'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.

Two zoogeographical regions separated by high mountain ranges are

Study of fossil evidences of evolution is called

$A$ : Blood group analysis shows the human beings are more closely related to apes than to monkeys.
$R$ : The blood groups $A$ and $B$ are found in apes but not in monkeys.