What is the phenomenon called when variation in gene frequency in a population occurs by chance rather than by natural selection?

Hybrid breakdown refers to the condition where the $F_2$ generation or subsequent generations of a hybrid cross are physiologically inferior or less viable compared to the $F_1$ generation.

Natural selection where more individuals acquire specific character value other than the mean character value,leads to:

Select the incorrect statements:
$I.$ Natural selection is essential for evolution.
$II.$ Natural selection does not include variations.
$III.$ The concept of natural selection was given by Hugo de Vries.
$IV.$ Mutation is a sudden inheritable change.
$V.$ The synthetic theory is also called the Neo-Darwinism theory of evolution.

Which of the following scientists believed in the concept of natural selection like Darwin?

According to the Neo-Darwinian theory,which of the following is responsible for the origin of new species?