Darwin judged the fitness of an individual by

When is genetic drift operative?

Natural selection can lead to . . . . . . .
$(a)$ stabilisation
$(b)$ genetic drift
$(c)$ directional change
$(d)$ disruption
Choose the correct answer from the options given below:
$(A)$ $(a)$, $(b)$ and $(c)$ only
$(B)$ $(a)$, $(c)$ and $(d)$ only
$(C)$ $(a)$, $(b)$, $(c)$ and $(d)$
$(D)$ $(a)$ and $(c)$ only

Which organisms are considered the most adapted in the environment?

$A$ scientist who worked on rats cut their tails up to $21$ successive generations,but at last,he found that the tails remained normal. Which scientist's theory is disapproved by this experiment?

Which one of the following is correct with respect to Allopatric speciation?