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(a) "The process of evolution… (Class 12 Biology 2025, Q23)

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BiologyCBSE Class 12 · Board exam 2025, set 57/7/1 · 3 marks · Short answer

CBSE Class 12 Biology 2025: question 23

Set in the CBSE Class 12 board exam of 2025, paper 57/7/1, section C, question 23, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q23 · 3 marks · Short answerEvolution

(a) "The process of evolution of different species in a given geographical area starts from a point and literally radiates to other geographical areas." Explain it with an example.
(b) Cite an example where more than one adaptive radiation has occurred in an isolated geographical area. Name the type of evolution your example depicts.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/7/1.

Hints

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

Hint 1

The word 'radiates' in the question is a clue to the term.

Hint 2

For (b), think of two groups of mammals in Australia that evolved look-alike forms.

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) This process is adaptive radiation: many species evolve from one ancestral stock in a geographical area and spread out into different habitats. It is a form of divergent evolution.

    ½ mark

  2. Example: Darwin's finches of the Galapagos Islands. From the original seed-eating finches, many varieties with altered beaks arose on the same island, including insectivorous and vegetarian finches. (Australian marsupials evolving from one ancestral stock is also correct.)

    1 mark

  3. (b) Example: in isolated Australia, adaptive radiation occurred twice, once in the Australian marsupials and once in the placental mammals. Each group gave rise to forms that resemble a form in the other group, e.g. the marsupial mole and the placental mole, or the flying phalanger and the flying squirrel.

    1 mark

  4. This depicts convergent evolution.

    ½ mark

Answer: (a) Adaptive radiation, e.g. Darwin's finches; (b) Australian marsupials and placental mammals, showing convergent evolution.

Where the marks are

  • Identifies adaptive radiation or divergent evolution (½ mark)
  • Correct example such as Darwin's finches or Australian marsupials (1 mark)
  • Example of more than one adaptive radiation: marsupials and placental mammals in Australia (1 mark)
  • Names convergent evolution (½ 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 paper2025 board paper, set 57/7/1: 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.