Darwin failed to provide an explanation for which of the following?

Allopatric speciation is due to

The ultimate source of organic evolution is:

Given below are two statements regarding evolution.
$Statement-I$: Isolation is the separation of a population of a particular species into smaller units, which prevents interbreeding between them.
$Statement-II$: Due to isolating mechanisms, divergence and speciation do not occur.
In the light of the above statements, choose the most appropriate answer from the options given below.

In the case of the peppered moth $(Biston \text{ } betularia)$, the dark-colored form became more prevalent than the light-colored form in England during the Industrial Revolution. This is an example of:

The frequency of a mutant gene in a population is expected to increase, if the gene is