Since the forest is full of berries and there is a town nearby, there is an abundance of different species of mice living there. Because mice are the owls' food source and the owls have no predators in the forest, the owls are overproducing and creating more offspring than what can survive.
A mutation occurred in the owl population, causing a change in the genes that altered the protein and made some owls to have larger claws than the other owls. When the owl reproduced, his dominant trait got passed to his offspring. This created variation in the population since some owls had small claws and others had large claws.
A tornado goes through the forest, wiping out most of the undergrowth, including the berry bushes! The lack of food in the forest causes the mice population to decline quickly, almost to extinction.
Changes in the Gene Pool
The owls with the larger claws trait can catch bigger prey and bring more food to their young than the owls with normal-sized claws. They can also build nests faster and fight back predators when flying isn't an option. These abilities cause an increase in the frequency of owls with bigger claws. The population of owls with normal claws declines because they have worse fitness and can't survive without mice in the forest. The larger claws trait is an adaptation that boosts the owls' fitness and allows them to survive and have better reproductive success.
The population of owls with big claws increases because they have the favorable trait, and therefore the advantage. Since they can survive for longer, they can reproduce more offspring and grow much more than the owls with normal claws. The pattern changes from owls with normal claws being born to owls with large claws being born. This is because the organisms with the best fitness survive.
After many generations, the normal claw genes disappear from the gene pool, leaving only the evolved owls with large claws in the forest. This happened because the large claw variation was more favorable than the normal claw trait.