Once upon a time in the forest of Evolutionland, lived peacefully the human species.
Differential survivial and reproduction
As time went by, the human species started to vary and different phenotypes appeared: the preppy human was no longer the only human, but the hairy human and the athletic human appeared, too.
But one day, the temperaute of the climate drastically changed and Evolutionland's average temperature started to drop.
Shift in gene pool
As the weather got colder and colder every day, the preppy humans and the athletic humans started to die and became a minority. However, due to their fur that kept them warm, the hairy humans were able to survive and reproduce.
Soon, the preppy humans and the athletic humans became extinct, but the hairy humans were able to thrive in the new conditions.
Finally, there was a shift in the species' gene pool as the hairy humans became the only type of humans who were able to adapt to their new environment. The hairy trait of the hairy humans was passed on from generation to generation and dominated the gene pool, as it gave the species a higher survival and reproduction rate.