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.
