In the cold environment there is a population of seals; some have thin blubber under their skin while the other has thick blubber under their skin.
Changes in gene pool
the seals with thin blubber are struggling to stay alive because their blubber isn't keeping in enough heat resulting i them freezing to death. The seals with the thicker blubber are staying warm and are able to live and reproduce.
The thick blubber seals will stay alive in this environment and pass the genes on to the next generation.
Since the seals with the thick blubber were the only ones to stay alive because they were able to keep warm in the cold environment, the genes pool in the population will produce all thick blubber seals.