Answer:
Natural selection is the process by which organisms that are better adapted to their environment are more likely to survive and reproduce, passing on their advantageous traits to their offspring. An example of natural selection can be seen in the difference between highland Tibetans and lowland Han in China.
Highland Tibetans have lived at high altitudes for thousands of years and have adapted to the low-oxygen environment. They have larger lung capacities, more efficient blood oxygen transport, and a higher red blood cell count compared to lowland Han. This adaptation has allowed Tibetans to live and thrive at high altitudes where the air is thin and oxygen is scarce. The ability to adapt to high altitudes is an advantageous trait that has been selected for over generations.
On the other hand, lowland Han have not lived at high altitudes for as long as Tibetans have, and thus have not developed the same adaptations. When lowland Han move to high altitudes, they often suffer from altitude sickness and other health problems due to the lack of oxygen. This is an example of natural selection at work, where the better-adapted population (Tibetans) have an advantage in their environment and are more likely to survive and pass on their advantageous traits to their offspring.