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

Genetic drift is observed only in which of the following?

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.

According to Neo-Darwinism,which of the following is responsible for evolution?

Present the views regarding the origin of variation and speciation.

$XX$ is a type of selection process in evolution. $XX$ promotes population changes in one particular direction. $XX$ favours small or large-sized individuals,and the mean size of the population changes in $XX$. Identify $XX$.