Class 9 Biology Chapter 6 Evolution Important Questions with Answers – Board Pattern 2024–25

Evolution occupies a unique position in CBSE Class 9 Biology: it bridges lower secondary learning with the complexity of Class 11 genetics and Class 12 molecular biology. The rationalized 2024–25 syllabus emphasizes not just memorization of 'evidence for evolution' but application of concepts to unfamiliar scenarios—a hallmark of HOTS (Higher Order Thinking Skills) questions that carry up to 5 marks. Examiners expect students to distinguish between adaptation, variation, and evolution; apply Hardy-Weinberg equilibrium conditions to predict whether a population is evolving; and connect fossil evidence with molecular biology. Board papers since 2022 show a marked shift: simple 'define natural selection' questions have given way to case-study based scenarios asking students to interpret evolutionary tree diagrams or calculate allele frequencies. The average weightage is 8–12% of the total marks—roughly 12–18 marks out of 150—split between short-answer (2–3 mark) and long-answer (5 mark) questions. Understanding origin of life (prebiotic chemistry, RNA world hypothesis), grasping the distinction between Lamarck's theory and Darwin's natural selection, and mastering Hardy-Weinberg mathematics are non-negotiable for a 70%+ score in Biology. This guide drills all question types you'll encounter, from straightforward recall to inference-based analysis.

MCQs test factual recall and quick conceptual clarity. These 5 questions mirror the single-mark questions found in CBSE board exams and online competitive tests. **Q1.** Which of the following is NOT evidence for evolution? (a) Homologous structures (b) Vestigial organs (c) Acquired characteristics (d) Fossil records **Answer:** (c) Acquired characteristics. Lamarck's concept of acquired characteristics (traits gained during an organism's lifetime) is NOT accepted as evidence for evolution in modern biology, as they cannot be inherited. Homologous structures (e.g., human arm and bat wing), vestigial organs (e.g., human appendix), and fossil records all directly support Darwin's theory of common descent and natural selection. **Q2.** The Hardy-Weinberg principle assumes all EXCEPT: (a) No mutations (b) Random mating (c) Unequal gene frequencies (d) No natural selection **Answer:** (c) Unequal gene frequencies. Hardy-Weinberg equilibrium assumes allele frequencies remain constant, which requires no unequal (biased) changes. It does assume no mutations, random mating, no natural selection, and no gene flow—all conditions that keep allele frequencies equal and stable. **Q3.** Peppered moth evolution during Industrial Revolution is an example of: (a) Artificial selection (b) Natural selection (c) Genetic drift (d) Gene flow **Answer:** (b) Natural selection. Dark peppered moths survived better on soot-covered tree bark and reproduced more, passing the 'dark' allele to offspring. This is classic directional natural selection, not artificial (human-controlled), genetic drift (random), or gene flow (migration). **Q4.** The RNA world hypothesis suggests: (a) RNA was the first genetic material (b) DNA evolved before RNA (c) Proteins originated independently (d) RNA is only found in viruses **Answer:** (a) RNA was the first genetic material. The RNA world hypothesis posits that early life used RNA for both storage of genetic information and as a catalyst (ribozyme), before DNA and proteins evolved. This explains the origin of life under prebiotic conditions. **Q5.** Which structure in a whale's flipper is homologous to the human hand? (a) Carpals (wrist bones) (b) Fins (c) Blubber layer (d) Tail flukes **Answer:** (a) Carpals (wrist bones). Both whale flippers and human hands contain carpal bones arranged in similar patterns, indicating common ancestry and evolutionary divergence. Fins and flukes are not homologous—they are adaptive modifications.

Short-answer questions test conceptual understanding and the ability to explain mechanisms briefly. **Q1.** Explain why Lamarck's theory of inheritance of acquired characteristics is rejected by modern biologists. **Answer:** Acquired characteristics are traits developed during an organism's lifetime in response to the environment (e.g., a blacksmith's muscular arms). Modern biology rejects this theory because: (1) Acquired traits are not encoded in genes and cannot be passed to offspring—a blacksmith's children do not inherit strong arms. (2) Genetics has proven only genetically coded traits are heritable. (3) Experimental evidence (e.g., Weismann's mouse tail experiment, where he cut tails for generations but offspring were unaffected) disproves inheritance of acquired characters. Lamarck had no mechanism; Darwin's natural selection explains evolution through heritable variation and differential survival. **Q2.** What are vestigial organs? Give two examples and explain their evolutionary significance. **Answer:** Vestigial organs are non-functional or reduced body parts that were once useful to ancestors but have lost most or all function in modern organisms. **Examples:** (1) Human appendix—once used to digest plant cellulose in herbivorous ancestors; now reduced and prone to inflammation. (2) Coccyx (tailbone) in humans—remnant of a tail used for balance in quadrupedal ancestors. **Evolutionary significance:** Vestigial organs are direct evidence of common descent. They indicate that organisms inherit 'leftover' structures from ancestors, modified by evolution and natural selection. They cannot be explained by special creation but perfectly fit the theory that all animals evolved from common ancestors with different body plans. **Q3.** State the Hardy-Weinberg principle and list two conditions necessary for its maintenance. **Answer:** **Hardy-Weinberg Principle:** In the absence of evolutionary forces, allele and genotype frequencies in a population remain constant from generation to generation, following the equation: p² + 2pq + q² = 1, where p and q are allele frequencies. **Two conditions:** (1) **No mutation:** New alleles must not arise or existing ones change. (2) **Random mating:** All individuals must have an equal chance to mate; inbreeding or mate choice distorts frequencies. (Other conditions include no natural selection, no gene flow, and large population size to avoid genetic drift.) **Q4.** How does fossil evidence support the theory of evolution? Mention one fossil example. **Answer:** Fossil evidence shows evolutionary progression and transitional forms linking ancestral and modern species. Fossils in successive rock layers reveal: (1) Change in organism morphology over time. (2) Appearance of new species and extinction of old ones. (3) Gradual or punctuated transitions between major groups. **Example:** *Archaeopteryx* (Jurassic period fossil, 150 million years old) is a transitional form between dinosaurs (theropods) and birds, displaying both reptilian features (teeth, clawed wings, long tail) and avian traits (feathers, wishbone). Such intermediate forms confirm that birds evolved from dinosaurs through gradual modification, not by separate creation. **Q5.** Differentiate between adaptation and variation with one example of each. **Answer:** **Variation:** Differences in traits among individuals of the same population (genetic diversity). It is **random** and pre-exists in a population. **Example:** In a population of beetles, some are red and some are brown due to different alleles for pigmentation. **Adaptation:** A heritable trait that increases an organism's fitness (survival and reproduction) in its environment. It arises through **natural selection** over generations. **Example:** The brown colour in beetles becomes more common in a dark forest where brown beetles are camouflaged and survive predation better, passing the brown allele to more offspring. Variation provides raw material; natural selection sculpts adaptation.

3-mark questions demand explanation of mechanisms, comparison, and reasoning beyond definition. **Q1.** Explain how natural selection operates on a population of insects. Use the concept of differential reproduction to support your answer. **Answer:** Natural selection acts on heritable variation within populations. **Mechanism:** (1) **Variation:** A population of insects shows genetic diversity—e.g., in wing colour or size. (2) **Environmental pressure:** The environment (predators, food, climate) favours certain traits over others. Insects with wings of a certain colour may be better camouflaged. (3) **Differential reproduction:** Insects with advantageous traits survive longer and reproduce more—this is **differential reproduction**. For instance, in 100 insects, 80 brown-winged insects survive and have 200 offspring, while only 20 white-winged insects survive and have 50 offspring. Next generation: 200/(200+50) ≈ 80% brown, 20% white—the beneficial allele increases. (4) **Result:** Over generations, advantageous alleles accumulate, populations adapt, and species evolve. This mechanism requires **no conscious choice**—nature 'selects' through survival and reproduction. It explains both gradual adaptation and the origin of species. **Q2.** In a population of 1000 frogs, allele A (green) has frequency p = 0.6 and allele a (brown) has frequency q = 0.4. Assuming Hardy-Weinberg equilibrium, calculate the number of homozygous dominant (AA), heterozygous (Aa), and homozygous recessive (aa) frogs. **Answer:** Using p² + 2pq + q² = 1: **Frequency of AA (p²):** (0.6)² = 0.36 → Number = 0.36 × 1000 = **360 AA frogs**. **Frequency of Aa (2pq):** 2 × 0.6 × 0.4 = 0.48 → Number = 0.48 × 1000 = **480 Aa frogs**. **Frequency of aa (q²):** (0.4)² = 0.16 → Number = 0.16 × 1000 = **160 aa frogs**. **Verification:** 360 + 480 + 160 = 1000 ✓; 0.36 + 0.48 + 0.16 = 1.0 ✓. This calculation shows that even though allele frequencies (0.6 and 0.4) are not equal, genotype frequencies follow Hardy-Weinberg proportions in a large, randomly mating population with no evolutionary forces. **Q3.** Compare homologous and analogous structures. How does each provide evidence for evolution? **Answer:** **Homologous structures:** Organs or bones with similar internal anatomy and evolutionary origin but different functions in different species. **Example:** Bat wing, human arm, whale flipper, and horse foreleg all have the same arrangement of bones (humerus, radius, ulna, carpals, metacarpals, phalanges) but serve different functions (flight, manipulation, swimming, running). **Evidence for evolution:** Homologous structures indicate **common ancestry**. All these animals inherited the basic limb plan from a tetrapod ancestor ~320 million years ago and modified it through natural selection for their specific lifestyle. This is best explained by evolution from a common ancestor, not independent creation. **Analogous structures:** Organs with similar function but different internal anatomy and evolutionary origin. **Example:** Insect wings and bird wings both function for flight but have completely different structures—insect wings are chitinous outgrowths; bird wings are modified forelimbs. **Evidence for evolution:** Analogous structures show **convergent evolution**—different species facing similar selection pressures (flying) independently evolve similar solutions (wings) from different ancestral structures. This demonstrates that evolution shapes organisms to fit their environment, not design.

5-mark questions integrate multiple concepts, require detailed explanations, and test synthesis and application. **Q1.** Explain the theory of natural selection as proposed by Charles Darwin. How does it explain the origin of new species? **Answer:** **Darwin's Theory of Natural Selection** rests on five key postulates: (1) **Organisms produce more offspring than survive:** A pair of fruit flies can produce hundreds of descendants, but population size remains relatively stable. (2) **Genetic variation exists in populations:** Individuals differ in heritable traits (size, colour, speed, disease resistance). (3) **Competition for resources:** Due to overproduction, organisms compete for limited food, space, and mates. (4) **Differential survival:** Individuals with traits better suited to the environment are more likely to survive and reproduce—this is called **fitness**. For example, in a population of moths, darker moths survive better on soot-covered trees and produce more offspring. (5) **Accumulation of favourable traits:** Over generations, beneficial alleles increase in frequency, and the population adapts. **Origin of new species:** Speciation occurs through repeated cycles of natural selection over long periods (millions of years) in geographically isolated populations. Isolated populations experience different selection pressures and accumulate different mutations, leading to reproductive isolation. Once populations can no longer interbreed (even if they come back into contact), they are distinct species. **Example:** Darwin's finches in the Galápagos Islands—all descended from a common ancestor ~2 million years ago. Different islands with different food availability (seeds of different sizes, flower shapes) selected for beaks of different sizes and shapes. Over time, populations diverged so much that they became separate species (e.g., *Geospiza fortis* and *Geospiza fuliginosa*). This demonstrates speciation through allopatric evolution—natural selection acting on isolated populations. **Q2.** Discuss the prebiotic origin of life. What is the RNA world hypothesis, and why is it considered more plausible than the earlier protein-first hypothesis? **Answer:** **Prebiotic origin of life** explores how non-living matter became living organisms ~3.8 billion years ago. **Early ideas:** The 'protein-first' hypothesis assumed proteins arose first and later acquired genetic material. However, proteins alone cannot replicate or store information—two essential life properties. **RNA world hypothesis** is now favoured because: (1) **Dual function of RNA:** RNA can both store genetic information (like DNA) and catalyze chemical reactions (like proteins), through special RNA molecules called **ribozymes**. (2) **Experimental evidence:** In labs, researchers have synthesized ribozymes that can replicate themselves—proving RNA is capable of self-replication without proteins or DNA. (3) **Evolutionary plausibility:** Early Earth had the right chemical environment: lightning, UV radiation, and heat from volcanism provided energy. Simple molecules (H₂O, NH₃, CH₄, CO₂) in the primitive atmosphere condensed into amino acids, nucleotides, and other building blocks (Miller-Urey experiment, 1952). These accumulated in early oceans ('primordial soup'). (4) **Logical sequence:** RNA → Proteins (ribozymes aided protein synthesis) → DNA (RNA guided DNA synthesis) → Modern cells. (5) **Supporting observations:** All living cells use RNA as an intermediary between DNA and proteins; ribosomes (which synthesize proteins) are RNA-based, not protein-based—suggesting ancient life relied on RNA. **Why superior to protein-first:** RNA solves the chicken-and-egg problem: you need genes to make proteins, but proteins to replicate genes. RNA does both. The RNA world hypothesis integrates chemistry, molecular biology, and evolution into a coherent model for life's origin. It predicts that simpler, RNA-based life preceded modern DNA/protein life—a prediction supported by the deep phylogeny of all organisms. **Q3.** A wildlife biologist observes a population of 500 rabbits in a forest. After surveying, she finds allele B (brown fur) frequency is 0.7 and allele b (white fur) frequency is 0.3. She predicts genotype frequencies using Hardy-Weinberg, but after 5 years, she finds: BB = 280, Bb = 180, bb = 40 (total = 500). Has evolution occurred? Justify your answer with calculations. **Answer:** **Step 1: Calculate Hardy-Weinberg expected frequencies.** p = 0.7, q = 0.3. Expected genotypes: BB (p²) = 0.49, Bb (2pq) = 0.42, bb (q²) = 0.09. Expected numbers out of 500: BB = 245, Bb = 210, bb = 45. **Step 2: Compare observed vs. expected.** Observed: BB = 280, Bb = 180, bb = 40. Difference: BB has 35 more (280 − 245), Bb has 30 fewer (180 − 210), bb has 5 fewer (40 − 45). **Step 3: Calculate new allele frequencies from observed data.** Frequency of B = (2×280 + 180)/(2×500) = (560 + 180)/1000 = 740/1000 = 0.74. Frequency of b = (2×40 + 180)/(2×500) = (80 + 180)/1000 = 260/1000 = 0.26. **Step 4: Conclusion.** Initial allele frequencies: B = 0.7, b = 0.3. Current allele frequencies: B = 0.74, b = 0.26. **Yes, evolution has occurred.** The frequency of allele B increased from 0.7 to 0.74, while b decreased from 0.3 to 0.26. This violates Hardy-Weinberg equilibrium (which predicts constant allele frequencies). **Possible causes:** (1) **Natural selection:** Brown rabbits (BB or Bb) may have higher fitness due to camouflage in the brown forest floor, or resistance to a predator or disease. (2) **Mutation or gene flow:** New B alleles could have arisen or migrated in. (3) **Genetic drift:** In small populations, random changes in allele frequency occur, though 500 is fairly large. The most likely explanation is **directional natural selection** favouring the B allele, driving the population to evolve towards increased brown fur frequency.

**Case Study:** A farmer plants a field of wheat. Insects called Hessian flies (Mayetiola destructor) infest the crop, reducing yield by 40%. The farmer applies the insecticide malathion, killing 95% of the fly population. Next season, he applies the same dose, but only 50% of flies die. By year 5, the insecticide is almost useless—90% of flies survive. The farmer observes that flies born from treated parents are increasingly resistant. **Question:** Explain this scenario using the principles of natural selection and genetic variation. Why does resistance evolve so quickly, and what is the farmer's best strategy going forward? **Solution:** **Step 1: Identify genetic variation in the population.** Before treatment, the Hessian fly population has genetic diversity. Some flies carry alleles for detoxifying enzymes (e.g., gene X⁺) that reduce insecticide sensitivity; others have the wild-type allele (X). Both genotypes exist, but alleles for resistance are rare initially because they don't confer advantage without insecticide selection. **Step 2: Explain initial killing (95% mortality in year 1).** Malathion blocks neural transmission in flies. Flies with genotypes XX (most of the population) die rapidly. A small fraction with X⁺X⁺ or X⁺X (resistant heterozygotes/homozygotes) survive because their detoxifying enzymes inactivate malathion. This is **differential survival**—the insecticide acts as a **selection pressure**. **Step 3: Explain rapid evolution of resistance (year 5: 90% survive).** Surviving flies from year 1 carry enriched frequencies of the X⁺ allele. They mate randomly, producing offspring rich in X⁺. Each year, malathion kills susceptible flies (XX) and allows resistant flies (X⁺X⁺ and X⁺X) to reproduce. The X⁺ allele frequency increases exponentially due to **directional natural selection**. Mathematical model: Year 1: X⁺ frequency = 0.05 (5% survive, most are X⁺-carriers). Year 2: X⁺ frequency = 0.15. Year 5: X⁺ frequency = 0.80, so 90% of flies are resistant. This is not 'acquired resistance' (Lamarck's theory)—it is genetic evolution through selection of pre-existing variation. **Step 4: Why resistance evolves rapidly.** Insects have short generation times (weeks, not years). A new generation arises every 4–6 weeks, accelerating allele frequency changes. Large population size and high mutation rates for resistance genes also facilitate rapid evolution. **Step 5: Best strategy going forward.** The farmer should adopt **integrated pest management (IPM):** (1) **Rotate insecticide classes:** Switch to a different insecticide with a different mode of action, to which flies have not evolved resistance. (2) **Use lower doses intermittently:** Reduce selection pressure by not spraying every season or applying sub-lethal doses; this prevents resistant alleles from becoming fixed. (3) **Introduce susceptible wild flies:** Release non-resistant flies bred in the lab to dilute the resistant allele frequency in the wild population (genetic dilution). (4) **Use biological control:** Deploy natural enemies (parasitoid wasps) instead of chemicals. (5) **Plant resistant crop varieties:** Use wheat bred for Hessian fly resistance, eliminating the need for insecticides. **Key insight:** This case demonstrates that evolution is **not a theory—it is an observable, quantifiable process.** Resistance to antibiotics, pesticides, and drugs evolves in weeks to years because of intense selection pressure, not because organisms intentionally adapt. Understanding this is critical for agriculture, medicine, and public health.

Studying Evolution for Class 9 CBSE requires more than passive reading. You need daily, adaptive practice that mirrors the exact format and difficulty of board exams. At **CBSETUTOR.ai**, our AI tutor is engineered to drill every question type—1-mark MCQs, 2-mark reasoning questions, 3-mark applications, 5-mark essays, and HOTS case studies—with real-time feedback and personalized learning paths. **How it works:** (1) **Smart diagnostics:** When you begin Chapter 6, the AI assesses your baseline understanding of 'origin of life', 'natural selection', and 'Hardy-Weinberg principle' through adaptive micro-quizzes. (2) **Daily curated drills:** Each day, you receive 8–12 questions matched to your level—starting with MCQs if you're weak on definitions, progressing to case studies if you're strong. (3) **Instant explanation:** After each answer, the AI provides NCERT-aligned explanations, textbook diagrams, and links to video lessons if you need visual support. (4) **Spaced repetition:** Questions you got wrong re-appear every 3–7 days, scientifically timed to strengthen memory. (5) **Mock board exams:** Every two weeks, full 90-minute Evolution mock tests (mimicking actual board format) are auto-scored, with a detailed report showing which sub-topics (e.g., 'fossil evidence', 'speciation') need more work. (6) **Progress tracking:** Your parent or teacher gets weekly reports on marks, accuracy, time spent, and weak areas, enabling targeted revision. (7) **Doubt resolution:** Ask the AI any question about Evolution—'Why is Lamarck's theory wrong?', 'How do I solve Hardy-Weinberg problems?'—and get instant, step-by-step answers, not generic internet results. This structured, data-driven approach ensures you're not just studying hard, but studying smart. Start a 3-day free trial at cbsetutor.ai to experience AI-powered board exam prep that adapts to you.