UPSC Civil Services (Main) Examination 2026 — Zoology Optional Paper I: Questions with Model Answers | Plutus IAS

08 Sep UPSC Civil Services (Main) Examination 2026 — Zoology Optional Paper I: Questions with Model Answers | Plutus IAS

The questions below are from Zoology Optional Paper I of UPSC Civil Services (Main) Examination 2026 (held 2026-08-30) — the actual paper, which is public. Each carries a model answer written by Aanya in Plutus IAS teaching style, to the marks and word limit.

Official source: official (upsc.gov.in).

Q1. Write short notes on the following in about 150 words each : 10×5=50 (a) Structure of gemmules in Porifera and their importance. 10 (b) Structure and functions of parathyroid gland. 10 (c) Retrogressive metamorphosis in *Herdmania*. 10 (d) Locomotion in echinoderms. 10 (e) Mechanism of parental care in reptiles. 10 (15 marks)

How to approach this question

The directive word “structure” asks for a precise description of anatomical or histological organization, while “importance,” “functions,” or “mechanism” demand the biological significance or physiological role. Each part should be answered in three layers: (1) concise definition/location, (2) detailed structural components, and (3) functional importance or process. The common mistake is to skip the functional link or to mix up cell types and hormones; avoid that by keeping the answer layered and labelled.

Model answer

(a) Structure of gemmules in Porifera and their importance

Gemmules are asexually produced dormant propagules in freshwater sponges (class Demospongiae) that ensure survival during adverse conditions. Structurally, a gemmule consists of an outer protective layer of spongin reinforced by siliceous or calcareous spicules, an inner membrane, a micropyle for excystment, and an internal mass of totipotent archaeocytes. The archaeocytes store nutrients and are surrounded by a collagenous coat that confers desiccation resistance. Upon return of favorable conditions, the micropyle opens, archaeocytes emerge, differentiate, and regenerate a new sponge. Thus, gemmules serve as both a dispersal and a perennation strategy, enabling sponges to colonize new habitats and survive drought or freezing.

(b) Structure and functions of parathyroid gland

The parathyroid gland is an endocrine organ located posterior to the thyroid gland in mammals; typically four glands (two superior, two inferior) comprise the organ. Histologically, chief cells (small, basophilic) synthesize and secrete parathyroid hormone (PTH), while oxyphil cells (larger, eosinophilic) increase with age and may modulate secretion. PTH raises blood calcium by stimulating osteoclast-mediated bone resorption, enhancing renal calcium reabsorption in distal tubules, and indirectly increasing intestinal calcium absorption via 1,25-dihydroxyvitamin D3 synthesis. Concurrently, PTH promotes renal phosphate excretion, maintaining the calcium–phosphate balance. Clinically, hyperparathyroidism causes hypercalcemia (nephrolithiasis, bone pain), whereas hypoparathyroidism leads to hypocalcemia (tetany, muscle spasms).

(c) Retrogressive metamorphosis in Herdmania

Herdmania, an ascidian, exhibits retrogressive metamorphosis in which the free-swimming tadpole larva transforms into a sessile adult with loss of chordate features. The larva possesses a notochord, dorsal nerve cord, and tail musculature, but during metamorphosis these structures regress: the notochord and tail muscles degenerate via apoptosis, the nerve cord reduces to a simple ganglion, and the pharynx enlarges to form the branchial basket. Concurrently, adhesive papillae attach the larva to substratum, the tunic thickens, and siphons develop for filter feeding. This retrogressive process is hormonally regulated by thyroid hormone analogues and thyroid hormone receptors, illustrating evolutionary reduction of chordate characters in favor of a sessile, filter-feeding lifestyle.

(d) Locomotion in echinoderms

Echinoderms employ a unique water-vascular system for locomotion. Sea stars move via tube feet: ampullae contract, forcing fluid into extensible podia that adhere to substratum; sequential contraction of longitudinal muscles retracts the podium, producing step-wise motion. Brittle stars use arm-flexion powered by articulated ossicles and muscles; arms push against the substrate in a rowing gait. Sea urchins walk on spines and tube feet, while crinoids elevate and retract arms to swim or crawl. The mutable collagenous tissue allows rapid stiffening or softening of connective elements, enabling flexible yet precise movement in varied environments.

(e) Mechanism of parental care in reptiles

Reptiles exhibit diverse parental care strategies that enhance offspring survival. Oviparous species such as crocodilians and some pythons guard nests and assist hatchlings; crocodile mothers carry hatchlings to water and protect them for weeks. Viviparous skinks and boas retain embryos and give birth to live young, effectively providing prenatal nutrition and protection. Some lizards (e.g., skinks) construct nests and remain to deter predators. Parental care is hormonally modulated by prolactin and oxytocin-like peptides, with prolactin stimulating brooding behavior and oxytocin promoting nest attendance. Environmental cues such as temperature and predation risk further fine-tune the duration and intensity of care, illustrating an adaptive continuum from no care to extensive guarding in reptiles.

Q2. (a) Write peculiar characters of phylum Annelida. Classify this phylum into different classes and mention features of each class with examples. 20 (b) Mention the structure of respiratory organs in Lamellidens and explain the mechanism of respiration in them. 15 (c) Discuss various flight adaptations in birds. 15 (15 marks)

How to approach this question

The directive word “mention” requires concise, factual information with clear classification and functional explanation. The examiner is testing (i) taxonomic knowledge of Annelida, (ii) structural-functional correlation in Mollusca, and (iii) adaptive morphology in Aves. A top answer must (1) list peculiar characters of the phylum, (2) classify into classes with diagnostic features and examples, and (3) describe respiratory structure and mechanism in Lamellidens and flight adaptations in birds. The common mistake is to mix up class-level features or to omit key examples and mechanisms.

Model answer

Peculiar characters of Phylum Annelida include metameric segmentation, a closed circulatory system, setae for locomotion, and a spacious coelom acting as a hydrostatic skeleton. They possess a dorsal cerebral ganglion and ventral nerve cord, and most forms exhibit metanephridial excretion.

Classification and features

• Polychaeta: Many setae, parapodia, marine; e.g., Nereis.
• Oligochaeta: Few setae, clitellum, terrestrial; e.g., Pheretima (earthworm).
• Hirudinea: No setae, suckers, ectoparasitic; e.g., Hirudo (leech).

Respiratory organs in Lamellidens are the ctenidia (gills). Structurally, ctenidia are paired, highly vascularized lamellae suspended in the mantle cavity. Mechanistically, water enters via the incurrent siphon, flows over the ctenidial filaments where O₂ diffuses into blood and CO₂ diffuses out, and exits through the excurrent siphon. The counter-current flow maximizes gas exchange efficiency.

Flight adaptations in birds include (i) skeletal adaptations—pneumatized, lightweight bones; (ii) respiratory adaptations—flow-through lungs with air sacs ensuring continuous oxygenation; (iii) muscular adaptations—large pectoral muscles and supracoracoideus for powerful downstroke and upstroke; and (iv) wing adaptations—cambered airfoil shape and alula for stall control. Flightless birds, such as ostriches, exhibit reduced wings, heavier skeletons, and powerful hind limbs for running, sacrificing flight efficiency for cursorial adaptation.

Q3. (a) Give a comparative account of aortic arches in vertebrates. 20 (b) Explain different ways of locomotion in Protozoa citing suitable examples. 15 (c) Describe the general features and life history of Nereis. 15 (15 marks)

How to approach this question

The directive word “give a comparative account” demands a point-by-point contrast across major vertebrate groups, not just a list. Examiners test your grasp of developmental homologies, adult functional specialisations, and evolutionary trends in the aortic-arch system. A top answer will (i) trace the embryonic origin of each arch, (ii) map its adult fate in fishes, amphibians, reptiles, birds and mammals, and (iii) highlight the key modification—loss or fusion—that produced the mammalian pattern. The common mistake is to stop at naming arches without explaining their adult derivatives or evolutionary significance.

Model answer

The aortic-arch system originates in the pharyngeal region of vertebrate embryos as six paired arteries that encircle the pharynx. Comparative embryology shows a conserved ground plan, but adult specialisation diverges sharply across classes.

• Fishes (e.g., Scoliodon): All six arches persist; the 3rd–6th arches supply gills, with the 3rd becoming the pretrematic artery, the 4th the afferent branchial, and the 5th–6th efferent branches. No systemic arch is formed.
• Amphibians (e.g., Rana): Arches I & II regress; III forms carotid arteries, IV becomes the systemic (right systemic retained in anurans), V is transient, and VI splits into pulmonary and cutaneous arteries. A ductus caroticus persists in some urodeles.
• Reptiles (e.g., Calotes): Paired systemic arches persist; III gives carotids, IV systemic, V rudimentary, VI pulmonary. The left systemic arch dominates in lizards and snakes, whereas both persist in crocodilians.
• Birds (e.g., Columba): Only the right systemic arch (IV) is retained; III forms carotids, VI becomes the pulmonary trunk, and left IV regresses. This asymmetry supports high-pressure systemic circulation.
• Mammals (e.g., Rattus): III forms internal carotids, IV asymmetrically—right becomes the brachiocephalic artery and left the definitive aortic arch, V disappears, and VI gives the pulmonary trunk and ductus arteriosus (which closes at birth). The carotid duct obliterates, completing the adult pattern.

Evolutionary synthesis shows progressive loss and fusion of embryonic arches to meet metabolic demands—air-breathing tetrapods convert branchial circuits into carotid, systemic and pulmonary channels, culminating in the fully divided mammalian heart. This pattern exemplifies how developmental constraints and functional optimisation shape vertebrate design.

Q4. (a) Explain types of fins in fishes and their role in locomotion. 20 (b) Describe the functions of various gonadal hormones in humans. 15 (c) Describe the general features, distribution and social organization in honey bees. 15 (15 marks)

How to approach this question

The directive word “Explain/Describe” tests your ability to define, classify and illustrate biological concepts with concrete examples. Examiners are looking for (1) a precise classification, (2) the functional anatomy of each category, and (3) ecological or evolutionary significance. The common mistake is to list facts without linking structure to survival advantage or to omit real species examples that show adaptive radiation.

Model answer

Fins in fishes are paired or unpaired appendages whose shape, size and placement determine locomotor efficiency and ecological niche occupancy. They are grouped as paired (pectoral, pelvic) and unpaired (dorsal, anal, caudal) fins, each specialising in thrust, stability, steering or braking.

1. Paired fins

• Pectoral fins: situated immediately behind the gill slits; act as hydrofoils for lift, steering and braking. In reef fishes (e.g., parrotfish) they also assist in precise station-holding in surge-prone habitats.
• Pelvic fins: located ventrally; provide pitch and roll stability. In gobies they are fused to form a sucker, enabling attachment to rocks in intertidal zones.

2. Unpaired fins

• Caudal fin: primary propulsor; its shape correlates with swimming mode. Rounded fins (e.g., goldfish) generate broad thrust for slow manoeuvring; forked fins (e.g., mackerel) reduce drag for sustained cruising; lunate fins (e.g., tuna) maximise thrust for high-speed pelagic migration.
• Dorsal & anal fins: prevent rolling and yaw. In catfish these fins are elongated and serrated, functioning as tactile sensors in turbid waters.

3. Specialised locomotor adaptations

• Flying fish (Exocoetus): enlarged pectorals and pelvic fins form rigid wings; rapid tail beats launch them into air to escape predators, gliding up to 40 m.
• Climbing perch (Anabas testudineus): labyrinthine accessory breathing organ and vascularised air-breathing labyrinth allow brief terrestrial excursions; pelvic fins act as anchors while the fish “walks” between ponds during monsoon retreat.

Conclusion: Fins exemplify modular evolution—minor morphological shifts yield disproportionate locomotor versatility, directly enhancing survival in heterogeneous aquatic and semi-terrestrial milieus.

Q5. Write short notes on the following in about 150 words each : 10×5=50 (a) Green house effect. 10 (b) Sign stimuli in animals and their significance. 10 (c) Gene therapy and its significance. 10 (d) Null hypothesis. 10 (e) Principle and applications of gel electrophoresis. 10 (15 marks)

How to approach this question

The directive word “short notes” demands concise, fact-rich answers that demonstrate conceptual clarity and application. Each note must be structured in three parts: (1) definition/principle, (2) mechanisms/examples/data, and (3) significance/outcomes. The common mistake is to write long paragraphs without clear sub-heads, which loses examiner time and marks.

Model answer

(a) Greenhouse effect

The greenhouse effect is the natural warming of Earth’s surface caused by certain atmospheric gases—greenhouse gases (GHGs)—that trap outgoing long-wave radiation. Primary GHGs include CO₂, CH₄, N₂O and water vapour; their rising concentrations (CO₂ ≈ 420 ppm in 2023) enhance the radiative forcing budget (+3.3 W m⁻² since pre-industrial times). Anthropogenic drivers—fossil-fuel combustion, deforestation, cement production—have amplified this effect, raising global mean temperature by ≈1.1 °C since 1850. The phenomenon is quantified via energy-balance models and satellite data (NASA CERES), while the IPCC AR6 assigns >95 % probability to human influence. Without mitigation, projected end-century warming (+2.5–4.5 °C) threatens ecosystems, agriculture and public health.

(b) Sign stimuli in animals and their significance

Sign stimuli are simple, species-specific cues that trigger fixed action patterns (FAPs) in animals, as formalised by Tinbergen and Lorenz. Examples include the red belly of a male stickleback eliciting attack, or the gaping mouth of a cuckoo chick evoking feeding by host birds. These stimuli act as “releasers” that activate innate neural circuits, ensuring rapid, predictable responses essential for survival and reproduction. Sign stimuli also underpin inter-specific communication—e.g., honeybee dances conveying food source direction—and are exploited in pest management via pheromone traps. Their study bridges ethology and neuroethology, revealing how hard-wired circuits interface with ecological pressures.

(c) Gene therapy and its significance

Gene therapy delivers therapeutic nucleic acids to correct genetic defects or modulate disease pathways. Delivery vectors—adenovirus, AAV, lentivirus—package genes into host cells; for instance, AAV-mediated transfer of the *F8* gene restores Factor VIII in Hemophilia A patients (Phase III trials, BioMarin BMN 270). Early success came with Luxturna (RPE65 gene for Leber congenital amaurosis) and CAR-T cells for CD19+ lymphomas. Beyond monogenic disorders, gene editing (CRISPR-Cas9) targets HIV co-receptors (CCR5Δ32) and sickle-cell disease (*BCL11A* enhancer). Regulatory frameworks (US FDA RMAT designation, India’s GT guidelines 2023) and ethical oversight ensure safety. Challenges—immune reactions, off-target edits, cost (>USD 2 M per patient)—are being addressed via tissue-specific vectors and base-editing platforms.

(d) Null hypothesis

The null hypothesis (H₀) is a default assumption stating that no effect or relationship exists between variables; it is tested against an alternative hypothesis (H₁) using statistical inference. For example, in a drug trial, H₀ posits “no difference in recovery rates between drug and placebo.” Researchers calculate p-values or confidence intervals; if p < 0.05 (α = 5 %), H₀ is rejected in favour of H₁. The null model anchors experimental design—e.g., ANOVA for multiple groups, chi-square for categorical data—and guards against confirmation bias. Misinterpretation of H₀ as “no effect ever” rather than “no detectable effect in this study” is a common pitfall; rigorous framing and pre-registration (e.g., OSF) mitigate p-hacking.

(e) Principle and applications of gel electrophoresis

Gel electrophoresis separates biomolecules—DNA, RNA, proteins—under an electric field based on size, charge and conformation. In agarose or polyacrylamide gels, nucleic acids migrate inversely to log(size); SDS-PAGE denatures proteins and imparts uniform negative charge, so mobility reflects molecular weight. Ethidium bromide or Coomassie staining visualises bands under UV or visible light; Southern, Northern and Western blots transfer separated molecules for hybridisation or antibody detection. Applications span genomics—RFLP analysis, STR profiling for forensics—and proteomics—identifying isoforms in cancer research. High-throughput platforms (e.g., Bioanalyzer, TapeStation) quantify yield and integrity, while CRISPR screens use pooled sgRNA libraries resolved by PAGE. Advances in pulsed-field and capillary electrophoresis expand resolution to megabase chromosomes and single-cell proteomes.

Q6. (a) Enumerate various population characteristics. Explain the following characteristics in detail : 20 (i) Natality and mortality (ii) Population dispersion (b) Describe one-way and two-way F-test and its significance in biological research. 15 (c) What is sericulture ? Describe rearing of mulberry silk worm. Add a note on types of silk cultured in India. 15 (15 marks)

How to approach this question

The directive word “Enumerate” calls for a concise list, while “Explain” demands elaboration with definitions, formulae, and real-world relevance. Examiners test your grasp of population ecology (natality, mortality, dispersion), statistical tools (F-test), and applied zoology (sericulture). A top answer must: (1) list population characteristics, (2) detail natality and mortality with ecological significance, (3) explain dispersion patterns with examples, (4) compare one-way vs two-way F-tests and their biological use, and (5) define sericulture, outline mulberry silkworm rearing steps, and list silk types in India. The common mistake is to treat natality and mortality as isolated rates without linking them to replacement rate or environmental resistance.

Model answer

Population characteristics include density, natality, mortality, immigration, emigration, age structure, sex ratio, dispersion, growth form, and biotic potential. Below, we elaborate natality, mortality, and dispersion, followed by F-tests and sericulture.

Natality and mortality

• Natality: Actual birth output measured as crude birth rate (CBR = births/1000/year) or ecological natality (realized births under environmental constraints). Maximum natality is the theoretical ceiling under ideal conditions; ecological natality is lower due to resource limitation and predation.
• Mortality: Deaths per 1000 individuals per year; includes age-specific mortality rates. Ecological mortality rises with density-dependent factors such as food shortage, disease, and predation.
• Replacement rate (2.1): Ensures population stability by offsetting infant mortality (~35/1000 in India, NFHS-5), maternal mortality (~130/100,000), and sex ratio imbalances (1020 females per 1000 males). India’s TFR fell from 5.9 in 1950 to 2.0 in 2021, yet regional disparities persist (Bihar 2.97 vs Kerala 1.7). Advanced healthcare alone does not guarantee lower fertility without concurrent improvements in female education, employment, and delayed marriage.

Population dispersion

• Random: Individuals distributed unpredictably (e.g., dandelions in a meadow).
• Uniform: Even spacing due to competition (e.g., territorial birds).
• Clumped: Aggregation around resources (e.g., elephants near waterholes).

One-way and two-way F-test

• One-way F-test: Compares variances among ≥3 groups against within-group variance; tests if group means differ significantly (e.g., effect of three diets on rat growth). Significance indicates at least one diet differs.
• Two-way F-test: Adds a second factor (e.g., diet and temperature) to partition total variance into main effects and interaction; reveals whether temperature modifies diet response. Biological significance lies in designing robust experiments, controlling Type I errors, and identifying interaction effects in ecology and physiology.

Sericulture and mulberry silkworm rearing

• Sericulture: Art and science of rearing silkworms (Bombyx mori) for silk cocoon production.
• Rearing steps: (1) Mountain rearing (chawki stage) at 25–26 °C, 80–85% RH; (2) Late-age rearing on mulberry leaves; (3) Mounting on mountages for spinning; (4) Harvesting cocoons on day 7–9; (5) Pupae processing and reeling for raw silk.
• Silk types in India: Mulberry (75% of production), Tasar (Antheraea mylitta, central India), Eri (Samia ricini, northeast), and Muga (Antheraea assamensis, Assam).

Conclusion: Natality and mortality underpin population trajectories, while dispersion patterns reflect resource heterogeneity. F-tests enable rigorous hypothesis testing in biological research, and sericulture exemplifies applied zoology integrating ecology, physiology, and economics to sustain rural livelihoods.

Q7. (a) Differentiate among alpha, beta and gamma diversity. Explain various factors which threaten the biodiversity. 20 (b) Describe instinctive behaviour and learned behaviour in animals taking suitable examples. 15 (c) What are transgenic animals ? Mention their applications in applied fields. 15 (15 marks)

How to approach this question

The directive word “Differentiate” tests your ability to contrast three closely related ecological concepts; the second part asks for an explanation of threats to biodiversity, which is a standard “factors + real-world examples” demand. A top answer must (1) define each diversity type with a clear metric, (2) contrast them with a one-line discriminator, and (3) list threats under four heads—habitat loss, climate change, overexploitation, and invasive species—illustrated by named case studies. The common mistake is to write a generic list of threats without linking them to measurable impacts on alpha, beta or gamma diversity.

Model answer

Alpha, beta and gamma diversity are hierarchical measures of biodiversity that capture variation at different spatial scales. Alpha diversity (α) is the species richness and evenness within a single habitat patch; it is quantified by indices such as Shannon or Simpson. Beta diversity (β) measures the turnover of species between two habitats and is calculated as β = γ/α, where γ is gamma diversity. Gamma diversity (γ) is the total species pool across a landscape or region. For example, a pristine Amazon stream may show high α (many fish species) but low β if adjacent streams are similar; conversely, the Western Ghats shows high β because each elevation band supports unique amphibians.

Threats to biodiversity operate through four major pathways. First, habitat loss—deforestation for agriculture in the Amazon has reduced α by 30 % in some bird guilds. Second, climate change shifts ranges and phenologies; rising sea surface temperature has caused coral bleaching in the Great Barrier Reef, lowering α from 1,500 to <500 species in some sectors. Third, overexploitation—illegal wildlife trade has driven the pangolin population in Southeast Asia down by 80 %, eroding γ diversity across multiple ecoregions. Fourth, invasive species—Lantana camara now occupies 40 % of India’s forests, reducing β by homogenising understorey composition. Conservation strategies must therefore integrate protected-area networks, climate-resilient corridors and strict enforcement of CITES listings to safeguard all three levels of diversity.

Q8. (a) Explain the following human genetic diseases : (i) Haemophilia (ii) Thalassemia 10+10 (b) Explain various biological rhythms with the help of suitable examples. 15 (c) Write the principle, working and applications of Transmission Electron Microscope (TEM). 15 (15 marks)

How to approach this question

The directive word is “Explain”, so the examiner is testing conceptual clarity and illustrative ability. For part (a) you must (i) define each disease, (ii) state the gene/chromosome involved, and (iii) outline clinical features and inheritance pattern. For part (b) you need to classify rhythms, give examples, and link them to physiological processes. The common mistake is to list only definitions without connecting them to real-world cases or mechanisms.

Model answer

(a) Haemophilia is an X-linked recessive bleeding disorder caused by mutations in the F8 gene (Haemophilia A) or F9 gene (Haemophilia B) on the X chromosome, leading to deficiency of coagulation factors VIII or IX respectively. Affected males (XY) express the disease; carrier females (XX) rarely show symptoms. Clinically, patients suffer prolonged bleeding after minor trauma, hemarthroses, and intracranial hemorrhage. Queen Victoria’s pedigree famously illustrates X-linked inheritance, and modern recombinant factor therapy or gene therapy (e.g., Roctavian for Haemophilia A) is now available.

Thalassemia is a group of autosomal recessive hemoglobinopathies caused by mutations in HBA1/HBA2 (α-thalassemia) or HBB (β-thalassemia) genes, reducing synthesis of α- or β-globin chains. Heterozygotes are carriers with mild anemia; homozygotes develop severe transfusion-dependent anemia, hepatosplenomegaly, and bone deformities. β-thalassemia major is prevalent in the Mediterranean and South Asia; prenatal diagnosis via chorionic villus sampling and pre-implantation genetic testing are standard. Gene editing (e.g., CRISPR-Cas9 in the CLIMB-111 trial) and lentiviral gene addition (e.g., Zynteglo) now offer curative options.

(b) Biological rhythms are endogenous cycles synchronized with environmental cues. Three main classes exist: circadian (≈24 h), ultradian (<24 h), and infradian (>24 h). The circadian rhythm is exemplified by the sleep-wake cycle governed by the suprachiasmatic nucleus (SCN) and melatonin secretion; disruption (e.g., shift work or jet lag) elevates cardiovascular risk. Ultradian rhythms include the 90-minute REM-NREM sleep cycle and pulsatile hormone release (e.g., LH every 60–120 min). Infradian rhythms include the 28-day menstrual cycle driven by estrogen and progesterone, and the annual circannual rhythm of hibernation in ground squirrels or seasonal affective disorder in humans. Chronotherapy leverages these rhythms—e.g., administering hypertension drugs in the morning to match the morning surge in blood pressure.

Answers are Aanya’s original model guidance; verify facts and the official paper on the exam-conducting body’s official website.

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