Previous year questions: Evolution

Q1 (CBSE Board 2026, 3 marks)

(a) How is the Hardy-Weinberg expression p squared plus 2pq plus q squared equal to one derived? (b) List any two factors that disturb genetic equilibrium.

Answer: The sum of all allelic frequencies in a population is one. Let A and a be the two alleles at a locus, with frequencies p and q, so p plus q equals one. The frequency of AA individuals is p squared, because the probability that an A allele on one chromosome of a diploid individual is paired with an A allele on the other chromosome is the product of the two probabilities. Similarly the frequency of aa is q squared, and the frequency of Aa is 2pq, because a heterozygote can receive A from the father and a from the mother or the reverse. Adding the three genotype frequencies gives p squared plus 2pq plus q squared, which equals the square of p plus q, that is one. Any two of these processes disturb the equilibrium: gene flow, random drift, mutation, recombination during gamete formation, and selection by the environment.

Q2 (CBSE Board 2026, 3 marks)

Explain the three different ways in which natural selection operates on the distribution of a trait in a population.

Answer: Stabilising selection favours the mean value of the character. Individuals with values near the mean survive and reproduce best, so more individuals acquire the mean value and the peak of the distribution becomes higher and narrower. Directional selection favours one extreme of the distribution. Individuals with values on one side of the mean do better, so the peak of the distribution shifts in that direction, as the dark winged moths became more common after industrialisation. Disruptive selection favours the values at both ends of the distribution. Individuals at the two extremes survive better than those in the middle, so two peaks form and the middle of the distribution becomes less common. The three modes are usually shown as graphs of the number of individuals against the character value.

Q3 (CBSE Board 2026, 1 mark)

The idea of use and disuse of organs for the evolution of organisms was proposed by:

(A) Charles Darwin (B) Thomas Malthus (C) Hugo de Vries (D) Lamarck

Answer: The correct option is (D). Lamarck explained evolution by the use and disuse of organs and by the inheritance of acquired characters, using the elongation of the giraffe's neck as his example. Darwin proposed natural selection, Malthus wrote on populations, and de Vries proposed mutation and saltation.

Q4 (CBSE Board 2025, 4 marks)

The most convincing evidence for tracing evolutionary relationships between humans and other animals comes from similarities at the molecular level. In the table below, the number of amino acid differences between the haemoglobin polypeptide of each animal and that of humans is given: Macaque 8, Dog 32, Bird 45, Frog 67, Lamprey 125.

(a) To which category of evolution, divergent or convergent, do the following relationships belong: (i) humans and Macaque, (ii) humans and Frog? (b) What do the biochemical similarities in haemoglobin suggest about the evolutionary relationship between humans, frog and lamprey? (c) Which of the two, Macaque or Lamprey, is more closely related to humans, and why? (Alternatively: which of the two, Dog or Frog, is more closely related to humans, and why?)

Answer: (a) Both relationships belong to divergent evolution, because the similarities come from a shared ancestor whose descendants diverged into different lines. (b) The similarities in the amino acid sequence of haemoglobin suggest that humans, frog and lamprey share a common ancestry, and the differences have accumulated since the lines separated. (c) Macaque is more closely related to humans than lamprey, because the number of amino acid differences between human and macaque haemoglobin is only eight, while between human and lamprey it is 125; fewer differences mean greater biochemical similarity and a more recent common ancestor. In the alternative, Dog is more closely related to humans than Frog, because 32 differences are fewer than 67.

Q5 (CBSE Board 2025, 2 marks)

A few stages in the evolutionary history of human beings are given: about 15 million years ago primates walking like gorillas and chimpanzees existed; about 3 to 4 million years ago man-like primates walked in Eastern Africa and fossils of their bones were discovered; about 2 million years ago an ancestor lived in the East African grasslands and ate fruits; about 1.5 million years ago a hominid had a brain size of 900 cc and probably ate meat. Name (a) one primate from about 15 million years ago, (b) one place in Eastern Africa where the fossils were discovered, (c) the ancestor that lived in the grasslands about 2 million years ago, and (d) the hominid found around 1.5 million years ago.

Answer: (a) Dryopithecus or Ramapithecus. (b) Ethiopia or Tanzania. (c) Australopithecines. (d) Homo erectus. Of the two primates, Ramapithecus was the more human and Dryopithecus the more apelike; Homo erectus had a larger brain than the earlier Homo habilis.

Q6 (CBSE Board 2024, 1 mark)

S.L. Miller in 1953, to support the theory of chemical evolution, created conditions in a closed flask that included:

(A) CH3, O2, NH3 and H2O vapour at 1800 degrees C (B) CH4, H2, NH3 and H2O vapour at 800 degrees C (C) CH4, CO2, H2 and H2O vapour at 1800 degrees C (D) CH4, NH4, SO2 and H2O vapour at 800 degrees C

Answer: The correct option is (B). Miller used methane, hydrogen, ammonia and water vapour at 800 degrees Celsius and passed an electric discharge through the mixture. Oxygen was not part of the mixture, because the early atmosphere was reducing and had no free oxygen. The experiment produced amino acids, and later similar experiments produced sugars, nitrogen bases, pigments and fats.

Q7 (CBSE Board 2024, 1 mark)

During the 1850s in the pre-industrialisation era in England, the expected effect of natural selection on the number of dark-winged moths as compared to white-winged moths was:

(A) more in number (B) less in number (C) both were equal in number (D) both were less in number

Answer: The correct option is (B). Before industrialisation, lichens covered the tree trunks and made the background pale, so the white winged moths were hidden while the dark winged moths were spotted by predators. The dark form was therefore at a disadvantage and was less numerous. After industrialisation, soot darkened the trunks and the proportions reversed.

Q8 (CBSE Board 2024, 1 mark)

A population is in Hardy-Weinberg equilibrium for a gene. If the frequency of allele A is 0.6, then the frequency of genotype Aa is:

(A) 0.21 (B) 0.42 (C) 0.48 (D) 0.32

Answer: The correct option is (C). If p equals 0.6, then q equals 0.4, and the frequency of the heterozygote is 2pq, which is 2 times 0.6 times 0.4, equal to 0.48. The frequencies of AA and aa are p squared equal to 0.36 and q squared equal to 0.16, and the three frequencies add up to one.

Q9 (CBSE Board 2023, 1 mark)

Select the option that gives the correct description of the process of natural selection with respect to the length of the neck of giraffe:

(A) Stabilising selection as giraffes with longer neck lengths are selected further (B) Disruptive selection as giraffes with smaller and longer neck lengths are selected (C) Directional selection as giraffes with longer neck lengths are selected (D) Stabilising selection as giraffes with medium neck lengths are selected

Answer: The correct option is (C). The giraffe population shifted towards one extreme, a longer neck, so the selection is directional. If the mean neck length had been favoured the selection would be stabilising, and if both the smallest and the longest necks had been favoured it would be disruptive.

Q10 (CBSE Board 2023, 5 marks)

(a)(i) Describe the phenomenon of adaptive radiation as explained by Darwin. (ii) Is human evolution an example of adaptive radiation? Give a reason in support of your answer.

Answer: (a)(i) Adaptive radiation is the process by which different species evolve in a given geographical area from a starting point and radiate into other habitats. Darwin observed many varieties of finches on the Galapagos Islands and conjectured that all of them evolved on the island itself from an ancestral seed-eating form. The beaks changed, so some forms became insectivorous and others vegetarian, and the different forms occupied different feeding habits. The Australian marsupials show the same pattern, with many different marsupials evolving from one ancestral stock within the Australian continent. (ii) No, human evolution is not an example of adaptive radiation. It is an example of progressive evolution: the lineage runs from Homo habilis to Homo erectus and then to Homo sapiens, with changes in brain size, posture and diet, rather than a branching into many species adapted to many different habitats.