A landslide has isolated a part of the cicada population from the main population. After a few generations, the new generations of cicadas exhibited dramatic gene differences from the old one. The possible mechanisms that can lead to it are:
A. Bottleneck effect
B. Disruptive selection
C. Migration
D. Gene flow
Let us rule out one by one.
Bottleneck effect occurs the population of a species suddenly goes down. This effect can alter the genetic variation present in the population. Hence is likely to effect the cicada population group that is small in number.
Disruptive selection won't happen as the same traits or phenotypes are present in both the split cicada populations. Landslide will not favour certain genes over others that lead to drastic changes in the gene composition of the two populations.
Migration is not present in the above scenario. Hence will not affect the genetic composition of the two populations of cicada.
Gene flow is the key factor that maintains the genetic variation in a population. When the population of cicada gets split, the gene flow gets affected. Certain traits that are present in one set of population goes missing in the other as the two groups are now not able to mate with each other.