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. Which mechanism is the most likely cause of these differences?

The bottleneck effect is the cause of the sudden shift in the genetic information of the population. Due to the landslide, suddenly high number of the members of the main population died, and this caused genetic drift for reducing the variation in the existing generation, which is now widely different from the original one.