It seems like the answer to "how did the categories get in the bucket" is 'they are whatever is left after the constraints of the system have removed everything else.'

For example, imagine you have a room full of different kinds of dice. A lever is pulled and any die that is not six-sided is removed from the room. All that's left are six-sided dice. You can no longer select for die that tend to land on seven, as the system has removed them as a possible option.

For evolution, the constraints would be the laws of physics. Whatever is left after the laws of physics have removed the impossibilites is what can be selected for.