What causes a sudden large change in a population?

Given below are the two statements.
Statement $I$: Directional isolation favours the intermediate forms and eliminates both phenotypic extremes.
Statement $II$: In directional isolation,more individuals acquire a value other than the mean character value.
In the light of the above statements,choose the correct answer from the options given below:

Which factors contribute to the success of organisms with variations?

Which is the most important factor for the evolution of a new species?

$A$ mutation is most likely to have a selective advantage in evolution if:

Darwin,in his 'Natural Selection Theory',did not believe in any role of which one of the following in organic evolution?