Zoology 2021 Paper II 50 marks Describe

Q4

(a) What is isolation ? Describe the major isolating mechanisms which lead to speciation. 20 (b) Enlist theories of origin of life. Explain the theory of biochemical evolution proposed by Oparin and Haldane. 15 (c) What is fossil data ? Discuss the chronological order of human evolution with suitable examples. 15

हिंदी में प्रश्न पढ़ें

(a) पृथक्करण क्या है ? उन प्रमुख पार्थक्य क्रियाविधियों का वर्णन कीजिए जो जाति उद्भवन की ओर ले जाती हैं । 20 (b) जीवन की उत्पत्ति के सिद्धांतों को सूचीबद्ध कीजिए । ओपेरिन एवं हेल्डेन द्वारा प्रस्तावित जैवरासायनिक विकास के सिद्धांत की व्याख्या कीजिए । 15 (c) जीवाश्म डाटा क्या है ? मानव विकास के कालानुक्रमिक क्रम की विवेचना उपयुक्त उदाहरणों के साथ कीजिए । 15

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Directive word: Describe

This question asks you to describe. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'describe' demands detailed, structured exposition of isolation mechanisms, biochemical evolution theory, and fossil-based human evolution. Allocate approximately 40% of time/words to part (a) given its 20 marks, 30% each to parts (b) and (c). Structure as: brief definitional introduction for each part → systematic description of mechanisms/theories/fossil sequence → integrated conclusion linking microevolution to macroevolutionary patterns.

Key points expected

  • Part (a): Definition of reproductive isolation and distinction between pre-zygotic (habitat, temporal, behavioral, mechanical, gametic) and post-zygotic (zygotic mortality, hybrid inviability, hybrid sterility) mechanisms with clear examples
  • Part (a): Explanation of how isolating mechanisms lead to speciation through cessation of gene flow and genetic divergence
  • Part (b): Enumeration of theories (special creation, spontaneous generation, panspermia, biochemical evolution/chemical evolution, modern synthetic theory)
  • Part (b): Detailed explanation of Oparin-Haldane hypothesis: reducing primitive atmosphere, organic soup formation, coacervates/protobionts, and heterotrophic origin preceded by chemosynthetic/metabolic evolution
  • Part (c): Definition of fossil data, types (body fossils, trace fossils, chemical fossils), and dating methods relevant to human evolution
  • Part (c): Chronological sequence: Dryopithecus → Ramapithecus → Australopithecus (A. afarensis 'Lucy', A. africanus) → Homo habilis → Homo erectus (Java Man, Peking Man) → Homo sapiens neanderthalensis → Homo sapiens sapiens (Cro-Magnon) with Indian context (Hathnora, Narmada hominid)
  • Part (c): Morphological trends: prognathism reduction, cranial capacity increase, bipedalism, tool use correlation with brain expansion

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%11Precise definitions of isolation (reproductive, not geographic alone), accurate distinction between pre- and post-zygotic mechanisms; correct Oparin-Haldane sequence (atmosphere → monomers → polymers → coacervates); accurate fossil chronology with correct geological epochs for each human ancestorBasic definitions correct but conflates geographic isolation with reproductive isolation, or misorders some fossil species; minor errors in Oparin-Haldane sequenceConfuses isolation types (geographic vs. reproductive), misrepresents Oparin-Haldane as biogenesis from life directly, or presents incorrect fossil sequence (e.g., placing Neanderthals before erectus)
Diagram / labelling18%9Clear labeled diagrams: isolation mechanism flowchart showing pre/post-zygotic branches; Miller-Urey apparatus sketch for part (b); cranial capacity comparison diagram or hominid evolutionary tree for part (c) with time scaleOne relevant diagram present but poorly labeled or missing key components; or diagrams mentioned but not drawnNo diagrams despite visual opportunities; or diagrams completely unlabeled and irrelevant to question demands
Examples & nomenclature20%10Specific examples for each isolation mechanism (e.g., Drosophila pseudoobscura for behavioral, mule for hybrid sterility); Indian fossil examples (Narmada hominid, Hathnora calvarium); correct binomial nomenclature throughout (Australopithecus afarensis, not just 'Lucy')Generic examples only (e.g., 'different flowering seasons' without naming species); some binomials correct but inconsistent; misses Indian fossil contextNo specific examples or incorrect examples (e.g., citing geographic isolation as reproductive mechanism); no binomial nomenclature used
Process explanation22%11Stepwise explanation of how isolation mechanisms operate at population genetic level; detailed chemical evolution stages with energy sources (UV, lightning, volcanic heat); clear progression showing anatomical changes in human evolution linked to functional adaptationsDescriptive but not mechanistic—lists mechanisms without explaining how they prevent gene flow; mentions coacervates without explaining formation; lists fossils without explaining evolutionary trendsNo process explanation—only definitions and lists; confuses cause and effect in evolutionary sequences
Evolutionary / applied context18%9Integration across parts: links isolation to speciation modes (allopatric, sympatric); connects biochemical evolution to modern RNA-world hypothesis; relates human fossil record to Out-of-Africa vs. Multiregional debate; mentions conservation implications of reproductive isolationSome integration attempted but superficial; mentions modern synthesis without elaboration; no connection between micro and macroevolutionThree isolated sections with no thematic linkage; no mention of modern evolutionary synthesis or contemporary relevance

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