Any change in the heritable characteristics of biological population over successive generation is known as Evolution.

Darwin’s theory of evolution – Natural Selection

Variation occurs within a population. Although Species produce a great number of offspring yet the population number remains the same. This constancy in the population number is because of the struggle for resources like food, shelter etc. This Is known as struggle for existence. It favors the fit individuals which survive and reproduce and eliminates the unfavorable ones. This is Natural Selection. These variations accumulate together and they form a new species over time