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Shown below are… (Class 12 Biology SQP 2025-26, Q25)

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BiologyCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2025-26: question 25

From the official CBSE sample question paper for 2025-26, section C, question 25, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q25 · 3 marks · Short answerCompetency-basedEvolution

Shown below are certain aquatic vertebrates, where natural selection has favoured them to develop certain characteristics which enable them to live in water.
A. Name and explain the phenomenon exhibited by the above animals.
B. Which one of the above is the most primitive one? What is its significance?

(For Visually impaired students: A. What are the basic conceptual similarities and differences between Lamarckian and Darwinian theory of evolution? B. Give an example that supports any one of the above two theories.)

Figure in the paper: Line drawings of six aquatic vertebrates, labelled salmon, shark, dolphin, ichthyosaur, blue whale and seal, all with streamlined bodies. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

Open one hint at a time and try again before the next.

Hint 1

The animals belong to different classes yet look alike – what is this pattern called?

Hint 2

Which of these animals is an extinct reptile?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. A. The animals show convergent evolution. Salmon and shark (fishes), ichthyosaur (reptile), and dolphin, whale and seal (mammals) are not closely related. Living in the same aquatic habitat, they evolved similar adaptive features: streamlined bodies, fins and flippers.

    1 mark

  2. These structures do the same job (swimming) but are not anatomically or ancestrally similar, so they are analogous structures. Natural selection in a similar habitat favoured similar adaptations in different groups, and analogy is the result of convergent evolution.

    1 mark

  3. B. The ichthyosaur is the most primitive (oldest extinct form) among these. Its significance: some land reptiles went back into water and evolved into fish-like reptiles (ichthyosaurs) about 200 mya, showing that reptiles re-adapted to aquatic life.

    1 mark

  4. (For visually impaired students) Similarity: Both Lamarck and Darwin held that organisms have heritable traits and that useful traits become more common over generations. Difference: Lamarck proposed inheritance of acquired characters, e.g. a giraffe stretching its neck passes a longer neck to its offspring. Darwin proposed natural selection acting on random, heritable variations, e.g. giraffes that were naturally long-necked survived and reproduced more.

Answer: A. Convergent evolution – analogous structures (streamlined body, fins/flippers) in unrelated animals sharing an aquatic habitat. B. Ichthyosaur – land reptiles returned to water ~200 mya as fish-like reptiles.

Where the marks are

  • Names convergent evolution (1 mark)
  • Explains analogous structures arising from similar habitat and function (1 mark)
  • Ichthyosaur with significance – reptiles returning to water about 200 mya (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Evolution, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.