CHAPTER 5 :- ORIGIN AND EVOLUTION OF LIFE

Q.25: Define evolution.

Ans.: Origin and gradual development of life one earth is termed as evolution.                                                 OR

                Evolution is descent with modifications.

 

Q. 26:  What is meant by organic evolution?

Ans.:        Organic evolution is slow, gradual, continuous and irreversible change through which the present day complex forms have descended from their simple preexisting forms of the post.

Q. 27: What is Darwinism?

Ans.: Theory of origin of species by Natural selection proposed by Darwin is known as Darwinism.

Q. 28: To which Island Darwin visited and which ship was used for his voyage?

Ans.         Darwin visited Galapagos Island and used H.M.S. Beagle ship for his voyage.

Q. 29: Which two scientists owe credit for Natural selection theory?

Ans.: The credit for Natural Selection theory goes to Charles Darwin and Alfred Russell Wallace.

Q. 30: Describe Darwin’s theory of Naturals Election State the objections raised against this theory?

Ans.:        Darwin’s theory of Natural selection is based on following principles:

A) Over production/Prodigality of numbers/Enormous fertility:

All organisms have ability to reproduce at more or less role (Differential reproduction).The organisms multiply in geometric ratio.

E.g. Insects lay thousands of eggs at a time. Cod fish lays several thousand eggs.  Female rabbit produced six young ones at a time.

Even a slowest breeding animal like elephant during its entire life span produces 6 young ones. If a pair of elephants is allowed to male freely i.e. without any natural check it will produce If million descendants in 750 years.

B) Relative constancy of numbers:

Even though the rate of reproduction is more all the individuals produced do not survive due to less availability of food, shelter, and other resources.

It forms the basis of food web and food chain.

C) Struggle for existence:

The competition among the organisms of same or different types for basic needs is known as struggle for existence.

The struggle for existence is of three different types which can be explained as follows:

a) Intraspecific struggle:

1) It is the struggle between individuals of the same species.

2) This is very acute type of struggle as the struggle is for food shelter, mating partner, etc.

3) It may lead to cannibalism i.e. eating animals of the own kind.

4) e.g. struggle between two deers, two horses etc.

b) Interspecific struggle:

1) It is the struggle between the members of different species.

2) The struggle may be for food, shelter etc.

3) e.g. struggle between rabbit and fox, fox and tiger.

c) Environmental struggle:

All the organisms struggle against environmental conditions like volcanoes, earthquake, flood, draught, heavy rains, etc.

Thus, climatic conditions and natural factors also help to restrict the number of organisms of a species.

d) Appearance of Variation and heredity:

1) No two individual on the earth surface except the twins are identical. They show differences among them.

2) These differences are known as variations.

3) These variations help some individuals to adjust with environmental conditions than the others.

4) Struggle for existence develops adaptive modifications.

5) Variations may be harmful, neutral or useful.

6) Darwin proposed that useful variations are passed on to their off spring and play major role in evolution.

7) It explains variations as favourable and unfavorable variation.

E) Survival of fittest or Natural selection:

1) The organisms which are provided with favourable variations would survive, because they are fittest to face their surrounding conditions, while the unfit are destroyed.

2) The nature selects the organisms which show favourable variations and are fit to survive, hence the theory is known as natural selection.

3) Only survival of fittest is not enough, but the organism must adopt with nature as nature is changing every time.

F) Inheritance of useful variations:

1) The organisms after getting fitted to the surrounding transmit their useful variations to the next generation while unfavorable variations are eliminated.

2) However Darwin was unable to differentiate between continuous and discontinuous variations.

G) Origin of new species or speciation:

1) Due to struggle for existence, variations and heredity organisms become better adapted to surroundings.

2) Nature helps the well adapted organisms by increasing rate of reproduction.

3) Due to sexual reproduction, recombination variations are passed onto next generations with better adaptation.

4) Now the new developed group differs from those organism which do not have variation and the entire new species of the organisms develop.

5) Formation of new species is called as speciation.

It is due to accumulation of favourable variations. Objections natural

Q.: Who coined the term survival of fittest?

Ans.: Herbert spencer coined the term survival of fittest.

 

Objections against natural selection theory:-

1) Inheritance of small variations: According to theory only useful variations are transmitted to the next generation but sometimes small variations which are not useful are also inherited.

2) Overspecialization of some organs: Theory does not explain overspecialized organs or their usefulness like tusk of elephant or antler of deer because these characters create hindrance to them and are not useful.

3) Vestigial organs: According to Darwinism, vestigial organs should not be present in organism but different animals or species show number of vestigial organs.

4) Arrival of fittest: Darwin explained survival of fittest but failed to explain arrival of fittest. Darwin was unable to explain the cause of variation and heredity.

5) Degeneration of organs: The theory does not account for the degeneration of certain organs in animals.

6) Discontinuous variation: The theory fails to explain the cause of sudden change in the body. Darwin nothing about heredity.

Q.: Give an account of Hugo De Vries of Mutation.

Ans. :       The Mutations theory is based on observations of experiment conducted by Hugo De Vries on the plant Oenothera lamarckiana.

The salient features of the theory can be given as follows:

1) Mutations or discontinuous variations are the raw material for evolution.

2) Mutations appear suddenly and become operational immediately.

3) Mutations do not revolve around mean or normal character of the species/

4) The same type of mutation can appear in number of individual of species.

5) All mutations are inherited from parents to off spring.

6) Mutation appearing all conceivable directions

7) Useful mutations are selected by nature lethal mutations are eliminated. Sometimes useless and less harmful mutations can persist in the progeny.

8) Single mutation may produce species, due to accumulation of characters.

9) Evolution is jerky and continuous process.

Q.: Explain various objections to Mutation theory.

Ans.: 1) The experimental material of Hugo De Vries was not a normal plant. It is a complex heterozygous form with chromosomal aberrations.

2) Natural mutations are not common as Hugo De Vries thought them to take place.

3) Maximum mutations are negatives or retrogressive.

4) Mutations are generally recessive and the characters taking part in mutation are dominant.

5) Mutation theory cannot satisfactorily and inter dependence of flowers; and pollinating agents.

6) Theory does not explain role of nature.