Okay, so an adaptations is a special charateristic that helps an animal thrive in a certain habitat, as you may or may not have known.
The most common way that adaptation happens is selective breeding. I am going to use RiverClan as an example here, because why not? Lets assume that RiverClan is just a regular colony of cats that formed the way that regular colonies of cats do. So, group of cats shows up. All of them are colorful and different. Some are red, some are brown, some are gray, you know the drill. So regularly formed RiverClan doesn't take care of each other like in the books, mothers care for each other's kits, but they don't hunt for each other unless they are friends with each other. Any cat that can't get fish dies. So any of those red and brown cats I mentioned earlier. So now are gray only RiverClan wants to learn how to swim to catch the fish and get around. Bunch of cats jump in the water, some get water logged due to their fur and drown.
This leaves normal colony RiverClan with gray, sleek furred cats. Now, these cats are competeing for food. All cats have webbing between their toes, but some cats more so. So the webbed toed cats get more food because they are faster swimmers. The cats that don't swim as well don't get as much food, making them seem unhealthy in comparison. Unhealthy = less likely to get a mate. Cats with webbed toes reproduce, with kittens that are likely even more webbed toed and sleek furred, while the others slowly get weeded out due to some other stuff
Im typing this all at like 1 am its probat not even readable but I tried.