Fact 1: Bottleneck effect occurs when a population suddenly gets much smaller.

Fact 2: Severe droughts have caused much of the rural population to urbanize.

Given the two facts above, answer the question "Severe droughts may cause " with 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 