CHAPTER 5 :- ORIGIN AND EVOLUTION OF LIFE

Q.: Explain the action of Natural selection with reference to industrial mechanism.

Ans. 1) Industrial melanosis the best example of natural selection.

2) The development of dark coloured Month in response to air pollution during industrial revolution in Britain was reported by Kettle well.

The result is due to random mutation in Month.

3) Before industrial revolution light coloured peppered month Biston betularia were in high number on trunk of trees.

4) They were camouflaged well with the lichen covered trees which helped them to escape from birds, their predators.

5) On the contrary, black coloured Moth i.e. Biston carbonaria were Conspicuous and become easy prey for birds.

6) After industrial revolution, release of smoke and soot killed lichens on trees due to which white moth became prominent or conspicuous.

7) Thus, natural selection has resulted in the establishment of a phenotypic trait in changing the environmental conditions.

Q.: How will you explain that natural selection is differential reproduction?

Ans.: 1) The difference in the rate and frequency of reproduction in the members of population is known as differential reproduction.

2) The better adapted individuals show high Rae of reproduction while less adapted individual breed at slower rate.

3) Due to this, proportionally more genes are contributed to the gene pool by better adapted highly reproducing organisms than the others.

4) If the differential reproduction continues for several generations the individual reproducing at the higher rate will become predominant with maximum contribution in the gene pool.

5) Any organism survive by producing, more off spring favourable variation  are preserved  by natural selection because of differential reproduction.

Q.: Define isolation. Explain different types of isolation in detail.

Ans.: 1) Isolation is the prevention of mating amongst the interbreeding groups due to geographical, ecological, physiological, behavioral, mechanical and genetic barriers.

2) Any factor which prevents interbreeding is known as isolating mechanism.

3) The two main types of isolation are as follows:

A) Geographical isolation:

1) The process takes place when the population is divided into subunits due to different factors like oceans, rivers, mountains, canyons, valleys deserts, forests etc.

2) This result in preventing individuals from interbreeding.

3) Geographical isolation or spatial isolation has played significant role in evolution and speciation.

4) In course of time, recombination and mutations occurring in the two groups may be incorporated in the gene pool of different groups.

5) The differences may become so much that the separated groups cannot interbreed when they come in contact again.

6) The two groups are considered as two different species.

7) For example, Darwin’s finches at Galapagos island. Different species of finches with varied feeding habit are found on different islands. The ancestral forms were from the mainland of South America 600 miles away.  These birds, perhaps blown by storms, evolved into different species on various islands.

B) Reproductive isolation:

1) The gene pool of a species is separate from all other. This is known as reproductive isolation and serves as a barrier to inter breeding.

2) It prevents breeding even of related populations present in same geographical area.

3) Reproductive isolation shows following types:-

A) Prezygotic or premating isolation: The main factors operating under this are as follows:

1) Mechanical Isolation: The morphology of genitalia or reproductive organs of both male and female of the two populations may be very complicated, unlike with the result of failure in mating.

2) Psychological isolation: The behaviors differences restrict random mating between male and female individuals in different species.

3) Seasonal isolation: Breeding period of mating individuals is different for members of different species.

4) Gametic Isolation: In free living aquatic forms where the fertilization is external the gametes produced by the different species usually do not  attract each other and this kind barrier is known as gametic isolation.

b) Post zygotic or post mating isolation:

The main factors operating under this mechanism are as follows:

I) Gamete mortality : Mating occurs i.e. sperm is transferred but egg is not fertilized.

II)  Zygote, mortality : Formed: Egg is fertilized but zygote dies.

III) Hybird mortality : Formed hybrid reaches maturity but fails to reproduce due to sterility.  E.g. Mule – Produced from male donkey and female horse.

Henny Produced from male horse and female donkey.