All of the grizzly bears in the forest were mostly the same size. They were all different colors, but the thing that made them the same was their size. They were all large.
After the bears came out of hibernation in the spring, they had lots of new offspring. One of these bears ended up looking a little different than all the other bears. He was skinnier than all the other ones. It was an inherited mutation.
As the bear grew up, they started to realize this bear was much more fit and agile than all the other larger bears. He could run to the fish better and he could run longer, so he was able to survive better. This allowed him to live longer and he outlived everyone.
On top of being faster than all the other bears, because he was able to outlive the other bears, he mated with lots of female bears, having lots of new offspring that looked like him.
When the bear mated, he had four offspring. Three of these cubs were skinny, and the other one was larger. This proves that he passed on his mutation to his offspring.
100 years later, the majority of the bears were skinny. Over the years the allele frequency had changed because the skinnier bears were living longer. Natural selection had selected the skinny trait, and from then on, most of the bears were fit.