We start with 100% of the population still intact and all the bears are thriving.
Then suddenly, a bear is born and raised thinner than the rest of the bears but at a cost of not being able to store food as well.
Suddenly there is a over production of thinner bears in the environment, but at that moment there is a fire that burns the forest down and are forced to move while some of the thinner bears die due to lack of food
The bears are forced to migrate to a new location with better food and water, and start reproducing.
And due to the fire and large better food and water, there is a less demand for thinner bears.
Then in the end the bears will be reproducing normal bears for now until there is a shift in the environment.