Each grizzly bear can produce more offspring than can survive. Only some of their offspring survive.
The variations in the bears have different claw sizes. Longer claws being able to help the bear more while the shorter claws can be difficult to survive with.
If there is less food in the environment, it creates competition between the bears. The bears with the larger claws will be more likely to survive because their large claws give them an advantage when getting food compared to bears with smaller claws.
The bears who have the smaller claws will be the ones who will most likely not survive. Their claws aren't as strong as the larger claws so it will be more difficult for them to catch food.
The variations are selected for and against. The ones with the helpful variations help them survive and get food. Unlike the ones who are unable because of the unhelpful variation. So, some do not survive.
The bear with large claws is able to reproduce because it survived throughout its life since it had a helpful variation of bigger claws. There are now more bears with larger claws since the smaller claws did not survive.