What is the core essence of Darwinism?

Hugo de Vries's experimental organism was

According to Darwin,organic evolution is due to:

Statement-$I$: The fitness according to Darwin,refers ultimately and only to reproductive fitness.
Statement-$II$: Those who are better fit in the environment leave less progeny than others.

Which one of the following is a cause of sympatric speciation?

The most likely reason for the development of resistance against pesticides in insects damaging a crop is