Natural Selection describes the process and the outcome, not intent. It is also situational, where different types of plants and animals fill different kinds of roles at different times because of overall pressures and opportunities from the environment.

Pandas eat bamboo, a very low energy source, in their environment because of environmental pressure. Kodiak bears get absolutely massive on their island because of the prevelance of seafood (whales) that wash up and provide an opportunity for more energy dense food sources. Neither are the result of design, just local environments.

Crocodiles survive extinction events because of their biology allowing for long periods without food and water. But they aren’t successful in cold climates for the same reason. They are vulnerable to other pressures like overhunting, but their other traits tend to discourage being eaten. Something still eat them though! Their immune system is crazy strong because it improved due to where they were successful, stagnant waters with a lot of bacteria.

Bats are the opposite, very fragile physically. But the species have adapted to communal living with fast reproduction and a strong immune system due to that proximity with each other to be massively successful on a global scale.

None of these are designed, and birds on islands being very different from those on mainland because they fit into different roles is a very easy way to see that there is nondesign, just adaptation given enough time.

Since the very beginning the life that was successful was the life that successfully reproduced. That just continued since that point in time, so all current life is also the life that successfully reproduces whether consciously or not.