Basic postulates of Darwinism :

(i) Enormous Power of Fertility : Every living organism has the power to reproduce rapidly. Hence there is a danger of population explosion, causing the shortage of the food and other facilities. As a result, there are chances of elimination of a number of individuals because the available resources are less and that resulted in competition among organisms where every individual tries to be fit better than the other. Darwin suggested that population of each species remains more or less constant because most of off springs die before themselves becoming capable of reproducing.

(ii) Struggle for Existence : Overpopulation resulted in struggle which Darwin called the struggle for existence.

It is of three types : 

(a) Intra-specific Struggle : The struggle within the species. 

(b) Inter-specific Struggle : The struggle between different groups of animals, e.g. carnivorous animals eat the lower herbivore animals. 

(c) Environment Struggle : Struggle with the adverse environment conditions. Such as extreme temperature, lack of sunlight, water, nutrients, etc.

(iii) Variations and Heredity : During competition, every individual tries to survive and become better fitted and shows variation. But all the variations are not significant from evolutionary point of view-some may be useful and some harmful. The useful variations are inherited in progeny of those organisms in which they arose and, therefore, the progeny have better chances of survival.

(iv) Survival of the Fittest or Natural Selection : During struggle for existence, only useful variations are selected by the organisms, and are adopted. Darwin stated that the variations are sorted out and selected naturally. Therefore, the organisms which possess such selected variations are said to be fittest.

(v) Origin of Species : Due to above facts, Darwin concluded that the struggle for existence leading to the survival of the fittest tend to successive generations to become better adapted to their environment. Thus, these forms are identified as new species.