Answer:
Adapting to living in high altitudes
Step-by-step explanation:
High altitudes means a lower oxygen partial pressure. Which means that the oxygen concentration is quite low and humans will need adaptation to deliver the required amounts of oxygen to the tissues.
People who live in high altitudes not only have a higher red blood cell count and haemoglobin content to help them carry enough oxygen to the body cells, the also have wider chests and bigger hearts. All which helps in achieving the body's demands.
Since those changes increase the human's chance of survival in those altered conditions, it's an adaptation.