NCERT Class 12 Biology Exemplar Chapter –7 Evolution Part 8

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Question 4:

What are the key concepts in the evolution theory of Darwin?

Answer:

  • Branching descent and natural selection are the two key concepts of Darwinian Theory of Evolution.

  • Branching descent – It is defined as a process by which new species originate from a single common descendant. Hence the various life forms can be represented as branches coming out of the same tree.

Example of branching descent

Example of Branching Descent

Example of branching descent

Example of branching descent

  • Natural Selection is a process in which heritable variations enabling better survival are selected and passed on from parent to progeny over generations. That is nature selects only those traits which help an organism to survive changed conditions.

  • Other important concepts in Darwin Theory are: Over-production of living beings, struggle for existence, survival of fittest.

Question 5:

Two organisms occupying a particular geographical area (say desert) show similar adaptive strategies. Taking examples, describe the phenomenon.

Answer:

Two organisms occupying the same areas would show convergent evolution.

This means that would try to adapt to the environment in a way that would benefit them.

For example: (let us consider the example taken, of a desert)

i. A desert has several types of drought resistant plants. Examples include Orchids, Agaves, Bryophyllum, Aloes, etc. All of them store mucilage and water but they’re different from each other morphologically.

ii. Similarly, wings of birds and butterflies, they perform similar function of flying but have different anatomically, i.e., different bone structures, different wing coverings, different shape and size. So in a similar habitat they are grouped due to similar adaptive features but have different ancestors.

Question 6:

We are told that evolution is a continuing phenomenon for all living things. Are humans also evolving? Justify your answer.

Answer:

  • Evolution is a dynamic process. This means that it is constantly happening.

  • Humans are evolving. Evolutionary changes appear after hundreds and thousands of years so they aren’t visible to us as they are happening.

  • Some examples of human evolution that has taken places are:

  • Appearance of mutation in the lactose tolerance gene. Earlier humans were unable to digest lactose containing products like milk and cheese. But now half of the world’s population can now consume lactose containing products. This development of lactose tolerance took about 20,000 years — which is like a minute in evolutionary time.

  • People living at extremely high altitude places have evolved in such a way that their red blood cells are able to carry more oxygen, preventing them from suffering from hypoxia. (body doesn’t get enough oxygen)

  • Another evolutionary change that happened can be observed in the population living in malaria prone regions.

  • People in these areas developed sickle cell anaemia to fight off the malaria causing protozoa (Plasmodium). E

Question 7:

Had Darwin been aware of Mendel’s work, would he been able to explain the origin of variations. Discuss.

Answer:

  • Darwin is credited with the theory of natural selection.

  • He talked about variations that occurred in organisms and these variations accumulated over time, giving rise to new organisms.

  • However, Darwin did not know about the source of these variations. (We now know that mutation, recombination, and immigration of genes are responsible for variations in a population.)

  • Mendel on the other hand was trying to find out what was causing the genetic and physical changes in the pea plants he was crossing.

  • He said that there were”factors” that were being passed down from generation to generation through gametes. (We now know that these “factors” are none other than genes.)

  • In both Mendel’s and Darwin’s theories, genes were playing a role but they remained unaware of it.

  • Were they aware of each other’s work, it is possible that Darwin could have explained the origin of variation.

  • Mendel’s work talked of “factors” being passed down and Darwin believed that changes were occurring and accumulating, so the possibility of Darwin explaining the source of variation is high.

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