Bottleneck effect occurs when a population suddenly gets much smaller, and severe droughts have caused much of the rural population to urbanize. Given these facts, severe droughts may cause  among the following options:
- evolution
 - population growth
 - forest is destroyed
 - species variation
 - major threat to health
 - Dehydration
 - the ground shaking
 - a bottleneck effect


 

        a bottleneck effect 