Fact 1: Gene flow occurs when individuals move into or out of a population.

Fact 2: Gene flow between populations is unidirectional.

Given the two facts above, answer the question "what is individuals moving into or out of a population?" with the following options: 
- Greenhouse gases
 - rapid expansion
 - Burning fuel and air
 - permineralization
 - peachleaf willow
 - competition
 - unidirectional
 - food and shelter


 

       unidirectional  