What is the possible reason for the development of resistance in pests against crop-destroying agents?

What is the core essence of Darwinism?

Given below are two statements regarding evolution.
Statement-$I$: Natural selection is the establishment of a phenotypic trait in changing environmental conditions.
Statement-$II$: Natural selection encourages those genes or traits that ensure highest degree of adaptive efficiency between population and its environment.
In the light of above statements, choose the most appropriate answer from the options given below.

An elephant produces six young ones in its life span of $100$ years. If this continues,then,in $750$ years,$19$ million elephants will be produced. It is $NOT$ possible in reality mainly due to . . . . . . .

Genetic drift operates in

Who proposed the mutation theory of evolution? Explain it.