NATURAL SELECTION THEORY
It was proposed by Charles Darwin. He is an English naturalist. Darwin 
Organisms, he studied essay on population, written by Malthus. This inspired Darwin to propose natural selection. Malthus said that the population increases geometrically --food and shelter increase-- arithmetically.
It was an accidental coincidence, that the Alfred Russell Wallace also got the same idea of natural selection. So, natural selection was actually proposed by Darwin and Wallace. But Darwin got credit as he produced more number of evidences for evolution. Darwin read his observations and the observations of wallace on July 1858 in London Linnaeus society. Darwin published the famous 19th century book, origin of species in 1859. Later he wrote two more books. They are Descent of man and Selection in relation to sex. Darwin is considered as the father of evolution.
Darwin proposed natural selection mainly on the basis of four facts and two inferences. The facts are
1. Over production (prodigality of production)
2. Constancy of population.
3. Variations.
4. Inheritance.
The inferences of Darwin are,
1. Competition (struggle for existence)
2. Natural selection (survival of the fittest)
1. OVER PRODUCTION
It states that every living organism has the capacity to produce more number of young ones. The population increase is in geometrical ratio. This increase can be seen in many animals.
A. Paramecium:
It undergoes binary fission three times for every two days. If all young ones survive and multiply at the above rate for five years. The volume of all paramecia would be ten times that of total earth.
B. Pearl oyster:
The female produces sixty million eggs per year. If all of them were fertilized, developed into young ones and reproduce at the above rate, for five years, the volume of all the individuals would be eight times that of the the size of earth.
C. Human being:
The population of human being is doubled for every ‘25’years. If the same rate continues for about 1000 years, there would not be even standing place for some humans.
The high rate of multiplication was observed in Frog, Fish, and Insects and even in plants also.
2. CONSTANCY OF POPULATION EVOLUTION
Even though the population increases enormously, the number of individuals in every generation remains constant. This constancy is mainly due to the following facts.
They are
(a) Many eggs will not be fertilized,
(b) Many zygotes will not be developed into young ones,
(c) Some young once will die,
(d) Some will not reproduce and
(e) Some may not reproduce at the above rate.
So, there is high mortality at every stage. Even the small population may be increased; it is kept constant due to struggle for existence
3. STRUGGLE FOR EXISTENCE
Even though the population increases enormously, the food and shelter will not be increased. Hence competition arises for their basic needs. The struggle is of three kinds. They are,
A. Intra specific struggle:
It is the struggle between the individuals of same species. This struggle is highly severe as their basic needs are same. Highest severe struggle can be seen in the individuals of same species living in the same niche. Eg-Man and Man; Dog and Dog; Rat and Rat;
B. Inter specific struggle:
It is the struggle between the individuals of different species.
Eg — Rat and Cat; Cat and Dog; Herbivores and Carnivores. In inter specific struggle one kind of population regulates another kind of population [Rat population is controlled by cat population.
C. Environmental struggle:
It is also called extra specific struggle. It is the struggle between the individuals and natural forces like earth quakes, volcanic eruptions, severe winter, serve cold etc. The environmental struggle causes high mortality. So competition acts as a natural check to limit the undue growth of a population.
4. VARIATIONS
The differences between the living organisms are called variations. These are raw materials for evolution. Presence of variations is the universal phenomenon. No two living organisms are identical except identical twins. Darwin recognized the presence of variations. But he did not explain the origin of variations. These are two types. They are (a) continuous, small, fluctuating and hereditary variations,(b) Discontinuous, large, sudden, non hereditable variations. Darwin said that the evolution is mainly due to small variations but not due to large variations. 
Examples:
The organisms which have protective coloration are better protected from enemies than the others.
Sheep which have dense hair are better protected in north pole than the other sheep from severe cold.
5. NATURAL SELECTION
It states that the nature creates struggle and selects those animals which won in competition.
The organisms which have useful variations will win in competition. Whereas the organisms which have no such useful variations will die.
Example:
Imagine that there are two rabbits ‘A’and’B. ‘A ’contains slightly longer legs and well developed muscles than ‘B’. Hence ‘A’ may be escaped easily from predator than ‘B’ by fast running. So, nature created struggle between predator and rabbits. The Nature selected ‘A’ to survive as it had useful variations.
Herbert Spencer called Darwin’s natural selection as survival of the finest.
Example of Natural selection:
The best example is industrial melanism. The moth, Biston betularia is two coloured. They are grey and black coloured. The grey coloured files were abundant in England before industrial revolution. Because they were protected as they had protective colouration (mixed with the back ground grey colour of bark). But the black files were conspicuous to birds, they were eaten by birds.
After industrial revolution, black coloured moths are more than the grey coloured flies. Because the black coloured moths had protective colouration. So, nature before industrial revolution selected grey coloured flies and the nature now selected black coloured flies.
6. Inheritances
It is the transmission of characters from one generation to successive generations. Darwin said that the useful variations are transmitted to the progeny with slight increase in every generation. Hence after many generations the useful variations become large. This lead to the formation of a new species.
previous topic about Lamarck theory
to know more about origin of life on earth
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