What causes a sudden large change in a population?

'$A$ brief reduction in size of a population,due to natural calamities,usually leads to random genetic drift'. For this statement,identify the correct example from the following.

Organic evolution would not have taken place:

In recent years,$mt-DNA$ and $Y$-chromosome $DNA$ sequences have been used for the study of human evolution because:

Which of the following phenomena supports Darwin's concept of natural selection in biological evolution?

Which organisms are considered the most adapted in the environment?