Principle #1 - Genetic Variation Variation exist in the Polar Bear's coats. Some polar bears have thin coats and other have thick ones.
Principal #2 - Overproduction & the Struggle for Existence Polar Bears will always produce more offspring than the environment can support. Not all cubs will survive.
Principle #3 - Differential Survival & Reproduction The thin coat polar bears are most likely to freeze to death than the ones with thick coats.
Principle #4 - Changes in Gene Pool Since the polar bears with thick coats have higher fitness in the cold weather they'll most likely have a higher frequency and pass down their thick coats to the next generations.
Over time the frequency for thick coats alleles will increase in the gene pool and no polar bear will ever freeze to death. :D