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Natural selection operates… (Class 12 Biology 2024, Q33a)

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BiologyCBSE Class 12 · Board exam 2024, set 57/5/1 · 5 marks · Long answer

CBSE Class 12 Biology 2024: question 33(a)

Set in the CBSE Class 12 board exam of 2024, paper 57/5/1, section E, question 33(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(a) · 5 marks · Long answerCompetency-basedEvolution

Natural selection operates in different ways in nature.
(i) Identify the type of natural selection depicted in the graph above. (1)
(ii) In England after industrialisation, the population of dark winged moths were more favoured than white winged moth. Explain. (2)
(iii) Anthropogenic action can enhance the rate of evolution. Explain with the help of an example. (2)

Figure in the paper: Graph of number of individuals vs phenotype/trait showing two overlapping bell curves: left curve labelled 'White Winged moth' and right curve labelled 'Dark Winged moth', the two peaks distinct (disruptive/directional shift of distribution). See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/5/1.

Hints

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

Hint 1

Look at which end of the trait range is favoured in the graph.

Hint 2

Think about what happened to the lichens and the colour of tree bark because of soot.

Hint 3

For (iii), think of what happens to a bacterial or insect population that is repeatedly exposed to the same chemical.

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. (i) The graph shows directional selection. Individuals at one extreme of the trait (dark-winged moths) are favoured, so the population peak shifts towards that extreme.

    1 mark

  2. (ii) Before industrialisation, tree trunks were covered with pale lichens. White-winged moths were camouflaged against them, while dark moths were easily seen and eaten by predators (birds).

    ½ mark

  3. After industrialisation, smoke and soot darkened the tree trunks. Lichens, being sensitive to pollution, could not grow on the trunks.

    ½ mark

  4. White moths now stood out against the dark bark, so predators picked them off and their numbers fell.

    ½ mark

  5. Dark (melanised) moths were camouflaged and survived to reproduce more. Natural selection therefore increased their proportion in the population.

    ½ mark

  6. (iii) Human activities create strong new selection pressures, so favoured variants rise in number within a few generations instead of over geological time.

    1 mark

  7. Example: heavy use of herbicides, pesticides, antibiotics or drugs kills the susceptible individuals. The few resistant weeds, insects, microbes or cancer cells survive and multiply, so resistant varieties become common within months or years. Industrial melanism in moths is a similar example.

    1 mark

Answer: (i) Directional selection; (ii) soot-darkened, lichen-free trunks camouflaged dark moths, while white moths were eaten by predators; (iii) e.g. excessive use of pesticides or antibiotics selects resistant organisms in a short time.

Where the marks are

  • Identifies directional selection (1 mark)
  • Smoke and soot darkened the tree trunks and lichens could not grow (½ mark)
  • White moths were no longer camouflaged and were eaten by predators (½ mark)
  • Their population decreased (½ mark)
  • Dark moths were camouflaged and selected, so their population increased (½ mark)
  • Explains that anthropogenic action speeds up selection of favoured or resistant forms (1 mark)
  • Gives a correct example, such as pesticide, herbicide or antibiotic resistance, or industrial melanism (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 paper2024 board paper, set 57/5/1: 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.