The main concept of the theory of Darwin which is based on facts and evidences is the natural selection and that operates in the transmission of species. The theory is based on the certain principles which we shall describe briefly and those are as follows:
A) Prodigality of Production : - reproduction is one of the fundamental property of living organisms. Darwin observed that each of the species reproduced at a terrific rate in a geometric ratio. Elephant is the slowest breeder amongst the animals. From a pair of elephant approximately three young one are produced. Mr. Darwin observed it and calculated that a single pair of elephant will be able to produce nineteen millions of descendants in just 750 years. A salmon fish produces 28,000,000 eggs in a season. If such immense number of eggs survive and mature and reproduce again with same number of eggs, what will be happen? Within few years the whole water space of the earth will be filled up only with the salmon fish. But it is the matter of natural selection for which the overcrowding of the species is resisting. The geometric multiplication will cause overcrowdings and the struggle for existence becomes inevitable.B) Struggle for existence: - Though the reproduction level is high, yet the food and spaces for living area never increase proportionately. As a result, there will be keen competition for food and shelter. so, this keen competition for survival amongst the living organisms is known as Struggle for existence. In Darwin’s theory of natural selection there are three kinds of struggle for existence, those are –
i) Intra-specific struggle : This is a struggle for food and space amonges the individuals of the same species. As for example, many seedlings are produces from a tree, which are distributed surrounding the tree. All these seedlings are trying to get light, air and raw materials for food from the surrounding soil. So, it is easy to understand the keen competition amongst the seedlings for procuring the above materials.
ii) Inter-specific struggle : In the theory of natural selection, this is a struggle for food and space between the member of different species. Innumerable instances are there in the living world. As for instance, tiger is hunting for deer which is their food. But it is quite natural that the deer is also trying to escape from the tiger. This type of struggle for existence is always going on.
iii) Environmental struggle : All living organisms are always struggling to be alive against the natural calamities such as flood, drought, storm, earthquake etc. In the living world, plants and animals are always facing those three kinds of struggles. As a result of struggles, death occurs in many organisms. Thus more or less a constant number of individuals of a species are always maintained.
C) Variations : Variation is another factor in the theory of natural selection. Darwin observed that there is a difference in the form and structure among the individuals of species through there is fundamental similarities. These differences among the individuals of a species are known as variations. These variations are the natural phenomena and act as a raw material of evolution. There are two kinds of variations, those are favorable and unfavorable. The favorable variation are the variations which helping the organism to adjust with the environment. On the other hand the unfavorable variations are not helping to struggle the organism and as a result the organism ultimately fail to survive.
D) Survival of the fittest : - As per theory of natural selection, continuous competition among the individuals are going on for their survival in nature. Only those individuals which have some favorable heritable variations in structural and functional features will adapt in new conditions. So, organisms with such favorable variations have better chance to survive and they will be considered as the natural selection. Consequently those favorable variations will be transmitted to their offspring. And, those individuals which have unfavorable variations will fail to adopt in the new environment and ultimately will be eliminated from nature. This is called as the survival of the fittest .
E) Natural selection and speciation : - The nature itself will decide, which one is fit and which one is unfit in nature. The good varieties will be selected by nature and the other will be eliminated. This is called natural selection. Natural selection operates amongst the fittest organism. Favorable variations are preserved in the organisms by natural selection and transmitted to the offspring. Repeated selection leads to slow and quite steady modifications of the structure and function in the subsequent generations. In course of time new forms are developed which are quite different from the primitive ones. These new forms are stabilized and leads to a new species. This are the process of natural selection and as a result the new species have evolved in nature by its operation.