Organisms produce more offsprings than that is needed. But not all survive until reproductive age. Most of them dies. And only some survives
The number of individuals in each species remains approximately constant over time
When the above two hypothesis are operational
Not all individuals survive
There is a struggle for existence.
Thus differential survival
Individuals of a species are not all alike
They vary in form and behavior
Some characteristics of individuals are heritable
Some varieties of heritable characteristics (which are called traits ) are more adaptive than others.
When the Hypothesis 3 and 4 are operational
Some kinds of individuals are more successful than the others. They have a greater chance of reproducing
Thus Differential Reproduction
These individuals with adaptive traits produce a greater proportion of the individuals in the new generation.
Those traits increase in frequency from generation to generation.