The main aspects of the theory of natural selection are as follows.

1. Rapid multiplication.

2. Limited food and space causing struggle for existence.

3. Natural selection or survival of the fit and elimination of the unfit.

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4. Origin of new species.

1. Rapid Multiplication:

Every living organism in this world repro­duces itself. The rate of increase of the organisms is always in a geometric proportion. Darwin illustrated this point with the example of the slowest breeder among animals, the elephant. An elephant starts breeding at its 30th year continues to do so until it is 90 years. It lives up to 100 years.

So in its life time it produces 6 offsprings. Darwin calculated that if all these survive, within about 750 years there would be about 19 million elephants. Oysters produce about 80 million eggs in a season. A pair of sparrows would produce 275 billion individuals in about 10 years if there is no death.

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2. Struggle for existence:

The rate of multiplication and existence of organisms are more or less common for all living things. But the food supply and space remain unchanged, in other words they are limited. Under these conditions, there starts a competition among the organisms to fulfil their needs of food and space. This is known as struggle for existence. It may be interspecific, that is between the members of different species or intraspecific between members of the same species or between them and the environmental factors.

3. Variations:

Every organism has its own specific characteristics and relationship with its environment, the variability caused by ‘variations’ may be large or small. Large variations are known as ‘macrovariations’. Small variations are termed as ‘microvariations’. Variations may be favourable or harmful. Those with useful variations survive while the others perish.

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4. Natural selection or Survival of the Fit:

The organisms with favourable variations are best adapted to the environment in which they live. They have a better chance of survival and perpetuation of their race. This principle is called Natural Selection or survival of the fit. While the organisms without any useful variations or with harmful variations are ill suited to the environment and hence they die and perish. This is known as the ‘elimination of the unfit.

An example of adaptation of a species to a changing environment is amply illustrated by the birds known as “Darwin’s Finches”. Darwin observed 13 species of this bird each with special adaptation for its particular mode of life. Those that fed on insects had long slender beaks by which they could peck small insects from small crevices and eat them On the other hand plant eaters had short strong beaks which were useful in breaking open the hard nuts.

Those living on volcanic islands covered with black ash possessed black feathers to merge with the surroundings. Darwin on observation of these argued that if a group of individuals belonging to the same species have different variations to suit the different environmental conditions in which they live, after continuing for several generations in this manner the individuals belonging to one and the same species origi­nally will become so different from their ancestors that they can now be classified as new species. This according to Darwin is the basis of the origin of new species.

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5. Origin of Species:

As already stated, the organisms with favourable variations transmit the same from generation to generation. With every generation the variations get distinct and refined. Due to the cumulative effect of these distinct variations, organisms emerge as new species in course of time. Thus new species are the evolutionary products of preex­isting forms, by developing useful variations.