Natural selection operates at the population level because organisms are the ones that have to adapt to a changing environment.
Example
The evolution of the peppered moth in England is an example of natural selection at work. Before the Industrial Revolution, the light-colored moths were better able to blend in with the trees and lichens in their environment. However, as the trees became darkened by soot from factories, the light-colored moths became easier for birds to spot. Over time, the frequency of the darker-colored moths increased because they had a higher survival rate.
