Which technique is used to prove that antibiotic-resistant bacterial mutants are pre-existing and not induced by the environment?

Which of the following types of isolation is important for speciation?

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

Genetic variation in a population arises due to

Which of the following was $NOT$ considered a part of Darwin's theory of natural selection in organic evolution?

The initiating force of evolution is