UPSC Zoology 2026 — Paper I

(a) A compact note with two explicit halves: (i) the STRUCTURE of a gemmule in Porifera (its layers, cell type, spicules, micropyle) and (ii) its IMPORTANCE (survival, dispersal, taxonomy, experimental value). Both halves must be present; a purely descriptive answer with no 'importance' loses the second half. (b) Two explicit halves: (i) STRUCTURE of the parathyroid gland (number, position, embryonic origin, histology) and (ii) FUNCTIONS, which essentially means the actions of parathyroid hormone (PTH) on bone, kidney and gut, its regulation and the consequences of hypo-/hyper-secretion. (c) Explain what retrogressive (backward) metamorphosis means in the ascidian Herdmania: describe the chordate characters of the free-swimming tadpole larva, list the changes that are LOST and those that are GAINED at settlement, and state why the phenomenon matters for chordate phylogeny. (d) Describe how echinoderms move: the water-vascular (ambulacral) system and the tube-foot mechanism as the basic device, and then the class-wise variants (Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, Crinoidea) with named examples. (e) Explain the MECHANISM (the actual behavioural and physiological devices) by which reptiles protect their eggs and young — nest site selection and construction, guarding, brooding and thermoregulation, hatchling communication and transport, and viviparity — with named species, and state its adaptive significance.

• Structure: mass of archaeocytes with reserve food, enclosed by a coat with a pneumatic layer of amphidisc spicules
• Micropyle as the exit for the cells
• Importance: survival in adverse seasons, asexual reproduction, dispersal
• Taxonomic and experimental value
• Structure: four small glands behind the thyroid, from third and fourth pharyngeal pouches, chief and oxyphil cells
• PTH raises blood calcium by acting on bone, kidney and gut
• Regulation by blood calcium through negative feedback, with calcitonin as antagonist
• Hypo- and hypersecretion effects
• Tadpole larva with notochord, nerve cord, gill slits and tail
• Changes lost: tail, notochord, nerve cord and sense organs
• Changes gained: attachment, siphons, tunic, enlarged pharynx and gill slits
• Significance for chordate affinity and the origin of vertebrates
• Water-vascular system path: madreporite to tube feet
• Tube-foot mechanism with ampulla, sucker and muscles
• Class-wise variants for Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, Crinoidea
• Named examples
• Nest making in turtles and crocodilians
• Guarding and brooding, as in the python and the king cobra
• Care of young, such as carrying hatchlings
• Viviparity and its adaptive significance

(a) Three linked tasks: (i) list the PECULIAR (diagnostic) characters of the phylum Annelida, (ii) CLASSIFY the phylum into its classes, and (iii) give the distinguishing features of EACH class WITH examples. All three must appear; a general essay on earthworms will not do. (b) Two parts: (i) the STRUCTURE of the respiratory organs of Lamellidens — the eulamellibranch ctenidia (demibranchs, lamellae, filaments, ostia, water tubes) and the mantle — and (ii) the MECHANISM of respiration, i.e. the ciliary water current through the mantle cavity and the gas exchange and blood route. (c) 'Discuss various flight adaptations' requires an organised, system-wise account — external form and feathers, skeleton, musculature, respiration, circulation, metabolism/excretion and sense organs — showing in each case HOW the feature serves flight, not a bare list.

• Peculiar characters: metameric segmentation, true coelom, closed circulation, nephridia, chaetae, ventral nerve cord
• Four classes: Polychaeta, Oligochaeta, Hirudinea, Archiannelida
• Distinguishing features of each class, such as parapodia, clitellum, suckers
• Examples for each class
• Eulamellibranch ctenidia: two demibranchs, descending and ascending lamellae, filaments
• Interlamellar junctions, water tubes and ostia
• Ciliary water current: inhalant siphon, ostia, water tubes, suprabranchial chamber, exhalant siphon
• Gas exchange in the gill blood and the role of the mantle
• External form: streamlined body, wings and feathers
• Skeleton: pneumatic bones, fusion, keeled sternum, furcula
• Muscles: pectoralis and supracoracoideus
• Respiration with air sacs, four-chambered heart, high metabolism, uric acid excretion

(a) A COMPARATIVE account: start from the basic six-paired embryonic aortic arch plan of vertebrates and trace, class by class from cyclostomes to mammals, which arches persist, which are lost, and what each becomes — ending with the evolutionary trend. (b) Explain the different MODES of protozoan locomotion — pseudopodial, flagellar, ciliary, metabolic/euglenoid, gliding and contractile/myonemal (and passive floating) — describing the mechanism of each and citing a named genus for every mode. (c) Two halves: (i) the GENERAL FEATURES of Nereis — habit, habitat, external morphology and the main organ systems — and (ii) its LIFE HISTORY — epitoky and the heteronereis phase, swarming, external fertilisation, spiral cleavage and the trochophore larva through to the young worm.

• Basic six pairs of embryonic arches between ventral and dorsal aortae
• Fishes: arches III–VI persist as gill arches with afferent and efferent parts
• Amphibians and reptiles: arch III carotid, IV systemic, VI pulmonary, and V lost
• Birds (right systemic arch) and mammals (left systemic arch), with the evolutionary trend
• Amoeboid movement with the types of pseudopodia and sol–gel mechanism
• Flagellar and ciliary movement with the mechanism
• Metabolic, gliding and contractile movement
• A named genus for each mode
• General features: habitat, head with eyes, tentacles and palps, parapodia, proboscis with jaws
• Organ systems in brief: closed blood system, nephridia, ventral nerve cord, separate sexes
• Epitoky and heteronereis formation, and swarming
• External fertilisation, spiral cleavage, trochophore and the young worm

(a) Two linked demands: (i) explain the TYPES of fins in fishes — median (dorsal, caudal, anal, adipose) and paired (pectoral, pelvic), their skeletal support, and the structural types of caudal fin; and (ii) explain their ROLE IN LOCOMOTION — propulsion, stability against roll, pitch and yaw, steering and braking — with modified/special functions. (b) Describe the FUNCTIONS of the various hormones produced by the human gonads — testicular (testosterone and the Sertoli-cell hormones) and ovarian (oestrogens, progesterone, inhibin, relaxin, AMH) — together with their source cells and hypothalamo-pituitary control. (c) Three named sub-demands on honey bees: (i) GENERAL FEATURES (systematic position and morphological adaptations), (ii) DISTRIBUTION (the Apis species of India and abroad, and where beekeeping is practised), and (iii) SOCIAL ORGANIZATION (the three castes, their origin and roles, division of labour, communication and caste determination). All three must appear.

• Median fins (dorsal, caudal, anal, adipose) and paired fins (pectoral, pelvic)
• Fin support: radials and fin rays, ceratotrichia and lepidotrichia
• Caudal fin types: protocercal, heterocercal, homocercal, diphycercal
• Role in propulsion, stability, steering and braking, with special modifications
• Testosterone: source Leydig cells under LH, male tract, spermatogenesis, secondary sexual characters
• Sertoli hormones: inhibin, androgen-binding protein and AMH
• Oestrogens and progesterone with their actions on the uterus and pregnancy
• Inhibin, relaxin and control by GnRH, FSH and LH with feedback
• General features: systematic position, mouthparts, pollen basket, wax glands, sting
• Distribution: A. dorsata, A. cerana, A. florea, A. mellifera and beekeeping regions
• Three castes: queen, drone, worker, with origin and roles
• Caste determination, division of labour and communication by dances and pheromones

(a) A compact note defining the greenhouse effect, distinguishing the natural from the enhanced (anthropogenic) effect, naming the greenhouse gases with their sources and relative warming potential, listing the ecological consequences and closing with mitigation/global agreements. (b) Define the sign (key) stimulus in classical ethology, relate it to the innate releasing mechanism, the fixed action pattern and the supernormal stimulus, give the standard experimental examples, and then state its biological SIGNIFICANCE — the second half explicitly asked for. (c) Define gene therapy, classify its types (somatic versus germline, ex vivo versus in vivo, augmentation versus silencing versus editing), name the vectors, cite landmark cases and approved products, and state its SIGNIFICANCE together with its limitations and ethical/regulatory position. (d) Define the null hypothesis in statistical testing, contrast it with the alternative hypothesis, explain the logic of testing it (test statistic, P-value, significance level, rejection or failure to reject), define Type I and Type II errors, and illustrate with a biological example. (e) Two explicit halves: (i) the PRINCIPLE of gel electrophoresis — migration of charged molecules in an electric field through a molecular sieve, and why DNA separates by size while proteins need SDS — and (ii) its APPLICATIONS in research, diagnosis and forensics.

• Definition: absorption and re-radiation of infrared radiation by greenhouse gases
• Natural effect (about 15°C against −18°C) against the enhanced human-caused effect
• Gases with sources: CO₂, CH₄, N₂O, CFCs, with relative warming potential
• Consequences and mitigation such as the Kyoto Protocol and the Paris Agreement
• Definition: simple stimulus releasing a fixed action pattern through the innate releasing mechanism
• Examples: stickleback red belly, gull chick pecking, goose egg rolling
• Supernormal stimulus
• Significance: quick innate response, species recognition
• Definition: introducing or editing genes to treat disease
• Types: somatic against germline, ex vivo against in vivo
• Vectors and named cases such as ADA-SCID, Luxturna and CRISPR therapy
• Significance and limitations
• Definition of H₀ as no effect and H₁ as alternative
• Test statistic, P-value and significance level
• Reject or fail to reject, with Type I and Type II errors
• A biological example
• Principle: charged molecules migrating in a gel in an electric field
• DNA separates by size on agarose
• Proteins by SDS-PAGE, with SDS giving uniform charge
• Applications in research, diagnosis and forensics

(a) Two distinct tasks: first ENUMERATE (list, briefly) the characteristics of a population; then EXPLAIN IN DETAIL two of them — (i) natality and mortality, including maximum versus realised natality, minimum versus realised mortality, specific rates, life tables and survivorship curves, and (ii) population dispersion, meaning the random, uniform and clumped internal distribution patterns with their causes, examples and measurement. (b) Describe the F-test in its one-way and two-way forms — the ratio of variances, the partitioning of sums of squares, degrees of freedom and the decision rule, the designs to which each applies, and the unique ability of the two-way test to detect interaction — and then state its significance in biological research. (c) Three explicit sub-parts: (i) define sericulture, (ii) describe the REARING of the mulberry silkworm Bombyx mori step by step from grainage to reeling, and (iii) add a note on the TYPES of silk cultured in India with their moth species, host plants and states.

• Enumeration of characteristics: density, natality, mortality, age structure, sex ratio, dispersion, biotic potential, growth form
• Natality and mortality: maximum against realised natality, minimum against realised mortality, specific rates
• Life table and survivorship curves of types I, II and III with examples
• Dispersion: random, uniform and clumped with causes, examples and the variance-to-mean ratio
• F as the ratio of variances with degrees of freedom and decision rule
• One-way ANOVA: partition into between and within sums of squares, F = MSB/MSW
• Two-way ANOVA: two factors and the interaction term
• Significance in biological research, with assumptions
• Definition of sericulture
• Rearing steps: grainage, incubation, brushing, feeding through five instars, mounting, harvesting, stifling and reeling
• Types of silk: mulberry, tasar, eri and muga with silkworm species
• Host plants and states for each type

(a) Two tasks: (i) DIFFERENTIATE alpha, beta and gamma diversity — Whittaker's inventory and differentiation diversity — with their definitions, measures and relationship, and (ii) EXPLAIN the various factors that threaten biodiversity, with concrete Indian and global examples. (b) Describe instinctive (innate) behaviour and learned behaviour with suitable examples: define and characterise each, give the categories of each (taxes/kineses/reflexes/fixed action patterns; habituation, classical and operant conditioning, imprinting, latent, insight and observational learning) with named workers and species, and close on the interaction between the two. (c) Two parts: (i) define transgenic animals and outline the METHODS by which they are produced, and (ii) mention their APPLICATIONS in applied fields — biomedicine, pharming, xenotransplantation, livestock, aquaculture, pest control and industry — with named examples, closing on concerns and regulation.

• Alpha diversity as within-habitat, beta as between-habitat turnover, gamma as regional, with γ = α × β and their measures
• Habitat loss, fragmentation and degradation as the main threat
• Overexploitation, invasive alien species, pollution, climate change and co-extinction
• Indian and global examples
• Instinct defined as inherited, stereotyped behaviour, with taxes, kineses, reflexes and fixed action patterns
• Learned behaviour defined, with habituation, conditioning, imprinting, latent, insight and observational learning
• Named examples and workers for each
• Interaction of instinct and learning
• Definition: animals with a foreign gene stably integrated and inherited
• Methods: pronuclear microinjection, retroviral vectors, embryonic stem cells, genome editing
• Applications: disease models, pharming, xenotransplantation, livestock, aquaculture, pest control
• Concerns and regulation

(a) Explain two named human genetic diseases in roughly equal depth: (i) HAEMOPHILIA — its types, X-linked recessive inheritance, the coagulation defect, clinical features and management — and (ii) THALASSAEMIA — its autosomal recessive globin-chain defect, alpha and beta forms, major versus trait, clinical picture, management and prevention in India. (b) Explain biological rhythms: define them and the associated terms (endogenous clock, zeitgeber, free-running period, entrainment), classify them by period into circadian, ultradian, infradian, circatidal, circalunar and circannual, illustrate each with named examples, describe the circadian clock mechanism, and state their adaptive significance. (c) Three named sub-parts on the transmission electron microscope: (i) PRINCIPLE — the electron wavelength and resolving power, scattering contrast; (ii) WORKING — the column from electron gun through condenser, specimen, objective and projector lenses to the viewing screen, under high vacuum, with specimen preparation; and (iii) APPLICATIONS.

• Haemophilia: X-linked recessive, types A, B and C, defect of factor VIII or IX, clinical features
• Haemophilia inheritance pattern with carrier females and management by factor replacement
• Thalassaemia: autosomal recessive, alpha and beta forms, major and trait, clinical picture
• Thalassaemia management and prevention: transfusion and chelation, screening and prenatal diagnosis in India
• Definition with endogenous clock, zeitgeber, free-running period and entrainment
• Types: circadian, ultradian, infradian, circatidal, circalunar, circannual
• A named example for each type
• Mechanism of the circadian clock and its significance
• Principle: electron beam of short wavelength, high resolving power and scattering contrast
• Working: electron gun, condenser, objective and projector lenses, screen, high vacuum
• Specimen preparation: fixation, embedding, ultrathin sectioning, heavy-metal staining
• Applications and limitations