Charles Robert Darwin published his theory of natural selection in his book ‘Origin of Species’ in 1859. Due to extreme popularity of the theory he is called ‘Father of the Evolution’ and his ideas are known as Darwinism. Darwin based his theory on certain observable facts and deduced some principles.
1. Enormous Fertility:
It is seen that animals and plants have enormous power to reproduce during breeding season. They reproduce offsprings in geometric ratio. An oyster produces 60-80 millions of eggs and a salmon lays 28,000,000 eggs in a year. A pair of flies can produce 191,010,000,000,000 descendants in a season. Thus, one may presume if all the offsprings survived and reproduced successfully the earth would have been filled with either one or other kind of animals. It is not so and populations of different species remain more or less constant. To explain this fact Darwin suggested struggle for existence.
2. Struggle for Existence:
In contrast to increase in the size of population, amount of food and space remains constant. Hence, there is an intense struggle among organisms for food, land, mate, etc. tp ensure living. This struggle is of three layers.
(a) Intraspecific struggle:
Struggle among individuals of the same species is severe since their requirement for existence is the same. It is called intraspecific struggle. For example, young seedlings spring up in clusters but when competition for food and sunlight becomes severs many die. Those grow faster have better chance to survive.
(b) Interspecific struggle:
Struggle between individuals of different species living together is called interspecific struggle. Different species have same food habit and so they compete among themselves. Moreover, the carnivorous animals prey upon the herbivorous ones.In turn, a secondary consumer may be killed and eaten up by a higher grade of carnivore. Thus, struggle for safety and struggle for getting prey animal is intense. Arnold said, ‘a slew is a slayer and in turn has been slain’.
(c) Struggle with the environment:
At times the environment changes drastically. To come out successful in the favourable conditions the organisms have to struggle. Physical processes like cyclone, earthquake, flood draught, extreme heat or cold add severity to the life of organisms. So they have to adopt in some manner in order to overcome or avoid these hazardous steps of the nature.
3. Variation and Heredity:
In order to better adapted to the ever-changing environment organisms tend to vary. Individuals of the same species vary among themselves. Even the offsprings from the same parents are not alike. Such differences are called variations. Darwin considered small and continuous variations to be essential for evolution.
Variations may be neutral, beneficial and harmful.
4. Survival of the Fittest:
The individuals or races with beneficial variations have better chance to come out successful in the struggle for existence. They are able to better withstand the hardship of the nature.Individuals with harmful variations fail in the competitions and are eliminated. So only the better variants are able to reproduce and transmit their characters to the next generation.
5. Natural Selection:
From above it appears as if nature weighs in the balance and selects better ones eliminating unwanted variants. Darwin explained long-necked giraffe had a better chance to survive over their short-necked kins and were better adapted to the changed surroundings.
6. Origin of New Species:
From the aforesaid facts and deductions, Darwin concluded that origin of new species takes place by natural selection. He summerised place by natural selection. He summerised that as a result of struggle for existence, variation and inheritance the offsprings of successive generations are expected to be better adopted for the next phase of struggle.
Darwin called it descent with modification.x the environment is ever changing and it demands new adaptations and hence new variations to appear in the organisms. As this natural process is continuous in the long-run descendants might differ in a greater way from their ancestors and can be called a separate species. In the same population different groups facing diverse environment may be adapted differently and they might give rise to more than one species.