Answer: 
Gas exchange
Explanation:
Gas exchange takes place in the millions of alveoli in the lungs and the capillaries that envelop them. As shown below, inhaled oxygen moves from the alveoli to the blood in the capillaries, and carbon dioxide moves from the blood in the capillaries to the air in the alveoli.
 
        
             
        
        
        
It is a perfect example of a natural selection.
(I'm not sure if this is the exact answer that you are looking for.)
        
                    
             
        
        
        
Small population size is your answer mate.
        
             
        
        
        
Hey, how are you doing?
The answer to your question would be: 

 i believe. 
Hope this helps, and if you need further help, contact me!
Thank you,
Darian D.