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Who was the originator of the theory of Natural Selection ? Explain the evolution of species by this theory.

Theory of Natural selection was given by Charles Darwin and Alfred Russel Wallace. 

Theory of Natural selection is based on the following points :

1. Overproduction : All living beings have an inherent tendency to produce offspring of their own kind in large number for the perpetuation of their race. This is called geometric increase. The number of their offspring is much more than can be supported by a particular environment and can possibly survive.

2. Limited Space and Food : The space in the universe remains constant. The ultimate source of food for plants and animals also remain constant. Therefore, the carrying capacity of the environment does not allow the population to grow indefinitely. Therefore, in spite of enormous reproductive potential of living beings under natural conditions the number of individuals of each species remains nearly constant over a long period of time.

3. Struggle for Existence : According to Darwin, individuals multiply in geometric ratio whereas space and food availability remains constant, so there is an intense competition and three fold struggle for existence. The struggle is intraspecific (between individuals of the same species), interspecific (between individuals of the different species), struggle (between living beings and changes of environment such as heat, cold flood, etc.)

4. Variations : The offsprings are similar to parents, but not identical i.e., they show some changes in shape, size, colour i.e. variations. No two individuals are identical except the identical or homozygotic twins. The everlasting competition among the organisms has compelled them to change in accordance with the conditions to utilize the natural resources and survive successfully.

5. Natural Selection and Survival of the Fittest : Darwin believed that as in artificial selection, man selects animals or plants with desired character, in the same way nature selects only those individuals which are with more favourable variations and are best adapted to the environment. The less fit and unfit organisms are left out by selection. This sorting out of individuals with useful variations is called Natural Selection by Darwin. Individuals with favourable variations survive reach adulthood and reproduce offsprings, others fail to do so. This is called differential reproduction. Darwin has called this differential reproduction as natural selection

6. Inheritance of Useful Variations : Individuals which survive the struggle for existence transmit their useful variations to the offsprings, which will also prove to be fit. Thus offsprings of selected individual are born fit to the environment.

7. Formation of New Species : According to Darwin new variations appear in energy generation and supplement the favourable variations inherited from parents. Thus, variations keep on accumulating and after a number of generations, the offsprings become markedly different forming a new species.