Why do populations evolve?

Populations evolve mainly because of their environment and what helps them to survive best in that particular environment. For example, in the North Pole a brown bear would not survive as well as a polar bear because polar bears have a white fur that allows them to creep up on their prey without their prey seeing them and running away. Also populations evolve because only the best survive, so populations evolve so that they can become the best and so that they can ensure their and their species' survival.