Origin and evolution of life Questions and Answers-4.

 Q. 1. Explain the action of lain the action of Natural selection with reference to industrial melanism.

                             OR                                 (3 marks)

Prior to industrialization there were far more light-coloured peppered moths than dark coloured ones in England. After industrialization this reversed. What does the above observation indicate? Explain giving reasons.

(3 marks) (Jan. ’19)

Ans. (1) Natural selection in action is best described by the example of industrial melanism.

(2) This phenomenon was first described by Kettlewell in U.K. He observed the development of dark coloured peppered moths (Biston carbonaria), in response to air pollution during

the industrial revolution in U.K.

(3) Before the industrial revolution light coloured peppered moths (Biston betularia) were abundant. The dark coloured moths were developed as a result of random mutation.

(4) These peppered moths were nocturnal in habit and during daytime they rested on the tree trunk.

(5) Their pale body colour of lighter moths camouflaged with the surrounding tree barks which were covered with lichen. This camouflage helped them to escape from the predatory birds. On the other hand, the black coloured moths resting on pale tree barks were very conspicuous and thus killed by the predatory birds. Their numbers were gradually reduced.

(6) However, after industrial revolution, the surrounding changed. In changed conditions melanic forms were at evolutionary advantage and hence the B. betularia were fast replaced by B. carbonaria.

Q. 2. What is meant by isolation? What role does it play in the evolution?                                                     (2 marks)

Ans. (1) Separation of a single population into several sub-units is called isolation.

(2) Due to isolation no interbreeding occurs between isolated groups.

(3) Each group may develop a set of new traits that lead to evolutionary changes.

(4) This ultimately results into the formation of new species or process of speciation.

Q. 3. Write a brief account of geographical isolation.

(3 marks)

Ans. (1) Geographical isolation results when an original population is separated into two or more groups by geographical barriers such as river, mountain, ocean, island formation, forests, etc.

(2) The separated groups are then exposed to different kinds of environmental factors. Each group is then acted upon by different set of evolutionary forces.

(3) Hence, each group acquires variations by developing mutations, recombination, etc.

(4) These variations are processed by Natural selection leading to progressive genetic divergence.

(5) Gradually the separated populations develop distinct gene pool which makes them unable to have interbreeding.

(6) Thus, the new species are formed by geographical isolation.

(7) Example of geographical isolation is given by Darwin's finches living on Galapagos islands. They differ not only among themselves but also from mainland finches in size, colour, beak and food habits.

Q. 4. Write a brief account of reproductive isolation.

(2 marks)

Ans. (1) Reproductive isolation is the phenomenon by which two groups of population are not allowed to interbreed with each other.

(2) The two populations may live close together but still due to genetic differences they remain isolated.

(3) Such isolation is brought about by the change in genetic material, gene pool and structure of genital organs.

(4) The reproductive isolation eventually leads to speciation.

Q. 5. What are the different types of premating or pre-zygotic isolating mechanisms?

(3 marks)

Ans. (1) Pre-mating isolating mechanisms do not allow individuals to mate with each other at all.

(2) By various mechanisms the two groups remain isolated. These mechanisms are of following types :

(i) Habitat isolation : Habitat isolation is the phenomenon in which members of population living in the same region occupe different habitats. Hence the poten mates do not interbreed among themse

(ii) Seasonal isolation : In seasonal isolat members of a population share the se region but attaining sexual maturity at tial

elves. lation, are the same aturity at the different times of the year. They thus remain isolated reproductively preventing interbreeding among themselves.

(iii) Ethological isolation : Ethological isolation is seen when members of two populations have different mating behaviours. This prevents interbreeding.

(iv) Mechanical isolation : Mechanical isolation is seen when the members of two populations have differences in the structure of reproductive organs. Due to such differences interbreeding is not

possible.

Q. 6. What are the different types of post-mating or post-zygotic isolating mechanisms?

(3 marks)

Ans. (1) In post-mating isolating mechanisms, the two individuals can mate but the result of mating is not favourable.

(2) Thus the populations remain isolated without the actual genetic exchange.

(3) Post-mating isolating mechanisms are divided into the following categories :

(i) Gamete mortality : In gamete mortality, there is death of gametes. Sperm transfer may take place but the egg is not fertilized due to gamete mortality.

(ii) Zygote mortality : In zygote mortality, the zygote is formed but it fails to thrive. Though the egg is fertilized the zygote does not survive.

(iii) Hybrid sterility : In this isolation, there is the formation of hybrid as the gametes or zygotes do not die but the hybrid formed is sterile. Sterile hybrid cannot contribute genetically to further generations.