NCERT Class 12 Biology Exemplar Chapter – 7 Evolution Part 5 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Short Answer Type Questions

Question 1:

Louis Pasteur՚s experiments, if you recall, proved that life can arise from only pre-existing life. Can we correct this as life evolves from pre-existent life or otherwise we will never answer the question as to how the first forms of life arose? Comment.


Louis Pasteur by careful experimentation demonstrated that life comes only from pre-existing life.

He performed the following experiment:

He took a pre-sterilised flasks and concluded life did not come from killed yeast. While in another flask open to air, new living organisms arose from ‘killed yeast’ .

This experiment of Louis Pasteur dismissed the Spontaneous generation theory once and for all.

However, the experiment of Louis Pasteur did not answer how the first life form came on earth.

Next Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (e. g. RNA, protein, etc.) when primitive atmosphere was present (i.e.. , high temperature, volcanic storms, reducing atmosphere containing , etc.) .

The experiment by S. L. Miller showed that organic molecules could develop from inorganic molecules leading to origin of life.

So Louis Pasteur՚s observation can be corrected by Miller՚s

Question 2:

The scientists believe that evolution is gradual. But extinction, part of evolutionary story, are ‘sudden’ and ‘abrupt’ and also group-specific. Comment whether a natural disaster can be the cause for extinction of species.


Natural disasters can be a cause for extinction of species.

Sudden, abrupt and group-specific extinction have occurred in nature because:

i. Changes in the environmental conditions like excessive cooling or warming, drought or rains.

ii. Natural disasters like floods, earthquakes, volcanoes, tsunamis, etc.

iii. Meteor hitting the earth led to large scale destruction.

Question 3:

Why is nascent oxygen supported to be toxic to aerobic life forms?


Nascent oxygen is a highly reactive substance; it is a permanent oxidising agent. It can easily react with various kinds of molecules like DNA, proteins present in the aerobic life forms.

This reaction will lead to unwanted mutation which may threaten the aerobic life forms.

Like if it reacts with DNA it can lead to mutation and defective proteins. Similarly if it reacts with proteins and enzymes they are degraded and many metabolic pathways may be impaired.

So, nascent oxygen supported to be toxic to aerobic life forms.

Question 4:

While creation and presence of variation is directionless, natural selection is directional as it is in the context of adaptation. Comment.


Variations occur randomly and spontaneously in each subsequent generation. These variations may be passed to next generation or may not be. The variation is not passed to next generation when the variation is not supported by the environment. So creation and presence of variation is directionless.

Now when a variation passes the natural selection test it is passed on to next generation. So natural selection is directional as it is in the context of adaptation. Natural selection is called so because it leads to only one path that is selection of better adapted individuals.

Natural selection leads to survival of fittest which survive in the prevailing conditions.

Question 5:

The evolutionary story of moths in England during industrialisation reveals, that ‘evolution is apparently reversible’ . Clarify this statement.


In the evolutionary story of moths in England, before industrialisation, it was observed that more white-winged moths on trees than dark-winged or melanised moths. But after industrialisation, there were more dark-winged moths in the same area, i.e.. , the proportion was reversed.

Before industrialisation period, the white moths survived because the tree bark was covered with white lichens, so they were unnoticed by predators.

In Post industrialisation period, the tree trunks became dark due to industrial smoke and soot. Under this condition the white-winged moth did not survive due to predators, dark-winged or melanised moth survived.

Hence ‘evolution is apparently reversible’ .

Question 6:

Comment on the statement that “evolution and natural selection are end result or consequence of some other processes but themselves are not processes” .


It is an on-going debate whether to call evolution and natural selection as an end result or a process.

When we talk about the world and the different organisms that we see today, we refer to evolution as being a process that result in all these changes. But, when we talk about life itself we say that evolution is a consequence of natural selection.

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