Principle #2 Overproduction & the Struggle for Existence:
Principle #3:Differential survival & Reproduction
There are two types of bears. There are skinny Polar bears. There is also bigger Polar bears.
Principal #4 Change in the Gene pool:
The skinny bears are chosen against their environment and the bigger bears are chosen for their environment. It is harder for the skinny bears to survive.
Skinny Polar bears do not survive as much as the bigger ones because they are not as well nutritioned as the bigger ones are causing them to be slower. Therefore bigger Polar bears survive and reproduce.
Over time the Polar bears will be mostly be all big. Skinny bears are not getting to reproduce so the bigger Polar bears will reproduce and make more of their population.