加灭绝风险。猎豹是一个现实世界的案例：约 10000 年前的瓶颈效应使猎豹几乎在遗传上完全相同，使它们对疾病和环境变化极为脆弱。维持遗传多样性因此是对一个物种进化未来的投资。

Bottleneck effect vs. founder effect: both reduce diversity, but the mechanism differs.瓶颈效应与奠基者效应：两者都减少多样性，但机制不同。 In this question (bottleneck), an existing large population is dramatically reduced by a catastrophe, then expands again. In the founder effect (Q3), a small group leaves the main population to colonize a new area. The genetic consequences are similar (reduced diversity, random allele frequency shifts), but the ecological context differs. Both are forms of genetic drift. A critical exam distinction: the bottleneck effect is more likely when asking about a catastrophe reducing an existing population; the founder effect is more likely when asking about colonization of a new habitat. The recovered horse population in this question is NOT the founder of a new population -- it is the remnant of the original population rebuilding in place, so bottleneck is the correct term.本题（瓶颈效应）中，一个现有的大种群因灾难骤然减少，然后再次扩大。在奠基者效应（Q3）中，一小群个体离开主种群迁入新地区。遗传后果相似（多样性减少，等位基因频率随机偏移），但生态背景不同。两者都是基因漂变的形式。一个重要的考试区分点：瓶颈效应在询问灾难减少现有种群时更适用；奠基者效应在询问新栖息地的殖民化时更适用。本题中恢复的马种群不是新种群的奠基者，而是原始种群在原地重建的残余，因此瓶颈效应是正确术语。