NCERT Class 12 Biology Exemplar Chapter – 7 Evolution Part 7 (For CBSE, ICSE, IAS, NET, NRA 2023)

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Long Answer Type Questions

Question 1:

Name the law that states that the sum of allelic frequencies in a population remains constant. What are the five factors that influence these values?


  • The law that states that the sum of allelic frequencies in a population remains constant is Hardy-Weinberg principle.
  • It states that in a given population the frequency of occurrence of alleles of a gene is supposed to remain fixed and the same through generations.
  • He represented it using the algebraic equation .
  • Five factors are known to affect allele frequency in populations i.e.. , Hardy-Weinberg equilibrium.
  • These are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.
  • Gene migration or gene flow – it is movement of alleles into a gene pool or out of a gene pool that allows gene frequencies to change in the original as well as in the new population.
  • Genetic drift – If the movement of alleles into a gene pool or out of a gene pool takes place by chance it is called Genetic Drift. This always influences frequencies of alleles and is inversely proportional to the size of the population. It sometimes result in the founder effect i.e.. , when a small group of individuals get isolated from a larger population to form a new population.
  • Mutation – A mutation is a change in the sequence of an organism՚s DNA. They are random and occur in all directions.
  • Natural Selection – is a process in which heritable variations enabling better survival are selected and passed onto the progeny. That is nature selects only those traits which help an organism to survive in the changed conditions. Those that are not fit for survival perish (die off) in the long run leading to evolution of new species.
  • Genetic recombination – Production of offspring with traits which differ from parents because of gene reshuffling or combination is called genetic recombination. This phenomenon occurs during gamete formation when chromosomes are passed from parents to progeny.

Question 2:

Explain divergent evolution in detail. What is the driving force behind it?


  • In Divergent evolution two species have common ancestors, but they develop different characteristics because of changes in the environment to which they adapt slowly and giving rise to new species.
  • For example: The bones of forelimbs of whale, bat, cheetah and man are similar in structure but serve different functions. The similarity in structure points towards a common ancestry but because their environments were different, they adapted accordingly.
  • Forelimbs of a whale are for swimming, forelimbs of bat are for flying and forelimbs of cheetah are for walking/running. But all these forelimbs have the same anatomy: humerus, radius, ulna, carpals, metacarpals and phalanges.
  • Divergent evolution can occur due to changes in abiotic factors (temperature, pH, sunlight level etc.) or biotic factors (living components of the ecosystem) .
  • The driving force behind the divergent evolution is adaptations to a new habitat and changed environmental conditions.

Question 3:

You have studied the story of Pepper moths in England. Had the industries been removed, what impact could it have on the moth population? Discuss.

Answer: Pepper moths had two variants – dark and light coloured moths.

  • In England, before industrialisation, it was observed that there were more white-winged moths than dark-winged or melanised moths. This was because the tree bark was covered with white lichens and white wings of the moth helped them in camouflaging. So they went unnoticed by predators and survived whereas dark-winged moths were easily visible to the predators that ate them.
  • But after industrialisation, there were more dark-winged moths in the same area.
  • This happened because in the post industrialisation period, the lichens did not survive due to increased pollution.
  • Soot covered the tree trunks making them dark.
  • This made the white-winged moth more visible to the predators that now picked them up instead of the dark moths. The dark moths were now able to camouflage because of the dark tree drunks.
  • Now, had the industries be removed, the pollution level would have gone down, allowing lichen to grow back and the number of white winged moths would have gone up again.