Members of population vary in physical, physiological and adaptive capacities. Individuals with adaptive variation survive only and those which do not have are perished.
Therefore, variations within a species enable the nature to exercise her selection.
(iii) Struggle for Existence:
Over-production and variations ensure a competition to obtain its iota of food, space and mate. This competition is called struggle for existence, which has inter-specific, intraspecific and environmental dimensions.
(iv) Survival of the Fittest/Natural Selection:
The ensuing competition is very tough. The natural conditions operating on the individual act as a selective force. Only those individuals with favorable variations are able to survive and are adapted to the available conditions.
(v) Inheritance of Useful Variations:
The individuals, who have been selected by nature, pass on their useful variations to the offspring of next generation.
The useful variations are preserved and accumulated in the individual of the species which ultimately lead to origin of a new species.