Answer:
Genetic Drift
Step-by-step explanation:
Genetic drift refers to the evolutionary agent that brings about the changes in the gene pool of the population by some chance event. In the given example, half of the polar bear population was wiped out of the habitat by any chance event such as natural calamity, epidemic disease or any other random events. Since no mutation or natural selection was responsible for the death of polar bears, it is an example of genetic drift.
Genetic drift is more effective in small populations. And the size of the population of the polar bear was also small to allow the genetic drift to cause the changes.